Rieppel, 1998

Zoological Journal of the Linnean Society (1998), 124: 1–41. With 8 figures

Article ID: zj970131

Corosaurus alcovensis Case and the phylogeneticinterrelationships of Triassic stem-groupSauropterygia

OLIVIER RIEPPEL

Department of Geology, The Field Museum, Roosevelt Rd. at Lake Shore Dr.Chicago, IL 60605-2496, U.S.A.

Received January 1997; accepted for publication July 1997

The holotype and other material of Corosaurus alcovensis Case from the Alcova Limestone ofCasper, Wyoming, was further prepared using a combination of chemical techniques.Anatomical revision resulted in the definition of new characters for the analysis of thephylogenetic interrelationships of Triassic stem-group Sauropterygia that document paraphylyof the Eusauropterygia. Corosaurus is the sister-taxon to a clade comprising Cymatosaurus,Pistosaurus, and by extension, plesiosaurs and pliosaurs.

1998 The Linnean Society of London

CONTENTS

Introduction . . . . . . . . . . . . . . . . . . . . . . . 1Material and methods . . . . . . . . . . . . . . . . . . . 2

The geological provenance of Corosaurus . . . . . . . . . . . . 3Systematic palaeontology . . . . . . . . . . . . . . . . . . 3Character definitions . . . . . . . . . . . . . . . . . . . . 5Cladistic analysis . . . . . . . . . . . . . . . . . . . . . 24Classification of the Sauropterygia . . . . . . . . . . . . . . . 33Discussion and conclusions . . . . . . . . . . . . . . . . . . 35Acknowledgements . . . . . . . . . . . . . . . . . . . . 38References . . . . . . . . . . . . . . . . . . . . . . . 38

INTRODUCTION

Corosaurus alcovensis Case, 1936, from the Alcova Limestone of Casper, Wyoming,was until recently the only Triassic stem-group sauropterygian known from the NewWorld. Case (1936) placed the genus in the suborder Nothosauroidea sensu Peyer(1933–34), noting its intermediate position between the Pachypleurosauridae andNothosauridae as defined by Peyer (1933–34). F.v. Huene (1948) placed Corosaurus,together with Simosaurus and Conchiosaurus (in fact a senior synonym of Nothosaurus:Rieppel & Wild, 1996; see also Rieppel & Brinkman, 1996), in his Simosauridae, a

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10024–4082/98/090001+41 $30.00/0 1998 The Linnean Society of London

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family he believed to be related to the Pistosauridae and, by extension, to plesiosaursand pliosaurs. Zangerl (1963: 120) considered Corosaurus to be “the most advancedmember of the Nothosauria”, a conclusion he believed to be in accordance with hisinterpretation of the Alcova Limestone as Late Triassic in age. The holotype (Case,1936) and further material pertaining to the taxon (Zangerl, 1963) was the subjectof a recent monographic treatment by Storrs (1991). Storrs (1991) placed the AlcovaLimestone in the uppermost Lower Triassic (Spathian); his cladistic analysis placedCorosaurus as sister-taxon of the Eusauropterygia sensu Tschanz (1989).

In view of our present knowledge of geographic occurrence of Triassic stem-group Sauropterygia in China, Europe and in the western United States, thephylogenetic relationships of Corosaurus are of crucial importance for the under-standing of the early paleobiogeographic history of the clade. The importance ofthe relative phylogenetic position of Corosaurus is emphasized by the fact that theearliest stem-group sauropterygians to occur in Europe are the pachypleurosaursDactylosaurus (Rieppel & Lin, 1995) and the eosauropterygians Cymatosaurus (E.v.Huene, 1944) and Nothosaurus (Meyer, 1847–55), all from the uppermost LowerTriassic (Rot, so2; Spathian), and hence of equivalent age to Corosaurus. Unfortunately,the stratigraphic control over the occurrence of stem-group sauropterygians in Chinais less precise.

Since the monographic treatment of Corosaurus by Storrs (1991), the Europeanstem-group sauropterygians have been subject to extensive reviews (e.g. Rieppel,1993a,b, 1994a, 1995, 1996; Rieppel & Lin, 1995; Rieppel & Wild, 1996), whichresulted in an expansion of the data matrix, and some changed definitions ofcharacters for the analysis of sauropterygian interrelationships as presented byRieppel (1994a). It is for this reason that the phylogenetic interrelationships ofCorosaurus will here be tested again against this expanded data set.

MATERIAL AND METHODS

The material of this analysis includes the holotype of Corosaurus alcovensis Case,1936, kept at the University of Wyoming in Laramie (UW 5485). The specimenconsists of several blocks of matrix which fit together and allow the restoration ofthe skeleton as preserved (Case, 1936, fig. 1; Storrs, 1991, fig. 3). This specimenhas been further prepared, and some of the skeletal elements have been completelyremoved from the matrix. Additional specimens included in this review is thematerial first described by Zangerl (1963; FMNH PR480, PR135), some of whichwas also further prepared, as well as the specimen collected in 1996 (FMNH 2018).

Mechanical preparation of Corosaurus embedded in the Alcova Limestone isextremely difficult and time consuming due to the hardness of the matrix. For thisreason, further preparation of Corosaurus involved a combination of chemical methods.After filling in cracks and sealing the bone surface with acryloid resin, the specimenwas exposed to the Waller solution, following the Waller method described by Blum,Maisey and Rutzky (1989). This method is designed to remove ferruginous matterfrom the matrix. After suitable exposure to the Waller solution, a thin layer ofpowdery matrix was removed by exposing the specimen to a solution of 5% bufferedformic acid. When the matrix ceases to react with the acid, the cycle begins againwith exposure of the specimen to Waller solution. The details of this procedure willbe published elsewhere (Passaglia & McCarroll, 1996).

TRIASSIC STEM-GROUP SAUROPTERYGIA 3

The geological provenance of Corosaurus

The holotype of Corosaurus alcovensis was collected in 1935 on a quarry dumpderived from an excavation in the Alcova Limestone in Jackson Canyon 9 milessouthwest of Casper, Natrona County, Wyoming (Case, 1936). Today, large partsof the Jackson Canyon, and the adjacent Goose Egg area, featuring massive outcropsof Alcova Limestone on top of Chugwater red shales and sandstones, are privateland and constitute the Jackson Canyon Wildlife Viewing Area, rendering accessdifficult. Southwest of the Jackson Canyon area is Bessemer Mountain, with vastexposures of Alcova Limestone which, again, are difficult to access today since theyare surrounded by private land.

A Field Museum field party visited the Alcova Limestone in 1948, and collectedFMNH PR480 and PR135 in outcrops 3 miles north-east of Freeland, NatronaCounty, Wyoming (Zangerl, 1963). In his monograph on Corosaurus and the AlcovaLimestone, Storrs (1991) added a third locality yielding Corosaurus on the south slopeof Muddy Mountain, south of Casper (outcrops located on the Milne Ranch; Storrs,1991, fig. 45).

A field party from the Field Museum returned to the outcrops of the AlcovaLimestone in the general neighbourhood of Casper in the summer of 1996 (Fig. 1).Vast outcrops of Alcova Limestone on federal land were intensively searched (underpermit # P96-WY-018), including an extensive ridge 4 miles north of Freelandextending northwards, the massive outcrops 3 miles northeast of Freeland first visitedby Zangerl and his crew in 1948, outcrops on the north slope of Muddy Mountain,as well as the exposures of the Alcova on the south slope of Muddy Mountainsearched by Storrs (1991). Corosaurus proved to be widespread throughout the AlcovaLimestone in the Casper area, but is very scarce and represented by very fragmentarymaterial only (Fig. 1). Only one partially articulated skeleton was found and collected,on the north slope of Muddy Mountain (FMNH 2018).

The stratigraphic correlation and hence determination of the relative geologicalage of the Alcova Limestone is rendered difficult by the general paucity of fossils,vertebrates (Corosaurus) and invertebrates alike (for a review see Storrs, 1991). Thepresence of the nothosauriform reptile Corosaurus has led some workers (Colbert,1957; Zangerl, 1963) to assign the Alcova to the Upper Triassic. Storrs (1991: 101)accepted a late Lower Triassic (Scythian, Spathian) or, perhaps, an early MiddleTriassic (Anisian) age for the Alcova Limestone.

SYSTEMATIC PALEONTOLOGY

Sauropterygia Owen, 1860Eosauropterygia Rieppel, 1994aCorosaurus Case, 1936Corosaurus alcovensis Case, 19361936 Corosaurus alcovensis, Case, p. 1.1948 Corosaurus, Huene, p. 41f1963 Corosaurus alcovensis, Zangerl, p. 117.1991 Corosaurus alcovensis, Storrs, p. 1.

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C A S P E R

Casper Mountain

Muddy Mountain

Freeland

JacksonCanyonBessemer

Mountain

GooseEgg

N

*

**

*

*

505

402

487

251

220

Figure 1. Localities (∗) in the Alcova limestone surveyed in summer 1996, yielding Corosaurus bone.

Holotype. UW 5485, skull and partial skeleton (Figs 2–6).

Horizon and Distribution. Alcova (Limestone) Member, Crow Mountain Formation,Chugwater Group, Triassic. Vicinity of Casper, east-central Wyoming.

Diagnosis. An eosauropterygian with a relatively short and unconstricted snout;postorbital region of skull subequal in length to preorbital region of skull; nasalselongated, only slightly shorter than frontals; upper temporal fossa subequal in sizeto orbit; frontals closely approaching the upper temporal fenestra; parietal skulltable weakly constricted; mandibular symphysis weakly enforced; maxillary fang(s)in anterior position; distinct coronoid process on lower jaw present; the posterioraspect of the dorsal tip of the neural spines distinctly broadened; sacral ribs fusedto respective vertebrae; transverse processes on caudal vertebrae distinctly elongated;dorsal part of medial surface of ilium ornamented by densely packed tubercles;pubis of distinctive shape, with convex anterior and concave posterior margin;deltopectoral crest on humerus weakly developed; medial gastral rib element fre-quently with two-pronged lateral process.


Figure 2. Skull of Corosaurus alcovensis Case (holotype, UW 5485), in dorsal view. Scale bar=5cm.

CHARACTER DEFINITIONS

The characters listed below are based on the data used previously in the analysisof the phylogenetic interrelationships of Simosaurus (Rieppel, 1994a). The data matrix

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pm

m

n

f

prf

pof

ppofpoju?

sq

so

op-eo

sq

q

Figure 3. Skull of Corosaurus alcovensis Case (holotype, UW 5485), in dorsal view. Scale bar=20mm.Abbreviations: f, frontal; ju, jugal; m, maxilla; n, nasal; op-eo, opisthotic-exoccipital (paroccipitalprocess); p, parietal; pm, premaxilla; po, postorbital; pof, postfrontal, prf, prefrontal; q, quadrate; so,supraoccipital; sq, squamosal.

relies heavily on the work of Gauthier, Kluge and Rowe (1988), further augmentedby the addition of characters taken from Evans (1988) and Storrs (1991, 1993).Additional references pertaining to the coding of non-sauropterygian taxa can befound in Rieppel (1994a). Codings for sauropterygians other than Corosaurus arebased on recent revisionary work (Rieppel, 1993a,b,c, 1994a,b,c, 1995, 1996, 1997;Rieppel & Lin, 1995; Rieppel & Wild, 1994, 1996). Coding for the postcranium ofCymatosaurus is based on the hypothesis that Proneusticosaurus Volz,1902, is a subjectivejunior synonym of Cymatosaurus Fritsch, 1894 (Rieppel & Hagdorn, 1997).

1. Premaxillae small (0) or large (1), forming most of snout in front of externalnares.

The premaxillae cover the entire snout region of the skull in front of the externalnares in Corosaurus (1) (Figs 2, 3).

2. Premaxilla without (0) or with (1) postnarial process, excluding maxilla fromposterior margin of external naris.


Figure 4. Dorsal vertebrae 27 through 31 of Corosaurus alcovensis Case (holotype, UW 5485). A, leftlateral view; B, dorsal view. Scale bar=2cm.

Corosaurus, as all Sauropterygia, lacks this process.3. Snout unconstricted (0) or constricted (1).In the ‘eusauropterygian’ genera Cymatosaurus, Germanosaurus, Nothosaurus and (most)

Lariosaurus, the snout is distinctly constricted at the level of the anterior margin ofthe external naris. A constriction of the snout is absent in pachypleurosaurs and inSimosaurus; in Pistosaurus, the snout gradually tapers to a blunt anterior tip. The snoutis constricted in Placodus, but tapering in all cyamodontoids except Henodus. A snoutconstriction is absent in Corosaurus. Indeed, the contours of the snout of Corosaurus(Figs 2, 3) resemble exactly those of the specimen of Lariosaurus described by Mazin(1985).

4. Temporal region of skull relatively high (0) or strongly depressed (1).Placodus, pachypleurosaurs, Corosaurus, Cymatosaurus, Simosaurus and Pistosaurus show

a distinctly lesser degree of depression of the temporal region of the skull than isobserved in the Germanosaurus–Nothosaurus–Lariosaurus clade.

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Figure 5. Postcranial elements of Corosaurus alcovensis Case. A, dorsal vertebra 26 in posterior view(UW 5485); B, dorsal vertebra in posterior view (FMNH PR245); C, left scapula in medial view (UW5485); D, right ilium in dorsomedial view (UW 5485). Scale bars: A=1cm; C, D=2cm.

5. Nasals shorter (0) or longer (1) than frontal(s).As in other ‘eusauropterygians’, the premaxillae form a combined posteromedian

process which extends far between the nasals, without reaching the anteromedialprocess of the frontals. The nasals taper anteriorly, forming a slender process whichlines the medial (dorsal) margin of the external naris up to a level somewhat in frontof the midpoint of the external naris. Taking their whole length into account, thenasals are only slightly shorter than the frontals (the right nasal is somewhat longerthan the left nasal). At the level of the Eosauropterygia (Rieppel, 1994a), the relativelylong nasals are autapomorphic in Corosaurus. The posterior tips of the nasals liebehind the level of the anterior margin of the orbit, as is the case in Eosauropterygiagenerally with the exception of Simosaurus (reduced nasals) and, possibly, earlypachypleurosaurs.

6. Nasals not reduced (0), somewhat reduced (1), or strongly reduced or absent(2).

The nasals of Corosaurus are unusually large in comparison to other Sauropterygia(Figs 2, 3). Cymatosaurus and Pistosaurus share strongly reduced or perhaps absentnasals (Rieppel, 1994a). The nasals are also reduced, although to a lesser degree,in Germanosaurus and Simosaurus.


Figure 6. Humerus of Corosaurus alcovensis Case (holotype, UW 5485).

7. Nasals do (0) or do not (1) enter external naris.The nasal variably enters the external naris or remains excluded therefrom in

Cymatosaurus; the nasal is excluded from the external naris in Pistosaurus. In all otherstem-group Sauropterygia, the nasal enters the external naris.

8. Nasals meet in dorsomedial suture (0), or are separated from one another bynasal processes of the premaxillae extending back to the frontal bone(s) (1).

This character is highly variable in Eosauropterygia (Sander, 1989; Rieppel &Wild, 1996), and variational studies in the pachypleurosaur genus Neusticosaurus haveshown that the nasals may or may not grow so as to cover the premaxillary-frontalcontact in dorsal view (Sander, 1989). The nasals always remain separated in thegenera Cymatosaurus, Germanosaurus, Pistosaurus, and Simosaurus with reduced nasals.The large nasals meet in Corosaurus (Figs 2, 3). The character is polymorphic inNothosaurus and Lariosaurus among ‘Eusauropterygia’.

9. The lacrimal is present and enters the external naris (0) or remains excludedfrom the external naris by a contact of maxilla and nasal (1), or the lacrimal isabsent (2).

The lacrimal is universally absent in Sauropterygia, including Corosaurus.10. The prefrontal and postfrontal are separated by the frontal along the dorsal

margin of the orbit (0), or a contact of prefrontal and postfrontal excludes the frontalfrom the dorsal margin of the orbit (1).

In Corosaurus, the prefrontal and postfrontal are separated by a distinct gap along

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the dorsal margin of the orbit (Figs 2, 3). The two bones approach each other moreclosely, however, than is the case in Germanosaurus, Lariosaurus, and Nothosaurus, whichshow relatively smaller pre-and postfrontals. A contact of prefrontal and postfrontalis variably present in Cymatosaurus, and is always present in Placodus.

11. Dorsal exposure of prefrontal large (0) or reduced (1).In the holotype of Corosaurus alcovensis, the right prefrontal is missing, but the

frontal shows a discrete facet which received its posteromedial (posterodorsal) tip.The left prefrontal is incompletely preserved, but the preserved parts indicate alarge dorsal exposure of the bone, and a broad contact with the adjacent nasal. Thenasal–prefrontal contact is a very variable character among sauropterygians withreduced (Nothosaurus: Rieppel & Wild, 1996) or not reduced (Simosaurus: Rieppel,1994a) prefrontals. This particular character is therefore not included in the presentanalysis. However, the dorsal exposure of the prefrontal is distinctly reduced inNothosaurus, Lariosaurus and Germanosaurus as compared to other sauropterygians.

12. Preorbital and postorbital region of skull: of subequal length (0), preorbitalregion distinctly longer than postorbital region (1), postorbital region distinctly longer(2).

In Corosaurus, the distance from the posterior margin of the orbit to the mandibularcondyle of the quadrate (postorbital skull) closely approximates the distance fromthe anterior margin of the orbit to the tip of the snout (preorbital skull), or is evenslightly shorter (note that the postorbital skull is separated from the preorbital skullin the holotype). In pachypleurosaurs, the preorbital skull is generally longer thanthe postorbital skull. In the ‘eusauropterygian’ genera Germanosaurus, Lariosaurus,Nothosaurus, and Simosaurus, the postorbital skull is distinctly elongated. In Cymatosaurus,the postorbital skull is relatively longer, but not as distinctly as in the Nothosauria.In Pistosaurus, the situation is less easily assessed, because the only skull still availablefor study lacks the tip of the snout. Illustrations of the lost specimen indicate,however, that the preorbital skull is relatively longer.

13. Upper temporal fossa absent (0), present and subequal in size or slightly largerthan the orbit (1), present and distinctly larger than orbit (2), or present and distinctlysmaller than orbit (3).

As the postorbital region of the skull is not distinctly elongated in Corosaurus, theupper temporal fenestra is relatively smaller (as compared to other ‘eusau-ropterygians’), and while still larger than the orbit, the discrepancy is not as strikingas in sauropterygians with an elongated postorbital skull. This is certainly a relativelyplesiomorphic trait of Corosaurus, indicating a lesser development of the dual jawadductor muscle system as compared to ‘eusauropterygians’ (Rieppel, 1994a). Thelongitudinal diameter of the (left) orbit is 26.5mm in the holotype of Corosaurusalcovensis, the longitudinal diameter of the (left) upper temporal fossa is approximately35mm. Dividing the longitudinal diameter of the upper temporal fossa by thelongitudinal diameter of the orbit yields a ratio of 1.32; corresponding values are1.6-2.0 for Cymatosaurus (1.3 for the juvenile specimen designated ‘specimen II’ ofC. gracilis by Schrammen, 1899, with relatively large orbits), 1.87 for Germanosaurus,1.7-2.0 for Lariosaurus, 2.1-3.9 for Nothosaurus, and 1.5-2.45 for Simosaurus.

14. Frontal(s) paired (0) or fused (1) in the adult.The frontals are paired in Corosaurus (Figs 2, 3). Among other Sauropterygia, the

frontals remain separate in Dactylosaurus, Cymatosaurus and Germanosaurus; they areincompletely fused in some specimens of the Serpianosaurus–Neusticosaurus clade (Carroll& Gaskill, 1985; Rieppel, 1989; Sander, 1989), and in Pistosaurus (Rieppel, 1994a,


fig. 38). The frontals are fused in Simosaurus, Nothosaurus, and Lariosaurus. Fusion offrontals is variable in Placodus (Rieppel, 1995).

15. Frontal(s) without (0) or with (1) distinct posterolateral processes.The frontal of Corosaurus shows a distinct posterolateral process which, together

with a posteromedial process, embraces an anterior process of the parietal. Thisarrangement results in a deeply interdigitating fronto-parietal suture (Fig. 3).

16. Frontal widely separated from the upper temporal fossa (0), narrowly ap-proaches the upper temporal fossa (1), or enters the anteromedial margin of theupper temporal fossa (2).

Corosaurus resembles Cymatosaurus and Germanosaurus by the presence of distinctposterolateral processes of the frontals which very narrowly approach the antero-medial margin of the upper temporal fossa (Figs 2, 3), and which enter the marginof the upper temporal fossa in some Cymatosaurus, as well as in Pistosaurus. Thefrontals do not approach the upper temporal fenestra as closely, and never enter itsanteromedial margin in pachypleurosaurs, Simosaurus, and the Nothosaurus–Lariosaurusclade.

17. Parietal(s) paired (0), fused in their posterior part only (1), or fully fused (2)in adult.

A distinct suture separates the parietals in front of the pineal foramen in the skullof Corosaurus. Behind the pineal foramen, the suture can be followed up to theposterior margin of the parietal table (Figs. 2, 3). The parietals are also paired alongtheir entire length in pachypleurosaurs. In Cymatosaurus, the parietals are eithercompletely fused, or they remain separate at least in front of the pineal foramen;the parietals also remain separate in front of the pineal foramen in Germanosaurusand Pistosaurus (as seen in the lost skull of ‘P. II’: Meyer, 1847–55). The parietalsare fully fused in Placodus, Simosaurus, Nothosaurus and Lariosaurus.

18. Pineal foramen close to the middle of the skull table (0), is displaced posteriorly(1), is displaced anteriorly (2), or is absent (3).

The relatively large pineal foramen of Corosaurus may lie somewhat in front ofthe midpoint of the parietal, but it is located well behind the fronto-parietal suture(Figs. 2, 3). In sauropterygians with an anteriorly displaced pineal foramen, thelatter borders on the fronto-parietal suture (Placodus, Pistosaurus).

19. Parietal skull table broad (0), weakly constricted (1), strongly constricted (2),or forming a sagittal crest (3).

The parietal skull table in Corosaurus is broad in front and immediately behindthe pineal foramen (Figs. 2, 3). More posteriorly, the parietal narrows somewhatbefore bifurcating and extending posterolaterally to meet the squamosal along theposterior margin of the broad upper temporal fossa. The parietal skull table isstrongly constricted, at least in its posterior part behind the pineal foramen, inCymatosaurus, and in the Nothosaurus–Lariosaurus clade. Among stem-group Sau-ropterygia, a sagittal crest is formed in one species of Nothosaurus (Rieppel & Wild,1994), in one species of Cymatosaurus (Rieppel & Werneburg, 1998), and in Pistosaurus.

20. Postparietals present (0) or absent (1).Postparietals are universally absent in Sauropterygia.21. Tabulars present (0) or absent (1).Tabulars are universally absent in Sauropterygia.22. Supratemporals present (0) or absent (1).Supratemporals are universally absent in Sauropterygia.23. The jugal extends anteriorly along the ventral margin of the orbit (0), is

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restricted to a position behind the orbit but enters the latter’s posterior margin (1),or is restricted to a position behind the orbit without reaching the latter’s posteriormargin (2).

Storrs (1991) reconstructed a splint-like jugal in Corosaurus, wedged in betweenthe postorbital and the posterior tip of the maxilla but without reaching the posteriormargin of the orbit. In fact, the posteroventral corner of the orbit is not preservedon the right side of the skull, and is very difficult to interpret on the left side dueto dorsoventral flattening of the skull which pushed the pterygoid–ectopterygoidflange up into the posteroventral corner of the orbit. Also, the lower end of the leftpostorbital is incompletely preserved. There is a triangular splint of bone lying abovethe posterior tip of the maxilla without reaching the posterior margin of the orbit(fig. 3), but it is impossible to determine whether this represents the jugal or afragment of the ectopterygoid. Above that splint lies a larger element which ananterior process following the lower margin of the orbit anteriorly up to the levelof its mid-point. Again, this bone could be the jugal, or the lower end of thepostorbital. If this bone is interpreted as part of the postorbital, Corosaurus would beautapomorphic with respect to that character, because no other sauropterygian isknown in which the postorbital extends anteriorly to the midpoint of the lowermargin of the orbit (except for an undescribed species of Nothosaurus from theMuschelkalk of Nahal Ramon, Israel). If the element is considered the jugal, itwould be represented in its plesiomorphic condition. I am inclined to consider theevidence inconclusive, and code the position of the jugal as unknown for Corosaurus.

24. The jugal extends backwards no farther than to the middle of the cheekregion (0), or nearly to the posterior end of the skull (1).

Due to the wide open ventral cheek region in Sauropterygia, the jugal does notextend far posteriorly. Where the cheek is closed (Placodus), the jugal does not extendbeyond the middle of the cheek region.

25. The jugal remains excluded from (0) or enters (1) the upper temporalarch.In some stem-group Sauropterygia (Simosaurus, the Nothosaurus–Lariosaurus clade,Germanosaurus, Pistosaurus, and Placodus), the jugal takes an integral part in theformation of the upper temporal arch.

26. Postfrontal large and plate-like (0), with a distinct lateral process overlappingthe dorsal tip of the postorbital (1), or postfrontal with a reduced lateral process andhence more of a narrow and elongate shape (2).

The postfrontal in Corosaurus has a distinctly triangular shape, with a strong lateralprocess overlapping the proximodorsal tip of the postorbital. A distinct lateral processon the postfrontal is also present in Placodus, Cyamodus, Germanosaurus, Simosaurus andPistosaurus, but it is reduced in pachypleurosaurs, in Nothosaurus (with the exceptionof Nothosaurus juvenilis: Rieppel, 1994c) and in Lariosaurus.

27. Lower temporal fossa absent (0), present and closed ventrally (1), present butopen ventrally (2). Since the presence of a ventrally open lower temporal fenestralogically implies the loss of the lower temporal bar, and hence the previous presenceof a ventrally closed temporal fenestra, this character is the only one of all multistatecharacters included in this analysis which was coded as ordered (as well as unordered)in the cladistic analysis (see discussion below).

This character assumes on an a priori basis that sauropterygians are derived fromdiapsids by the loss of the lower temporal arch (Kuhn-Schnyder, 1967), rather thathaving undergone ventral emargination of the cheek region.


28. Squamosal descends to (0), or remains broadly separated from (1) ventralmargin of skull.

The left part of the occiput is well preserved in Corosaurus, and shows thesquamosal to descend far down towards the ventral margin of the skull laterally,leaving only the mandibular condyle of the quadrate exposed.

29. Quadratojugal present (0) or absent (1).As noted by Storrs (1991), the lateral edge of the laterally descending flange of

the squamosal, covering the quadrate in lateral view, is finely broken, but thefragments appear all to be in place. Although the presence of a quadratojugal washypothesized by Case (1936), the character cannot be assessed unequivocally.

30. Quadratojugal with (0) or without (1) anterior process.In Sauropterygians with a widely open cheek region, the quadratojugal never

bears a distinct anterior process. This character is coded as unknown for those taxawhich lack the quadratojugal.

31. Occiput with paroccipital process forming the lower margin of the posttemporalfossa and extending laterally (0), paroccipital processes trending posteriorly (1), orocciput plate-like with no distinct paroccipital process and with strongly reducedposttemporal fossae (2).

The renewed preparation of the holotype of Corosaurus alcovensis further exposedthe left paroccipital process, which extends posterolaterally, and which seems tohave loosely articulated in a distinct notch in the lower margin of the occipitalexposure of the squamosal with its distal tip (fig. 3).

32. Squamosal without (0) or with (1) distinct notch to receive distal tip ofparoccipital process.

The notch in the squamosal receiving the distal tip of the paroccipital process isalso observed in Cymatosaurus (Rieppel, 1994a, fig. 39; Rieppel & Werneburg, 1997),but is otherwise currently unknown in other sauropterygians.

33. Mandibular articulations approximately at level with occipital condyle (0) ordisplaced to a level distinctly behind occipital condyle (1), or positioned anterior tothe occipital condyle (2).

If placed in situ, the left mandibular condyle of the quadrate lies distinctly behindthe level of the occipital condyle in the holotype of Corosaurus alcovensis.

34. Exoccipitals do (0) or do not (1) meet dorsal to the basioccipital condyle.The left exoccipital is well exposed on the left side of the occipital condyle in the

holotype of Corosaurus alcovensis, and indicates that the exoccipitals were not in contactdorsal to the basioccipital, as is also the case in all other adequately preservedSauropterygia.

35. Supraoccipital exposed more or less vertically on occiput (0), or exposed moreor less horizontally at posterior end of parietal skull table (1).

The supraoccipital is not exposed in articulation with the skull table in Corosaurus.The bone slants sharply in a posteroventral direction in Placodus, Simosaurus, Cy-matosaurus and Pistosaurus, as was most probably also the case in Corosaurus. Inpachypleurosaurs, and in the Nothosaurus–Lariosaurus clade, the supraoccipital isbroadly exposed between the posteriorly diverging temporal processes of the parietal,indicating a more horizontal orientation of the bone.

36. Occipital crest absent (0) or present (1).In Corosaurus, the parietal and squamosal form a distinct occipital crest, which is

also present in Nothosaurus, Lariosaurus, and Pistosaurus. This occipital crest is absent

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in Placodus, pachypleurosaurs, Simosaurus, Germanosaurus, and in some, but not all,Cymatosaurus.

37. Quadrate with straight posterior margin (0) or quadrate shaft deeply excavated(concave) posteriorly (1).

Corosaurus shares with ‘eusauropterygians’ sensu Tschanz (1989) the straight posteriormargin of the quadrate.

38. Quadrate covered by squamosal and quadratojugal in lateral view (0), orquadrate exposed in lateral view (1).

As in other sauropterygians, the descending process of the squamosal does notcompletely cover the quadrate in lateral view in Corosaurus.

39. Dorsal wing of epipterygoid broad (0) or narrow (1).The base of the epipterygoid is well preserved in Corosaurus, but its dorsal wing is

broken (Case, 1936). Still, what is preserved of the bone indicates a narrow dorsalprocess. The dorsal wing of the epipterygoid is also narrow in Placodus (Rieppel, 1995),perhaps in pachypleurosaurs (Carroll & Gaskill, 1985, although the unequivocalidentification of the epipterygoid is very difficult in this material), in Cymatosaurus(Rieppel & Werneburg, 1997), and in Pistosaurus (Rieppel, 1994a, fig. 38). The dorsalwing of the epipterygoid is broad in Simosaurus, Nothosaurus and most probably inLariosaurus (Rieppel, 1994b). The epipterygoid is unknown in Germanosaurus.

40. Lateral conch on quadrate absent (0) or present (1).41. Palatobasal articulation present (0) or absent (1).An immovable palatobasal articulation is characteristic of Sauropterygia in general

(Storrs, 1991; Taylor, 1992; Taylor & Cruickshank, 1993; Rieppel, 1994a). Thepalate of Cymatosaurus shows the absence of interpterygoid vacuities and no indicationof a movable palatobasal articulation, although the squamosal shows a distinct notchon the ventral margin of its occipital flange, which must have received the distal tipof the paroccipital process in a loose articulation. This latter feature, together withthe fact that the otico-occipital segment of the braincase (prootic, opisthotic,exoccipital) has dropped out of the dermatocranium in all Cymatosaurus skulls available,might indicate a metakinetic skull in that genus. However, a beautifully preservedspecimen of Cymatosaurus (Rieppel & Werneburg, 1997) shows fusion of the basi-cranium with the dermal palate, indicating an immovable palatobasal articulation.The same condition is assumed for Corosaurus, although neither the basicranium,nor the dermal palate are exposed. Interpterygoid vacuities are known in Pistosaurus(Edinger, 1935) and the plesiosaur–pliosaur clade, but in no other sauropterygian.Again, their presence in Corosaurus remains unknown (Storrs, 1991).

42. Basioccipital tubera free (0) or in complex relation to the pterygoid, as theyextend ventrally (1) or laterally (2).

This character was discussed extensively by Rieppel (1994b, 1995; Nosotti &Pinna, 1993). For lack of adequate preparation, this character cannot be assessedfor Corosaurus.

43. Suborbital fenestra absent (0) or present (1).Renewed preparation of the holotype of Corosaurus alcovensis did expose bone in

the floor of the right orbit, indicating that the suborbital fenestra is absent, as in allother Triassic stem-group Sauropterygia.

44. Pterygoid flanges well developed (0) or strongly reduced (1).Pterygoid-ectopterygoid flanges are well developed in Placodus (Rieppel, 1995),

Nothosaurus giganteus (Rieppel & Wild, 1996), Cymatosaurus (Rieppel, 1997) andPistosaurus (Rieppel, 1994a, fig. 38), but reduced in other Eosauropterygia. Storrs


(1991) described a strongly developed pterygoid-ectopterygoid flange in Corosaurus,as may, indeed, be indicated by breakage of the bone at the posterior end of theleft maxilla.

45. Premaxillae enter internal naris (0) or are excluded (1).This character cannot be assessed for Corosaurus.46. Ectopterygoid present (0) or absent (1).Fragments of the ectopterygoid are exposed at the posterior tip of the left maxilla

in Corosaurus (Case, 1936). The exposed left transverse process of the pterygoidretains parts of the ectopterygoid attached at the pterygoid-ectopterygoid suture.Among Sauropterygia, the ectopterygoid appears to be absent in pachypleurosaurs.

47. Internal carotid passage enters basicranium directly (0) or through quadrateramus of pterygoid (1).

Simosaurus and Nothosaurus show a peculiar passage of the internal carotid artery,which coming up from the neck enters the skull through a foramen on the posterioraspect of the quadrate ramus of the pterygoid (Rieppel, 1994b). The same derivedcourse of the internal carotid is observed in a specimen of Cymatosaurus (Rieppel &Werneburg, 1997). For lack of adequate preservation, this character remains unknownfor pachypleurosaurs, Germanosaurus, Lariosaurus, Pistosaurus, and Corosaurus.

48. Retroarticular process of lower jaw absent (0) or present (1).The lower jaw of Corosaurus (FMNH PR 246) shows a very prominent retroarticular

process. The chorda tympani foramen is located posteroventromedial to the man-dibular articular facet.

49. Distinct coronoid process of lower jaw absent (0) or present (1).Corosaurus (FMNH PR 246) is unusual among stem-group Eosauropterygia as it

shows not only a well developed coronoid process on the lower jaw, but also aprominent lateral exposure of the coronoid bone. In other eosauropterygians (exceptfor some plesiosaurs), the coronoid process is weakly developed (pachypleurosaurs)or virtually absent, and the coronoid bone located predominantly on the medialside of the mandible at the anterior margin of the adductor fossa (Rieppel, 1994a).Unfortunately, the lower jaw of Cymatosaurus remains very incompletely known.

50. Surangular without (0) or with (1) strongly projecting lateral ridge definingthe insertion area for superficial adductor muscle fibres on the lateral surface of thelower jaw.

The external jaw adductor muscle inserts into the dorsal surface of the surangular.In most reptiles, particularly those with an incomplete ventral margin of the dermalcovering of the cheek region, the insertion of superficial fibers extends downwardson to the lateral surface of the lower jaw. A usually weakly expressed ridge on thesurangular commonly delineates the ventral margin of the insertional area (Rieppel& Gronowski, 1981). Corosaurus shows an unusually well developed lateral ridge onthe surangular, forming an almost horizontal shelve for the insertion of superficialjaw adductor muscle fibres. In that respect, Corosaurus resembles Simosaurus, Nothosaurusand Lariosaurus among sauropterygians (Rieppel, 1994a). Unfortunately, the lowerjaw of Cymatosaurus remains incompletely known.

51. Mandibular symphysis short (0), somewhat enforced (1), or elongated and‘scoop’-like (2).

In contrast to pachypleurosaurs, which lack enlargement of the mandibularsymphysis, Placodus and the ‘Eusauropterygia’ (with the exception of Simosaurus) sharean elongated, ‘scoop’-like symphysis. The mandibular symphysis of Corosaurus is

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slightly enforced and ‘scoop’-shaped, but less so than in Cymatosaurus, Nothosaurus andLariosaurus (Rieppel, 1994a).

52. Splenial enters the mandibular symphysis (0), or remains excluded therefrom(1).

The splenial may be excluded from the mandibular symphysis in Corosaurus (Storrs,1991, fig. 9C), but the character cannot be determined unequivocally on the basisof available material.

53. Teeth set in shallow or deep sockets (0) or superficially attached to bone (1).As in all Sauropterygia, the dentition of Corosaurus is thecodont.

54. Anterior (premaxillary and dentary) teeth upright (0) or strongly procumbent(1).

Corosaurus resembles the ‘Eusauropterygia’ sensu Tschanz (1989) with stronglyprocumbent anterior dentary and maxillary teeth.

55. Premaxillary and anterior dentary fangs absent (0) or present (1).In the holotype of Corosaurus alcovensis, five premaxillary teeth are preserved, and

they gradually decreased in size from front to back. The anterior dentary teeth evenexceed the anterior premaxillary teeth in size (Case, 1936; Storrs, 1991). Thepresence of premaxillary fangs in Pistosaurus is coded on the basis of alveolar size inthe figure of the (now lost) skull provided by Schrammen (1899).

56. One or two caniniform teeth present (0) or absent (1) on maxilla.Typically in ‘Eusauropterygia’ sensu Tschanz (1989), the maxillary bears one or

two enlarged fangs located between the external naris and the orbit. This part ofthe maxilla shows no enlarged teeth in Corosaurus. However, the (preserved) an-teriormost tooth on the maxilla is distinctly larger than the third tooth (the secondalveolus is empty but large again). The succeeding maxillary teeth (a minimum of10 is preserved) are all distinctly smaller than the anteriormost (preserved) tooth,and decrease in size from front to back. The conclusion, therefore, is that Corosaurusshares the presence of one, perhaps two enlarged fangs, only these are located in amore anterior position, lateral to the external naris, in an unconstricted snout.

57. The maxillary tooth row is restricted to a level in front of the posterior marginof the orbit (0), or it extends backwards to a level below the posterior corner of theorbit and/or the anterior corner of the upper temporal fossa (1), or it extendsbackwards to a level below the anterior one third to one half of the upper temporalfossa (2).

Among stem-group Sauropterygia, Simosaurus, Nothosaurus and Lariosaurus show theposterior extension of the maxillary tooth row to a level well behind the anteriormargin of the upper temporal fossa. In Corosaurus, the teeth extend to the posteriorcorner of the orbit.

58. Teeth on pterygoid flange present (0) or absent (1).No tooth fragments can be detected in the area of the crushed ectopterygoid–

pterygoid flange in the holotype of Corosaurus alcovensis, and pterygoid teeth aretherefore considered to be absent as in all other Sauropterygia.

59. Vertebrae notochordal (0) or non-notochordal (1).Non-notochordal vertebrae are generally characteristic of Eosauropterygia (Riep-

pel, 1994a); the non-notochordal nature of the vertebrae of Corosaurus was ascertainedby complete separation of the 26th and 27th dorsal elements through a combinationof chemical methods of preparation.

60. Vertebrae amphicoelous (0), platycoelous (1) or procoelous/opisthocoelous(2).


The vertebrae of ‘Eusauropterygia’ are generally platycoelous; in Corosaurus, thedorsal vertebrae a deeply amphicoelous.

61. Dorsal intercentra present (0) or absent (1).62. Cervical intercentra present (0) or absent (1).63. Ventral surface of cervical centra rounded (0) or keeled (1).The holotype of Corosaurus shows the cervical vertebrae in lateral view only, their

lower edge concealed by cervical ribs. FMNH PR 135 includes a transversallysectioned cervical element with somewhat irregular contours. It can be interpreted,however, as showing a keeled ventral surface of the centrum, as is the case in allother adequately preserved Sauropterygia.

64. Zygosphene-zygantrum articulation absent (0) or present (1).Storrs (1991) identified a zygosphene-zygantrum articulation between the 29th

and 30th dorsal vertebrae of the holotype of Corosaurus alcovensis. The string of dorsalvertebrae 26 through 31 has been completely removed from the surrounding matrix(Fig. 4). These vertebrae are very tightly articulated with one another, which makesit exceedingly difficult to ascertain the presence of an accessory intervertebralarticulation. Separation of the 26th (fig. 5A) from the 27th element exposed a badlyeroded base of the neural spine in both elements, again rendering assessment of thecharacter equivocal. The posterior surface of a separated neural arch (FMNH PR245, fig. 5B) is relatively well preserved. It shows the base of the neural spinecarrying a distinct posterior projection above the postzygapophyses. Below thisposteriorly projecting spine, and above the postzygapophyses, is situated a small yetsomewhat compressed pit. FMNH PR2018 allowed the preparation of an evenbetter preserved posterior surface of an isolated dorsal neural arch. The neural spinecarries a distinct ridge on its posterior surface, protruding into a posteriorly projectingtip at its base. Again a small yet distinct and deep pit is located below this spine,and between the postzygapophyses. If this pit is, indeed the zygantrum (as is assumedhere on grounds of parsimony), it is undivided by a vertical septum, as is also thecase in Cymatosaurus (Rieppel & Hagdorn, 1997). The zygantrum of Simosaurus andNothosaurus is distinctly different, however, in that it is both broader and wider,internally divided by a vertical septum, and not recessed below a posterior basalprojection of the neural spine.

65. Sutural facets receiving the pedicels of the neural arch on the dorsal surfaceof the centrum in the dorsal region are narrow (0) or expanded into a cruciform or‘butterfly-shaped’ platform (1).

The partially preserved neural arch referred to above (FMNH PR 245; Fig. 5B)is distorted so as to show the larger part of the base of the pedicels. These aresomewhat expanded and would have met the dorsal surface of the centrum on abroadened ‘butterfly-shaped’ platform. In contrast to other sauropterygians, theneural arches seem to separate less easily form their respective centra in Corosaurus.The neural suture remains distinct throughout the vertebral column, however.

66. Transverse processes of neural arches of the dorsal region relatively short (0)or distinctly elongated (1).

Corosaurus shows an autapomorphic elongation of the transverse processes on theproximal caudal vertebrae (Case, 1936; Storrs, 1991), but although distinct, thetransverse processes of the dorsal vertebrae are not distinctly elongated to an extentthat would be similar to that seen in Placodus or archosauromorph reptiles.

67. Vertebral centrum distinctly constricted in ventral view (0) or with parallellateral edges (1).

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The vertebral centra are not constricted in ventral view in Dactylosaurus, in theSerpianosaurus–Neusticosaurus clade (Rieppel & Lin, 1995), and they are not or onlyslightly constricted in Nothosaurus and Lariosaurus. The centra are distinctly constrictedin Corosaurus as well as in Placodus and Simosaurus.

68. Distal end of transverse processes of dorsal vertebrae not increasing in diameter(0) or distinctly thickened (1).

The transverse processes on the dorsal vertebrae of Corosaurus show a distinctexpansion into a vertically oriented, oblong or oval articular head. This characteris shared by Pistosaurus (Sues, 1987) among the sauropterygians included in thisanalysis. A relatively high, vertically oriented and oval articular surface is alsoobserved on the transverse processes of dorsal vertebrae of pachypleurosaurs,Simosaurus, Nothosaurus (Rieppel & Wild, 1996), Lariosaurus, and some isolated vertebraereferred to Cymatosaurus (Rieppel & Hagdorn, 1997), but in all these taxa thetransverse processes are distinctly shorter and overall stouter than in Corosaurus andPistosaurus, and a distal expansion is, if at all, only very weekly expressed.

69. Zygapophyseal pachyostosis absent (0) or present (1).Zygapophyseal pachyostosis, resulting in a ‘swollen’ appearance of pre- and

postzygapophyses, is present in all pachypleurosaurs, as well as in some species ofNothosaurus and all species of Lariosaurus. It is absent in all other Sauropterygia,including Corosaurus.

70. Pre-and postzygapophyses do not (0) or do (1) show an anteroposterior trendof increasing inclination within the dorsal and sacral region.

The pre-and postzygapophyses of Corosaurus show a more or less horizontalorientation in the anterior dorsal region; in the three sacral vertebrae, the exposedzygapophyses show an inclined articular surface (Storrs, 1991). A similar change ofzygapophyseal orientation within the vertebral column is also known in Placodus(Rieppel, 1995; the trend is reversed in the sacral region in Placodus) and SimosaurusRieppel, 1994a), but not in pachypleurosaurs, nor in the Nothosaurus–Lariosaurusclade.

71. Cervical ribs without (0) or with (1) a distinct free anterior process.Cervical ribs are well exposed in the holotype of Corosaurus alcovensis, and they

show a distinct free anterior process (Storrs, 1991), as is generally the case in stem-group Sauropterygia.

72. Pachyostosis of dorsal ribs absent (0) or present (1).Pachyostosis of the dorsal ribs is absent in Dactylosaurus, present in the Serpianosaurus–

Neusticosaurus clade, and it is also present in some Nothosaurus and some Lariosaurusspecies. It is absent in all other Sauropterygia, including Corosaurus.

73. The number of sacral ribs is two (0); three (1); four or more (2).The three sacral ribs of Corosaurus are firmly fused to their respective vertebrae,

and they show a marked distal expansion. This distinguishes them from the firstcaudal rib (transverse process), which is turned anteriorly, but lacks the distalexpansion and probably did not contact the ilium.

74. Sacral ribs with (0) or without (1) distinct expansion of distal head. The sacralribs of Corosaurus show a distinct distal expansion, as is also the case in Simosaurusand Placodus among stem-group Sauropterygia.

75. Sacral (and caudal) ribs or transverse processes sutured (0) or fused (1) to theirrespective centrum.

Corosaurus is unusual among Sauropterygia by showing sacral ribs (or transverseprocesses: see Rieppel, 1993d) fused to their respective centrum. This condition is


not known to occur in Placodus, pachypleurosaurs, or in any of the ‘eusauropterygians’included in this study with the possible exception of Nothosaurus giganteus (Rieppel &Wild, 1996, fig. 55).

76. Cleithrum present (0) or absent (1).The cleithrum is universally lacking in Sauropterygia.77. Clavicles broad (0) or narrow (1) medially.The best preserved and fully exposed clavicle of Corosaurus shows the element to

taper to a blunt tip medially (Storrs, 1991, fig. 11B), as is also observed in Placodusand some (as yet undescribed; kept in private collections) specimens of Nothosaurus(Rieppel & Wild, 1996). In other eosauropterygians, the clavicles are broad medially.

78. Clavicles positioned dorsally (0) or anteroventrally (1) to the interclavicle.The anteroventral position of the clavicles with respect to the interclavicle was

recognized as a sauropterygian synapomorphy by Carroll and Gaskill (1985).79. Clavicles do not meet in front of the interclavicle (0) or meet in an interdigitating

anteromedial suture (1).This character cannot be assessed unequivocally in the holotype of Corosaurus

alcovensis. Further preparation of the right clavicle has revealed a greater width ofthe anteromedial margin than figured by Storrs (1991, fig. 11A), resulting in greatersimilarity of the element with the clavicle shown in figure 11B by Storrs (1991).Unfortunately, the medial tip of the right clavicle of the holotype remains obscuredby an overlapping rib. Based on the specimen figured by Storrs (1991, fig. 11B),the absence of a medial interdigitating suture is assumed (as by Storrs, 1991, in hisreconstruction of the pectoral girdle).

80. Clavicles without (0) or with (1) anterolaterally expanded corners.The right clavicle of the holotype of Corosaurus alcovensis shows a distinctly expanded

anterolateral corner, which is also seen in Dactylosaurus among pachypleurosaurs,and in Simosaurus, Nothosaurus and Lariosaurus among stem-group ‘eusauropterygians’.

81. Clavicle applied to the anterior (lateral) (0) or to the medial (1) surface ofscapula.

The position of the clavicle medial rather than anterior (lateral) to the scapulawas recognized as a sauropterygian synapomorphy by Carroll and Gaskill (1985).The left scapula of the holotype of Corosaurus alcovensis has been completely freedfrom the surrounding matrix (Fig. 5C), and reveals a mediolaterally compressedstate of preservation. The anterior slope of the glenoidal portion of the scapula iscomplete and smooth, however, and it delineates the lateral margin of an articularfacet which is located on the anteromedial aspect of the scapula, much as in otherEosauropterygia (Rieppel, 1994a).

82. Interclavicle rhomboidal (0) or T-shaped (1).As indicated by Case (1936) and Storrs (1991), the interclavicle is a triradiate

element approaching a T-shaped structure modified by some reduction of theposterior stem.

83. Posterior process on (T-shaped) interclavicle elongate (0), short (1), or rudi-mentary or absent (2).

The posterior stem of the interclavicle of Corosaurus is still about as long as eitheranterior lateral process, which corresponds to similar proportions of the interclaviclein Simosaurus. The posterior stem is much more reduced in Placodus or in theNothosaurus–Lariosaurus clade.

84. Scapula represented by a broad blade of bone (0), or with a constrictionseparating a ventral glenoidal portion from a posteriorly directed dorsal blade (1).

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The scapula of Corosaurus is strongly constricted at the transition from the triangular(and laterally compressed) glenoidal portion and the posterodorsal blade or process.With respect to this character, Corosaurus resembles other Eosauropterygia moreclosely than Placodus, where the scapula is a broad plate of bone as in plesiomorphicamniotes.

85. The dorsal wing or process of the eosauropterygian scapula tapers to a blunttip (0) or is ventrally expanded at its posterior end (1).

The posterodorsal process on the scapula of the holotype of Corosaurus alcovensis isbroken at its posteroventral edge, but the curved margin of the remaining boneclearly indicates some distal expansion of the scapular blade (Fig. 5C; see also Storrs,1991, fig. 12A). This is a character which Corosaurus shares with Pistosaurus (Sues,1987) among the ‘Eusauropterygia’ included in this analysis. In pachypleurosaurs,Simosaurus, Nothosaurus and Lariosaurus, the posterodorsal process of the scapula neverexpands distally, but is either parallel edged (Simosaurus), or tapers to a blunt tip.

86. Supraglenoid buttress present (0) or absent (1).The supraglenoid buttress is uniformly absent in Sauropterygia.87. One (0) or two (1) coracoid ossifications.Corosaurus shares a single coracoid ossification with all other Sauropterygia.88. Coracoid of rounded contours (0), slightly waisted (1), strongly waisted (2), or

with expanded medial symphysis (3).The coracoid of Corosaurus is an almost rectangular plate with only slightly convex

anterior and posterior margins. This contrasts with the shape of the coracoid inother stem-group Eosauropterygia, which has strongly concave anterior and posteriormargins, resulting in a distinctly waisted shape. Pistosaurus approaches plesiosaurswith an expanded medial symphysis between the coracoids (Sues, 1987).

89. Coracoid foramen enclosed by coracoid ossification (0), or between coracoidand scapula (1).

The scapula and coracoid of Corosaurus are not well enough known to allow theunequivocal assessment of this character. It appears, however, that a slight indentationon the ventral margin of the posterior part of the glenoidal portion of the scapulamight indicate the position of the coracoid foramen between scapula and coracoid,as is characteristic of other stem-group Sauropterygia.

90. Pectoral fenestration absent (0) or present (1).See Storrs (1991) for further comments on this character.91. Limbs short and stout (0) or long and slender (1).92. Humerus rather straight (0) or ‘curved’ (1).The ‘curved’ humerus has been used as a ‘nothosaurian’ character for a long

time, and was used by Storrs (1991) in support of his hypothesis that Placodontiais the sister-group of ‘Eusauropterygia’, coding a straight and slender humerus forpachypleurosaurs. Assessment of the character is somewhat complicated by sexualdimorphism of the humerus in pachypleurosaurs. It is, however, rather difficult todistinguish any significant difference in shape of the humerus of adult Dactylosaurus(sex y: Rieppel, 1993b; 1994a; Rieppel & Lin, 1995) and adult representatives ofthe Serpianosaurus–Neusticosaurus–clade (Carroll & Gaskill, 1985; Rieppel, 1989, Sander,1989) from a typical humerus of Nothosaurus (see Rieppel, 1994a, fig. 58). The curvedhumerus is therefore coded present for pachypleurosaurs (as in Rieppel, 1994a).However, sauropterygian humeri differ in the relative development of the de-ltopectoral crest.

93. Deltopectoral crest well developed (0) or reduced (1).


The humerus of Corosaurus appears more distinctly curved than that of most otherSauropterygia due to a lesser development of the deltopectoral crest (fig. 6). Ahumerus with a well developed deltopectoral crest (pachypleurosaurs sex y, with theexception of Keichousaurus; Simosaurus, Nothosaurus, Cymatosaurus) shows an angulationof the proximal part of the preaxial margin of the humerus. Below the deltopectoralcrest, the anterior margin of the humerus may be straight or even somewhat concave,resulting in a slightly waisted mid-diaphyseal region. In Corosaurus, the deltopectoralcrest is not prominent, and the preaxial margin of the humerus evenly convex,resulting in a more strongly curved appearance of the humerus. This humeralmorphology is shared by some Lariosaurus, and by Placodus among the Sauropterygiaincluded in this analysis.

94. Insertional crest for latissimus dorsi muscle prominent (0) or reduced (1).The postaxial margin of the humerus of Corosaurus is evenly concave, with no

indication of a projecting crest for the insertion of the latissimus dorsi muscle. Thiscontrasts with the condition observed in Nothosaurus (Rieppel, 1994a, fig. 58F),particularly in large specimens (Rieppel, 1994a, fig. 59B), where the crest for theinsertion of the latissimus dorsi muscle projects into the concavity of the posteriormargin of the humerus. This degree of development of the crest for the insertionof the latissimus dorsi muscle is not known in Placodus (Rieppel, 1994a, fig. 60),pachypleurosaurs (e.g. Rieppel & Lin, 1995, fig. 14), Simosaurus (Rieppel, 1994a),Lariosaurus and Pistosaurus (Sues, 1987), but it does occur in humeri attributed toCymatosaurus (Rieppel, 1994a, fig. 57).

95. Humerus with prominent (0) or reduced (1) epicondyles.The humerus of Corosaurus shows strongly reduced epicondyles as is typical of

most Sauropterygia with the exception of humeri attributed to Cymatosaurus (Rieppel,1994a, fig. 57), Dactylosaurus (Rieppel & Lin, 1995, fig. 14; see also Rieppel, 1993b,fig. 8), and some Neusticosaurus (Rieppel & Lin, 1995).

96. The ectepicondylar groove is open and notched anteriorly (0), open withoutanterior notch (1), or closed (2) (i.e. ectepicondylar foramen present).

The ectepicondylar groove is distinct in Corosaurus, and deeply notched distally.This character is variable among other Sauropterygia.

97. Entepicondylar foramen present (0) or absent (1).The entepicondylar foramen is present in Corosaurus, as in most stem-group

Sauropterygia with the exception of Placodus, Simosaurus, and Pistosaurus.98. Radius shorter than ulna (0), or longer than ulna (1) or approximately of the

same length (2).The left radius and left ulna of the holotype of Corosaurus alcovensis have been

completely removed from the matrix. Both elements are broken at their proximalend, such that their individual length cannot be established. The radius appearsgenerally as a more robust bone with a triangular cross-section compared to theflattened ulna. The ulna, on the other hand, shows a greater degree of terminalexpansion, and a greater degree of concavity of its preaxial margin (compared tothe postaxial margin of the radius). If the two bone fragments are placed in a naturalposition, it appears from the degree of their curvature that the two elements wouldbe of similar length (see also Storrs, 1991).

99. Iliac blade well developed (0), reduced but projecting beyond level of posteriormargin of acetabular portion of ilium (1), reduced and no longer projecting beyondposterior margin of acetabular portion of ilium (2), or absent, i.e. reduced to simpledorsal stub (3).

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The right ilium of Corosaurus (FMNH PR243; Storrs, 1991, fig. 15C) has beencompletely removed from the surrounding matrix (fig. 5D). It closely resembles theilium of Placodus, Simosaurus and Nothosaurus in being partitioned into a roughlytriangular or pyramidal ventral (acetabular) portion, and a reduced dorsal bladecarrying a small anterior and a distinct posterior projection. The posterior processof the dorsal blade of the ilium projects distinctly beyond the posterior margin ofthe bone in Corosaurus, as is also the case in Placodus, but not in Simosaurus andNothosaurus.

The medial surface of the ilium, facing dorsomedially in the articulated skeleton(as indicated by articulated material of Lariosaurus (Ceresiosaurus) calcagnii: H. Lanz,pers. comm.; see also Nothosaurus (Paranothosaurus) amsleri: Peyer, 1939, pl. 67), isfinished perichondral bone with an interesting ornamentation consisting of numerous,densely packed tubercles in the dorsal region, between the broad ventral (acetabular)portion and the dorsal wing. A similarly distinct ornamentation, probably relatedto muscle insertions, has until now not been observed in any other stem-groupsauropterygian. The acetabular surface of the ilium is unfinished, and appearssomewhat eroded.

100. Pubis with convex (0) or with concave (1) ventral (medial) margin.The pubis of Corosaurus is autapomorphic (at the level of the Sauropterygia) by

having a convex anterior and a concave posterior margin. It therefore lacks the‘waisted’ appearance (concave anterior and posterior margins) typical for the pubisof other stem-group Sauropterygia other than Placodus. In contrast to Corosaurus (aswell as Placodus and Cymatosaurus [syn. Proneusticosaurus]), which shows a rounded(convex) ventral (medial) margin of the pubis, Simosaurus and Nothosaurus havedeveloped a distinct concavity in the ossified ventral margin of the pubis. A similar,if shallower concavity is observed in Serpianosaurus, and in some Lariosaurus.

101. Obturator foramen closed (0) or open (1) in adult.The obturator foramen in the pubis of Corosaurus is open, i.e. not completely

enclosed within the pubis. The same condition is observed in Cymatosaurus (syn.Proneusticosaurus), in some adults of the Serpianosaurus–Neusticosaurus clade, and inLariosaurus. The obturator foramen is usually closed in adults of Simosaurus andNothosaurus (Rieppel, 1994a), and in Placodus.

102. Thyroid fenestra absent (0) or present (1).The thyroid foramen is present in Corosaurus as a consequence of the concave

posterior margin of the pubis and the concave anterior margin of the ilium.103. Acetabulum oval (0) or circular (1).104. Femoral shaft stout and straight (0) or slender and sigmoidally curved (1).The femur of Corosaurus is distinctly longer than the humerus, and of a more

gracile appearance. It is a slender, sigmoidally curved bone with a cylindricaldiapophysis, as is generally the case in stem-group Sauropterygia.

105. Internal trochanter well developed (0) or reduced (1).The femur of Corosaurus shows a distinct internal trochanter (Storrs, 1991), similar

to the internal trochanter of Placodus (Rieppel, 1994a, figs 62, 63) and Cymatosaurus(syn. Proneusticosaurus). The internal trochanter is more strongly reduced in otherSauropterygia.

106. Intertrochanteric fossa deep (0), distinct but reduced (1), or rudimentary orabsent (2).

In Corosaurus, the internal trochanter is set off from the proximal articular


head by a very shallow intertrochanteric fossa only, as is also the case in othersauropterygians with the exception of Placodus (Rieppel, 1994a, figs 62, 63).

107. Distal femoral condyles prominent (0) or not projecting markedly beyondshaft (1).

The distal articular head of the femur shows weakly developed articular condylesvaguely separated from one another but not projecting beyond the femoral shaft.

108. Anterior femoral condyle relative to posterior condyle larger and extendingfurther distally (0) or smaller/equisized and of subequal extent distally (1).

109. The perforating artery passes between astragalus and calcaneum (0), orbetween the distal heads of tibia and fibula proximal to the astragalus (1).

FMNH PR 480 shows a partially preserved pes with two large, circular bones inthe proximal tarsal row. The larger of these must be the astragalus, the smaller onethe calcaneum. The astragalus of Nothosaurus and Simosaurus (Rieppel 1994a, figs 34,64) shows a distinct concavity on the proximal edge, which may articulate with thedistal end of the tibia and/or indicate the passage of the perforating artery proximalto the astragalus (Rieppel, 1994a, fig. 64). Such a concavity is absent on the astragalusof Placodus and Dactylosaurus, but it is present in the Serpianosaurus–Neusticosaurus clade,and in Lariosaurus. However, in no sauropterygian is there evidence of even an illdefined foramen located between astragalus and calcaneum for the passage of theperforating artery.

110. Astragalus without (0) or with (1) a proximal concavity.The astragalus may show a well defined articular facet for the tibia on its proximal

margin in a variety of amniotes. In sauropterygians, the astragalus generally showsa reduced degree of ossification, and in articulated tarsi is often located in anintermedium position distal to the spatium interosseum. The concavity on itsproximal margin may therefore indicate the passage of the perforating artery(Rieppel, 1994a, fig. 64). A concavity is absent on the proximal margin of theastragalus in Corosaurus.

111. Calcaneal tuber absent (0) or present (1).112. Foot short and broad (0) or long and slender (1).113. Distal tarsal 1 present (0) or absent (1).114. Distal tarsal 5 present (0) or absent (1).115. Total number of tarsal ossifications four or more (0), three (1) or two (2).The partially preserved tarsus of FMNH PR 480 shows the fourth distal tarsal in

articulation with the calcaneum. There is no evidence of any other distal tarsalbone, and considering the state of preservation, three tarsal ossifications is consideredthe natural number.

116. Metatarsal 5 long and slender (0) or distinctly shorter than the othermetatarsals and with a broad base (1).

In the partially preserved tarsus of FMNH PR 480, three metatarsals and onephalanx are preserved. There is, however, no way to identify the exact homologyof the metatarsals, although the fifth would be expected to be shorter than the onespresent.

117. Metatarsal 5 straight (0) or ‘hooked’ (1).118. Mineralized sternum absent (0) or present (1). See deBraga and Rieppel

(1997) for further discussion of this character.119. The medial gastral rib element always only has a single (0) lateral process,

or may have a two-pronged lateral process (1).Isolated median gastral rib elements of Corosaurus may show a two-pronged lateral

O. RIEPPEL24

process. The same character is observed in Simosaurus and Nothosaurus (Rieppel,1994a), but it has never been observed in pachypleurosaurs, and it also does notoccur in Cymatosaurus (syn. Proneusticosaurus). Two-pronged lateral processes also occuron medial gastral elements of Microleptosaurus schlosseri Skuphos, 1893, but this taxonis too incompletely known to be entered in this phylogenetic analysis (Rieppel, 1996).

CLADISTIC ANALYSIS

The data matrix shown in Table 1 was analyzed using the software packagePAUP version 3.1.1 developed by David L. Swofford (Swofford, 1990; Swofford &Begle, 1993). The heuristic search option implemented invariably employed randomstepwise addition (10 replications unless noted otherwise), and branch swapping (onminimal trees only) was effected by tree bisection an reconnection. All searches wererun with all multistate characters unordered unless noted otherwise. An initialanalysis included sauropterygian taxa only in search for an unrooted network.Excluding all non-sauropterygian taxa from the analysis rendered a large numberof characters uninformative (1, 2, 5, 9, 15, 20, 21, 22, 24, 27, 28, 30, 34, 38, 40,41, 42, 43, 47, 48, 52, 53, 58, 59, 61, 62, 63, 64, 65, 66, 71, 73, 76, 78, 81, 84,86, 87, 88, 89, 90, 91, 92, 102, 103, 104, 107, 108, 109, 111, 112, 113, 114, 116,117, 118), and yielded one single most parsimonious network (Fig. 7A) which wouldnot support a monophyletic Eusauropterygia (Tschanz, 1989), Nothosauriformes(Storrs, 1991) or Eosauropterygia (Rieppel, 1994a), no matter where the root isplaced. Including all 23 terminal taxa in a search for an unrooted network renderedcharacter 63 the only uninformative one, and yielded three most parsimoniousnetworks with lack of resolution among archosauromorph taxa only. The segmentof that network including Sauropterygia (Fig. 7B) potentially supports a monophyleticEosauropterygia, but would not support a monophyletic Eusauropterygia or No-thosauriformes no matter where the root is placed.

Rooting the tree always assumed monophyly of the ingroup and paraphyly of theoutgroup. Rooting the Sauropterygia (ingroup) on all other taxa included in theanalysis (character 63 uninformative) yielded three most parsimonious trees (MPTs)with a tree length (TL) of 446 steps, a Consistency Index (CI) of 0.655, and aRetention Index (RI) of 0.697. Relationships within Sauropterygia are fully resolved.Tree topology reads as follows (see also fig. 8): (Placodus ((Corosaurus (Cymatosaurus,Pistosaurus)) ((Dactylosaurus, Serpianosaurus–Neusticosaurus) (Simosaurus (Germanosaurus (No-thosaurus, Lariosaurus)))))). The strict consensus tree of the three MPTs reveals lack ofresolution to be restricted to archosauromorph taxa.

Identical (and fully resolved) ingroup interrelationships were found by rooting theSauropterygia (ingroup) on the outgroup taxa used by Storrs (1991), i.e. Capto-rhinidae, Araeoscelidia, Younginiformes and Claudiosaurus. The analysis yielded onesingle MPT (TL=251; CI=0.697; RI=0.726), but many of the characters wererendered uninformative by the deletion of all other terminal taxa from the analysis(2, 5, 24, 28, 34, 40, 53, 63, 66, 111, 116, 117).

Treating Diapsida (including Testudines: Rieppel & deBraga, 1996) as ingroup,and rooting the analysis on Captorhinidae (character 63 uninformative) yieldedthree MPTs (TL=446; CI=0.655; RI=0.697), with lack of resolution restricted tothe archosauromorph clade. Sauropterygian interrelationships are fully resolved andidentical to those indicated above.


T 1. Data matrix for the analysis of the phylogenetic relationships of Triassic stem-groupSauropterygia.

Corosaurus-Data-141 1 2 3 4 5 6 7 8 9 10

1 Ancestor 0 0 0 0 0 0 0 0 0 02 Captorhinidae 0 0 0 0 0 0 0 0 0 03 Testudines 0 0 0 0 0 0 0 0 1 0 & 14 Araeoscelidia 0 0 0 0 0 0 0 0 0 05 Younginiformes 0 0 0 0 0 0 0 0 1 06 Kuehneosauridae 0 0 0 0 0 0 0 0 1 07 Rhynchocephalia 0 0 0 0 0 0 0 0 1 & 2 08 Squamata 0 0 0 0 0 0 0 0 1 & 2 0 & 19 Rhynchosauria 1 1 0 0 0 0 0 0 1 010 Prolacertiformes 1 1 0 0 1 0 0 0 1 011 Trilophosaurus 1 ? 0 0 0 0 0 0 ? 112 Choristodera 1 1 0 0 1 0 0 0 1 013 Archosauriformes 1 1 0 0 1 0 0 0 1 014 Claudiosaurus 0 0 0 0 0 0 0 0 1 015 Dactylosaurus 1 0 0 0 0 0 0 0 2 016 Serpiano-Neustico 1 0 0 0 0 0 0 0 & 1 2 017 Simosaurus 1 0 0 0 0 1 0 1 2 018 Nothosaurus 1 0 1 1 0 0 0 0 & 1 2 019 Lariosaurus 1 0 1 1 0 0 0 0 & 1 2 020 Corosaurus 1 0 0 0 0 0 0 0 2 021 Cymatosaurus 1 0 1 0 0 2 0 &1 1 2 0 & 122 Germanosaurus 1 0 1 1 0 1 0 1 2 023 Pistosaurus 1 0 0 0 0 2 1 1 2 024 Placodus 1 0 1 0 1 0 0 0 2 1

Corosaurus-data-142 11 12 13 14 15 16 17 18 19 20

1 Ancestor 0 0 0 0 0 0 0 0 0 02 Captorhinidae 0 0 0 0 0 ? 0 0 & 2 0 03 Testudines 0 0 & 2 0 0 0 ? 0 3 0 14 Araeoscelidia 0 1 1 0 1 0 0 0 0 05 Younginiformes 0 1 1 0 1 0 0 0 0 06 Kuehneosauride 0 0 1 0 1 0 0 2 0 17 Rhynchocephalia 0 0 1 0 & 1 0 & 1 0 0 & 1 0 & 2 0 & 2 18 Squamata 0 0 & 1 & 2 1 0 & 1 0 0 0 & 1 0 & 2 & 3 0 & 2 19 Rhynchosauria 0 0 1 0 0 0 1 3 3 110 Prolacertiformes 0 1 1 0 0 & 1 0 0 & 1 2 & 3 0 111 Trilophosaurus 0 1 1 0 0 1 0 3 3 112 Choristodera 0 2 1 0 0 0 0 3 1 113 Archosauriformes 0 0 & 1 1 0 & 1 0 & 1 0 & 1 0 & 1 0 & 3 0 & 2 1

& 214 Claudiosaurus 0 1 1 0 1 0 0 0 0 115 Dactylosaurus 0 1 3 0 1 0 0 0 0 116 Serpiano-Neustico 0 1 3 0 1 0 0 0 0 117 Simosaurus 0 2 2 1 0 0 2 1 1 118 Nothosaurus 1 2 2 1 1 0 2 1 2 & 3 119 Lariosaurus 1 2 2 1 1 0 2 1 2 120 Corosaurus 0 0 1 0 1 1 0 0 1 121 Cymatosaurus 0 2 2 0 1 1 & 2 1 & 2 0 2 & 3 122 Germanosaurus 1 2 2 0 1 1 1 1 1 123 Pistosaurus 0 1 2 0 1 2 1 2 3 124 Placodus 0 0 2 0 & 1 1 0 0 & 2 2 0 1

O. RIEPPEL26

T 1—continued

Corosaurus-Data-143 21 22 23 24 25 26 27 28 29 30

1 Ancestor 0 0 0 0 0 0 0 0 0 02 Captorhinidae 1 0 0 0 0 0 0 0 0 03 Testudines 1 0 & 1 0 0 0 ? 0 1 0 04 Araeoscelidia 0 0 0 0 0 1 0 & 1 0 0 05 Younginiformes 0 & 1 0 0 0 0 1 1 0 0 06 Kuehneosauridae 1 1 0 0 0 1 2 1 1 17 Rhynchocephalia 1 0 & 1 0 & 1 1 0 1 1 & 2 0 0 18 Squamata 1 0 & 1 0 & 1 0 0 1 2 1 1 ?9 Rhynchosauria 1 1 0 1 0 1 1 1 0 010 Prolacertiformes 1 0 & 1 0 0 & 1 0 1 2 1 0 & 1 111 Trilophosaurus 1 ? 0 ? 0 1 0 ? 0 ?12 Choristodera 1 1 0 0 0 1 1 1 0 & 1 013 Archosauriformes 1 0 & 1 0 1 0 1 1 1 0 014 Claudiosaurus 1 1 0 0 0 1 2 0 0 115 Dactylosaurus 1 1 ? 0 0 2 2 0 0 116 Serpiano-Neustico 1 1 0 0 0 2 2 0 0 117 Simosaurus 1 1 0 0 1 1 2 0 0 118 Nothosaurus 1 1 1 & 2 0 1 1 & 2 2 0 0 & 1 119 Lariosaurus 1 1 1 & 2 0 1 2 2 0 ? ?20 Corosaurus 1 1 ? 0 0 1 2 0 ? ?21 Cymatosaurus 1 1 1 0 0 1 2 0 1 ?22 Germanosaurus 1 1 2 0 1 1 2 0 ? ?23 Pistosaurus 1 1 ? 0 1 1 2 0 1 ?24 Placodus 1 1 0 0 1 1 2 0 0 0

Corosaurus-Data-144 31 32 33 34 35 36 37 38 39 40

1 Ancestor 0 0 0 0 0 0 0 0 0 02 Captorhinidae 0 0 0 ? 0 0 0 0 0 03 Testudines 0 0 0 & 2 0 0 0 1 1 0 04 Araeoscelidia 0 0 0 1 0 0 0 0 ? 05 Younginiformes 0 0 0 1 0 0 1 1 ? 06 Kuehneosauridae 0 0 0 1 0 1 1 1 ? 17 Rhynchocephalia 0 & 1 0 0 1 0 0 & 1 1 1 1 0 & 18 Squamata 0 0 0 & 1 & 2 0 & 1 0 & 1 0 & 1 1 1 1 19 Rhynchosauria 1 0 0 0 0 0 1 1 1 010 Prolacertiformes 1 0 0 1 0 0 1 1 ? 011 Trilophosaurus 1 0 2 ? 0 ? 1 1 0 012 Choristodera 1 0 1 1 0 1 1 1 ? 013 Archosauriformes 0 & 1 0 0 & 1 & 2 1 0 0 & 1 1 1 ? 014 Claudiosaurus 0 0 0 ? 0 0 0 0 ? 015 Dactylosaurus 2 0 0 ? 1 0 1 1 1 016 Serpiano-Neustico 2 0 0 ? 1 0 1 1 1 017 Simosaurus 2 0 1 1 0 0 0 1 0 018 Nothosaurus 2 0 0 & 1 1 1 1 0 1 0 019 Lariosaurus 2 0 0 ? 1 1 0 1 0 020 Corosaurus 1 1 1 1 0 1 0 1 1 021 Cymatosaurus 1 1 1 ? 0 0 & 1 0 1 ? 022 Germanosaurus ? ? ? ? ? 0 0 1 ? 023 Pistosaurus 1 ? 1 ? 0 1 0 1 1 024 Placodus 1 0 0 1 0 0 1 1 1 0


T 1—continued

Corosaurus-Data-145 41 42 43 44 45 46 47 48 49 50

1 Ancestor 0 0 0 0 0 0 0 0 0 02 Captorhinidae 0 0 0 0 0 1 0 0 0 03 Testudines 0 & 1 0 1 0 0 & 1 1 0 0 & 1 0 04 Araeoscelidia 0 0 1 0 0 0 0 0 0 05 Younginiformes 0 0 1 0 0 0 0 1 1 06 Kuehneosauridae 0 0 1 0 ? ? 0 1 0 07 Rhynchocephalia 0 0 1 0 0 0 0 1 1 08 Squamata 0 0 1 0 0 & 1 0 0 1 1 09 Rhynchosauria 0 0 1 0 1 0 0 1 1 010 Prolacertiformes 0 0 1 0 0 0 0 1 0 & 1 011 Trilophosaurus 0 0 1 0 0 0 0 1 1 012 Choristodera ? 0 1 0 1 0 0 1 0 013 Archosauriformes 0 & 1 0 1 0 0 & 1 0 & 1 0 1 0 & 1 014 Claudiosaurus 0 0 1 1 0 0 0 0 0 ?15 Dactylosaurus 1 ? 0 1 0 1 ? 1 0 016 Serpiano-Neustico 1 ? 0 1 0 1 ? 1 0 017 Simosaurus 1 2 0 1 0 0 1 1 0 118 Nothosaurus 1 2 0 0 & 1 1 0 1 1 0 119 Lariosaurus 1 ? 0 1 ? 0 ? 1 0 120 Corosaurus 1 ? 0 0 ? 0 ? 1 1 121 Cymatosaurus 1 ? 0 0 1 0 1 ? 0 ?22 Germanosaurus 1 ? ? ? ? ? ? ? ? ?23 Pistosaurus 1 ? 0 0 ? 0 ? ? ? ?24 Placodus 1 1 0 0 1 0 0 1 1 0

Corosaurus-Data-146 51 52 53 54 55 56 57 58 59 60

1 Ancestor 0 0 0 0 0 0 0 0 0 02 Captorhinidae 0 0 0 0 ? 0 0 0 0 03 Testudines 0 1 ? ? ? ? ? 0 0 & 1 0 & 24 Araeoscelidia 0 1 0 0 0 0 0 0 0 05 Younginiformes 0 ? 0 0 0 1 0 0 0 06 Kuehneosauridae 0 ? 0 0 0 1 0 1 1 07 Rhynchocephalia 0 1 1 0 0 1 0 1 0 & 1 08 Squamata 0 1 1 0 0 1 0 1 1 0 & 29 Rhynchosauria 0 0 0 ? 0 1 0 1 1 010 Prolacertiformes 0 1 0 0 0 1 0 0 & 1 1 0 & 211 Trilophosaurus 0 ? 0 ? 0 1 0 1 1 1 & 212 Choristodera 0 1 0 0 0 1 0 0 1 113 Archosauriformes 0 1 0 0 0 1 0 0 & 1 1 0 & 1 & 214 Claudiosaurus 0 1 0 0 0 1 0 0 ? 015 Dactylosaurus 0 ? 0 0 0 1 0 1 1 016 Serpiano-Neustico 0 ? 0 0 0 1 0 1 1 017 Simosaurus 0 1 0 1 0 1 2 1 1 118 Nothosaurus 2 1 0 1 1 0 2 1 1 119 Lariosaurus 2 ? 0 1 1 0 2 1 1 120 Corosaurus 1 ? 0 1 1 0 1 1 1 021 Cymatosaurus 2 1 0 1 1 0 1 1 1 122 Germanosaurus ? ? 0 1 1 1 1 ? ? ?23 Pistosaurus ? ? 0 1 1 0 1 1 1 124 Placodus 2 0 0 1 0 1 0 1 0 0

O. RIEPPEL28

T 1—continued

Corosaurus-Data-147 61 62 63 64 65 66 67 68 69 70

1 Ancestor 0 0 0 0 0 0 0 0 0 02 Captorhinidae 0 0 0 0 ? 0 0 0 0 03 Testudines 1 0 & 1 1 0 0 0 0 0 0 04 Araeoscelidia 0 0 1 0 ? 0 0 0 0 05 Younginiformes 0 0 1 0 0 0 0 0 0 06 Kuehneosauridae 1 1 1 0 ? 1 0 0 0 17 Rhynchocephalia 0 & 1 0 1 1 0 0 0 0 0 08 Squamata 1 0 1 0 & 1 0 0 0 0 0 0 & 19 Rhynchosauria 0 0 1 0 ? 0 0 0 0 0 & 110 Prolacertiformes 0 & 1 0 & 1 1 0 ? 0 0 0 0 011 Trilophosaurus 0 0 1 0 ? 0 ? 0 0 012 Choristodera 1 0 1 0 1 0 0 0 0 013 Archosauriformes 0 & 1 0 & 1 1 0 0 1 0 0 0 0 & 114 Claudiosaurus 0 0 1 0 ? 0 0 0 0 015 Dactylosaurus 1 1 1 1 1 0 1 0 1 016 Serpiano-Neustico 1 1 1 1 1 0 1 0 1 017 Simosaurus 1 1 1 1 1 0 0 0 0 118 Nothosaurus 1 1 1 1 1 0 1 0 0 & 1 019 Lariosaurus 1 1 1 ? 2 0 1 0 1 020 Corosaurus 1 1 1 1 1 0 0 1 0 121 Cymatosaurus 1 1 ? 1 1 0 0 0 ? ?22 Germanosaurus ? ? ? ? ? ? ? ? ? ?23 Pistosaurus ? ? ? 1 1 0 0 1 0 ?24 Placodus 1 1 1 0 0 1 0 0 0 1

Corosaurus-Data-148 71 72 73 74 75 76 77 78 79 80

1 Ancestor 0 0 0 0 0 0 0 0 0 02 Captorhinidae 0 0 0 0 0 ? 0 0 0 03 Testudines 0 0 0 0 0 1 0 0 0 04 Araeoscelidia 0 0 0 0 0 0 0 0 0 05 Younginiformes 0 0 0 0 0 & 1 0 & 1 1 0 0 06 Kuehneosauridae ? 0 0 0 1 ? ? ? ? ?7 Rhynchocephalia 0 0 0 0 1 1 1 0 0 08 Squamata 0 0 0 0 1 1 0 & 1 0 0 09 Rhynchosauria 0 0 0 0 0 1 1 0 0 010 Prolacertiformes 1 0 0 0 1 1 1 0 0 011 Trilophosaurus 1 0 0 0 ? 1 1 0 0 012 Choristedera 1 0 1 0 0 1 1 1 0 013 Archosauriformes 1 0 0 & 2 0 0 & 1 1 1 0 0 014 Claudiosaurus 1 0 0 0 1 1 1 0 0 015 Dactylosaurus 1 0 1 1 0 1 0 1 1 116 Serpiano0Neustico 1 1 1 1 0 1 0 1 1 017 Simosaurus 1 0 1 0 0 1 0 1 1 118 Nothosaurus 1 0 & 1 1 1 0 & 1 1 0 & 1 1 1 119 Lariosaurus 1 0 & 1 2 1 0 1 0 1 1 120 Corosaurus 1 0 1 0 1 1 1 1 0 121 Cymatosaurus ? 0 ? ? 0 1 ? ? ? ?22 Germanosaurus ? ? ? ? ? 1 ? ? ? ?23 Pistosaurus 1 0 ? ? ? 1 ? ? ? ?24 Placodus 1 0 1 0 0 1 1 1 0 0


T 1—continued

Corosaurus-Data-149 81 82 83 84 85 86 87 88 89 90

1 Ancestor 0 0 0 0 0 0 0 0 0 02 Captorhinidae 0 0 ? 0 ? 0 1 0 0 03 Testudines 0 1 0 0 ? ? 0 0 1 04 Araeoscelidia 0 1 0 0 ? 0 1 0 0 05 Younginiformes 0 0 & 1 0 0 ? 1 0 0 0 06 Kuehneosauridae ? ? ? 1 ? 1 0 0 0 07 Rhynchocephalia 0 1 0 0 ? 1 0 0 0 08 Squamata 0 1 0 & 1 0 ? 1 0 0 0 09 Rhynchosauria 0 1 0 0 ? 1 0 0 0 & 1 010 Prolacertiformes 0 0 ? 0 ? 1 0 0 0 011 Trilophosaurus 0 1 0 0 ? 1 0 0 0 012 Choristodera 0 1 0 0 ? 1 0 0 0 013 Archosauriformes 0 1 0 0 ? 1 0 0 0 & 1 014 Claudiosaurus 0 1 0 0 ? 1 0 0 0 015 Dactylosaurus 1 ? ? 1 0 1 0 2 1 116 Serpiano-Neustico 1 0 & 1 2 1 0 1 0 2 1 117 Simosaurus 1 1 1 1 0 1 0 2 1 118 Nothosaurus 1 0 & 1 2 1 0 1 0 2 1 119 Lariosaurus 1 ? ? 1 0 1 0 2 1 120 Corosaurus 1 1 1 1 1 1 0 1 1 121 Cymatosaurus ? ? ? ? ? ? 0 ? ? ?22 Germanosaurus ? ? ? ? ? ? 0 ? ? ?23 Pistosaurus ? ? ? 1 1 1 0 3 ? ?24 Placodus 1 1 1 0 ? 1 0 0 1 1

Corosaurus-Data-140 91 92 93 94 95 96 97 98 99 100

1 Ancestor 0 0 0 0 0 0 0 0 0 02 Captorhinidae 0 0 0 0 0 0 0 0 0 03 Testudines 0 0 0 0 0 & 1 0 & 2 1 0 1 & 2 04 Araeoscelidia 1 0 0 0 0 0 0 0 0 05 Younginiformes 1 0 0 0 0 0 & 2 0 0 & 1 0 06 Kuehneosauridae 1 0 0 0 0 2 1 2 0 07 Rhynchocephalia 1 0 0 0 0 2 0 0 0 08 Squamata 1 0 0 0 0 0 & 2 1 0 0 09 Rhynchosauria 1 0 0 0 0 0 1 0 0 010 Prolacertiformes 1 0 0 0 0 0 1 1 & 2 0 011 Trilophosaurus 1 0 0 ? 0 0 1 0 0 012 Choristodera 1 0 0 0 0 0 & 2 1 2 0 013 Archosauriformes 1 0 0 0 0 & 1 0 1 0 0 014 Claudiosaurus 0 0 1 1 1 1 0 0 0 015 Dactylosaurus 0 1 0 1 0 0 0 1 3 016 Serpiano-Neustico 0 1 0 1 0 & 1 0 & 1 0 1 & 2 3 0 & 117 Simosaurus 0 1 0 1 1 1 1 2 2 118 Nothosaurus 0 1 0 & 1 0 & 1 1 0 & 1 0 2 2 119 Lariosaurus 0 1 1 1 1 1 0 2 3 0 & 120 Corosaurus 0 1 1 1 1 0 0 2 1 021 Cymatosaurus 0 0 0 0 0 0 0 ? ? 022 Germanosaurus ? ? ? ? ? ? ? ? ? ?23 Pistosaurus 0 1 ? 1 1 1 1 2 0 ?24 Placodus 0 1 1 1 1 0 1 2 1 0

O. RIEPPEL30

T 1—continued

Corosaurus-Data-141 101 102 103 104 105 106 107 108 109 110

1 Ancestor 0 0 0 0 0 0 0 0 0 02 Captorhinidae 0 0 0 0 0 0 0 0 0 03 Testudines 0 1 0 0 0 0 1 1 1 04 Araeoscelidia 0 0 0 0 0 0 0 0 0 05 Younginiformes 0 0 1 1 0 1 0 1 0 06 Kuehneosauridae 0 1 1 1 0 1 0 1 ? 07 Rhynchocephalia 0 1 1 1 0 1 0 1 1 08 Squamata 0 1 1 1 0 1 0 1 1 09 Rhynchosauria 0 0 1 1 0 1 0 1 1 010 Prolacertiformes 0 0 & 1 1 1 0 1 0 1 0 011 Trilophosaurus 0 0 1 1 0 1 0 1 0 012 Choristodera 0 0 1 1 0 1 1 1 ? 013 Archosauriformes 0 0 1 1 0 1 & 2 0 & 1 1 0 014 Claudiosaurus 0 0 1 1 0 1 1 1 0 015 Dactylosaurus 0 1 1 ? 1 ? ? ? ? 016 Serpiano-Neustico 0 & 1 1 1 1 1 2 1 1 1 117 Simosaurus 0 1 1 1 1 2 1 1 1 118 Nothosaurus 0 1 1 1 1 2 1 1 1 119 Lariosaurus 1 1 1 1 1 2 1 1 1 120 Corosaurus 1 1 1 1 0 1 1 1 1 021 Cymatosaurus 1 1 1 1 0 1 1 1 ? ?22 Germanosaurus ? ? ? ? ? ? ? ? ? ?23 Pistosaurus ? ? ? 1 1 ? ? ? ? ?24 Placodus 0 1 1 1 0 1 1 1 1 0

Corosaurus-Data-142 111 112 113 114 115 116 117 118 119

1 Ancestor 0 0 0 0 0 0 0 0 02 Captorhinidae 0 0 0 0 0 0 0 0 03 Testudines 0 0 0 & 1 1 0 1 1 0 ?4 Araeoscelidia 0 1 0 0 0 0 0 1 05 Younginiformes 0 1 0 0 0 1 0 1 06 Kuehneosauridae ? ? ? ? ? ? ? ? ?7 Rhynchocephalia 0 1 0 & 1 1 0 1 1 1 08 Squamata 0 1 1 1 0 1 1 1 ?9 Rhynchosauria 1 1 0 1 0 1 1 ? 010 Prolacertiformes 0 & 1 1 0 & 1 1 0 1 0 & 1 1 011 Trilophosaurus 1 1 0 1 0 1 1 ? ?12 Choristodera 1 1 0 1 0 1 1 ? ?13 Archosauriformes 1 1 0 & 1 1 0 1 1 1 014 Claudiosaurus 0 1 0 0 0 0 0 ? 015 Dactylosaurus ? ? ? ? ? ? ? 0 016 Serpiano-Neustico 0 0 1 1 2 0 0 0 017 Simosaurus 0 0 1 1 1 0 0 0 118 Nothosaurus 0 0 1 1 1 0 0 0 119 Lariosaurus 0 0 1 1 0 0 0 0 020 Corosaurus 0 0 1 1 1 ? ? 0 121 Cymatosaurus ? ? ? ? ? ? ? 0 022 Germanosaurus ? ? ? ? ? ? ? 0 ?23 Pistosaurus ? ? ? ? ? ? ? 0 ?24 Placodus 0 0 1 1 2 0 0 0 0


EOSAUROPTERYGIA

EUSAUROPTERYGIA

Corosaurus

Serpiano-Nesuticosaurus

Dactylosaurus

Placodus Cymatosaurus

Pistosaurus

Larisaurus

NothosaurusGermanosaurusSimosaurus

A

EOSAUROPTERYGIA

Corosaurus

Serpiano-NesuticosaurusDactylosaurus

Placodus

Cymatosaurus

Pistosaurus

Larisaurus

NothosaurusGermanosaurusSimosaurus

All otherDiapsida

B

Figure 7. A, unrooted network for ingroup taxa only; B, unrooted network for ingroup and outgrouptaxa. For further discussion see text.

Treating all terminal taxa as ingroup (Reptilia), and rooting the search on an all-0-ancestor (all characters informative) yielded three MPTs (TL=450; CI=0.651;RI=0.713), again with fully resolved sauropterygian interrelationships, and withlack of resolution restricted to the archosauromorph clade.

In the final analysis (search options as indicated above, 100 replications), character27 was treated as ordered (for reasons discussed in the definition of that character),and all terminal taxa (Reptilia, monophyletic ingroup) were rooted on the all-0-ancestor. (Treating character 27 as unordered decreased tree length by one step,but increased the number of most parsimonious trees by one, decreasing resolutionwithin the archosauromorph clade.) The analysis yielded two MPTs (TL=451;CI=0.650; RI=0.713), with fully resolved sauropterygian interrelationships asshown in the strict consensus tree (fig. 8). Bootstrap values (1000 replications) anddecay indices (the number of steps beyond the most parsimonious tree it takes tobreak a node) are shown in Figure 8.

Placodus comes out as sister-taxon to a monophyletic Eosauropterygia. However,the Eusauropterygia sensu Tschanz (1989) become paraphyletic due to altered

O. RIEPPEL32

Coros

auru

s

Serpi

ano-

Neusti

cosa

urus

Dactyl

osau

rus

Placo

dus

Cymat

osau

rus

Pistos

auru

s

Larisa

urus

Nothos

auru

s

Germ

anos

auru

s

Simos

auru

s

53%DI : 3

51%DI : 4

92%DI : 3

53%DI : 4

56%DI : 3

<50%DI : 2

94%DI : 3

60%DI : 2

100%DI : >4

Figure 8. Strict consensus tree of two equally parsimonious trees (TL=451; CI=0.650; RI=0.713),showing fully resolved sauropterygian interrelationships. For further discussion see text.

relationships of the Pachypleurosauria. Indeed, the Eosauropterygia divide into twomajor lineages. One clade includes Corosaurus as sister-taxon to Cymatosaurus andPistosaurus [(Corosaurus (Cymatosaurus, Pistosaurus))]. Pachypleurosaurs, on the otherhand, turn out to be the sister-group of the Nothosauria, a clade including Simosaurusand the Nothosauridae [(Pachypleurosauria (Simosaurus (Germanosaurus (Nothosaurus,Lariosaurus))))].

DELTRAN character optimization will tend to push synapomorphies up the tree,with the consequence that fewer characters will diagnose more inclusive clades, butsubclades will tend to preserve diagnostic characters as reversal is less likely to occurthan with ACCTRAN character optimization. For this reason, the listing of diagnosticcharacters below will be based on DELTRAN character optimization (unlessotherwise noted). Unequivocal synapomorphies (with a consistency index of 1.0) areindicated with an asterisk.

Sauropterygia: 1(1), 9(2)∗, 13(2), 27(2), 28(0), 31(1), 39(1), 41(1)∗, 43(0), 54(1),58(1), 62(1), 71(1), 73(1), 78(1), 81(1)∗, 83(1), 90(1)∗, 92(1), 94(1), 95(1), 98(2),116(0), 117(0). Eosauropterygia: 47(1)∗, 64(1), 65(1), 80(1), 84(1), 97(0), 115(1).Nothosauroidea (Pachypleurosauria plus Nothosauria): 30(1), 31(2), 44(1), 77(0),79(1)∗, 88(2)∗, 105(1), 106(2). Nothosauria (Simosaurus plus Nothosauridae): 8(1),12(2), 17(2), 18(1), 19(1), 25(1), 37(0), 39(0), 42(2)∗, 50(1), 57(2), 60(1), 96(1), 99(2),100(1)∗, 110(1). Nothosauridae (Germanosaurus plus Nothosaurinae): 3(1), 4(1)∗, 11(1)∗,23(2)∗, 55(1). Nothosaurinae (Nothosaurus plus Lariosaurus): 14(1), 19(2), 35(1), 36(1),51(2), 56(0), 67(1), 74(1). Pistosauroidea (Corosaurus plus Pistosauria): 32(1)∗, 33(1),36(1), 37(0), 55(1), 56(0), 57(1), 85(1)∗, 101(1). Pistosauria (Cymatosaurus plus Pistosaurus):6(2), 8(1), 16(2), 17(1), 19(3), 29(1), 60(1).


CLASSIFICATION OF THE SAUROPTERYGIA

The increased data set for the analysis of sauropterygian interrelationships resultsin a new inclusive hierarchy of ingroup taxa with some important paleobiologicalconsequences. The formal classification retrieved form the hierarchy of sauro-pterygian taxa reads as follows:

1. Sauropterygia Owen, 1860

Definition. A monophyletic taxon including the Placodontia and the Eosauropterygia.

Diagnosis. Premaxilla large, forming most of snout in front of external nares;lacrimal absent; upper temporal fossa distinctly larger than orbit (reversed inpachypleurosaurs); lower temporal fossa open ventrally; squamosal descends towardsventral margin of skull; paroccipital process trending posterolaterally; dorsal wingof epipterygoid narrow; palate akinetic; suborbital fenestra absent; mandibularsymphysis enforced and ‘scoop’-like (51[2], ACCTRAN; DELTRAN characteroptimization renders this a synapomorphy of Nothosaurinae, convergent in Placodusand Cymatosaurus); anterior premaxillary and dentary teeth procumbent; teeth onpterygoid flange absent; cervical intercentra absent; cervical ribs with free anteriorprocess; three or more sacral ribs; clavicles positioned anteroventrally to interclavicle;clavicles applied to medial surface of scapula; posterior stem of interclavicle short;pectoral fenestration present; humerus curved; insertional crest for latissimus dorsimuscle reduced; humerus with reduced epicondyles; radius and ulna of equal length;fifth metatarsal long and slender; straight fifth metatarsal.

1.1. Eosauropterygia Rieppel, 1994a

Definition. A monophyletic taxon including the Pachypleurosauria, Nothosauria,Corosaurus and the Pistosauria.

Diagnosis. Basioccipital tubera in complex relation to pterygoid as they extendlaterally (42[2]; ACCTRAN); internal carotid enters quadrate ramus of pterygoid;zygosphene-zygantrum articulation present (64[1]); pedicels of neural arch receivedon ‘butterfly’-shaped platform on centrum; clavicles with anterolaterally expandedcorners; scapula constricted to separate a ventral glenoid portion from posterodorsalblade or process; entepicondylar foramen present; three tarsal ossifications.

1.1.1. NothosauroideaDefinition. A monophyletic taxon including the Pachypleurosauria and the No-thosauria.

Diagnosis. Quadratojugal without anterior process; occiput plate-like, with stronglyreduced posttemporal fossae (unknown in Germanosaurus); supraoccipital horizontallyexposed (35[1], ACCTRAN; unknown in Germanosaurus); pterygoid-ectopterygoidflanges strongly reduced (unknown in Germanosaurus); clavicles broad medially (un-known in Germanosaurus); clavicles meet in anteromedial suture (unknown in Ger-manosaurus); coracoid strongly waisted (unknown in Germanosaurus); internal trochanterreduced (unknown in Germanosaurus); intertrochanteric fossa rudimentary or absent(unknown in Germanosaurus).

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1.1.1.1. Nothosauria

Definition. A monophyletic taxon including Simosaurus and the Nothosauridae.

Diagnosis. Nasals separated by premaxilla–frontal contact; postorbital skull distinctlylonger than preorbital skull; parietal fully fused in adult; pineal foramen displacedposteriorly; parietal skull table weakly constricted; jugal enters upper temporalarch; quadrate with straight posterior margin; dorsal blade of epipterygoid broad;surangular with distinctly projecting lateral flange for the insertion of superficial jawadductor muscle fibres; maxillary tooth-row extending backwards to level beyondthe anterior corner of the upper temporal fossa; vertebrae platycoelous; ectepicondylargroove open without anterior notch; posterior dorsal process does not project beyondposterior margin of acetabular portion of ilium; pubis with concave ventral (medial)margin; astragalus with proximal concavity; medial gastral rib element may havetwo-pronged lateral process (119[1], ACCTRAN).

1.1.1.2. NothosauridaeDefinition. A monophyletic taxon including Germanosaurus and the Nothosaurinae.

Diagnosis. snout constricted; temporal region of skull strongly depressed; reduceddorsal exposure of prefrontal; jugal restricted to a position behind the orbit withoutentering the latter’s posterior margin; premaxillary and anterior dentary fangspresent.

1.1.1.3. NothosaurinaeDefinition. A monophyletic taxon including Nothosaurus and Lariosaurus.

Diagnosis. Frontals fused in adult; parietal skull table strongly constricted (at leastposteriorly); occipital crest present; maxillary canines present; dorsal centra notconstricted in ventral view; sacral ribs without distal expansion.

1.1.2. PistosauroideaDefinition. A monophyletic taxon including Corosaurus and the Pistosauria.

Diagnosis. Frontal closely approaches upper temporal fossa (16[1], ACCTRAN); jugalrestricted to position behind orbit, but enters the latter’s posterior margin (23[1],ACCTRAN); quadratojugal absent (29[1], ACCTRAN); occipital exposure of squa-mosal with distinct notch receiving the distal end of the paroccipital process;mandibular articulation displaced to a level distinctly behind the occipital condyle;occipital crest present; quadrate with straight posterior margin; premaxillary andanterior dentary fangs present; maxillary canines present; maxillary tooth rowextends posteriorly to a level below the posterior margin of orbit or anterior marginof upper temporal fossa; posterodorsal wing of scapula ventrally expanded at itsposterior end; obturator foramen open in adult.

1.1.2.1. PistosauriaDefinition. A monophyletic taxon including Cymatosaurus, Pistosaurus, and plesiosaursand pliosaurs.


Diagnosis. Nasals strongly reduced (or absent); nasals may fail to enter external naris(7[1]. ACCTRAN; reversal in some Cymatosaurus), nasals separated by premaxilla-prefrontal contact; frontal enters anteromedial margin of upper temporal fossa(reversal in some Cymatosaurus); parietal incompletely fused in adult (completely fusedin some Cymatosaurus); vertebrae platycoelous.

DISCUSSION AND CONCLUSIONS

The revision of the phylogenetic interrelationships of the Sauropterygia as detailedabove partially falsifies the elegant evolutionary scenario of their adaptation to asecondary aquatic existence first proposed by Sues (1987; see also Rieppel, 1989,1994a; Tschanz, 1989). The concept of the monophyletic Eusauropterygia withpachypleurosaurs as their sister-group, with Corosaurus and Simosaurus as their mostbasal clades, and with plesiosaurs and pliosaurs as their crown group clades, permittedthe hypothesis of a progressive morphological adaptation of Sauropterygia as theirsuccessively more derived clades moved away from lagoonal or shallow waternearshore habitats towards the open sea. The new phylogeny, indicating paraphylyof the Eusauropterygia, a basal dichotomy within the Eosauropterygia, and showingthe Pachypleurosauria as sister-group to the Nothosauria, may turn a number ofcharacters formerly believed to be plesiomorphic in pachypleurosaurs and in Si-mosaurus into reversals. These characters involve snout constriction, the elongationof the postorbital skull and the enlargement of the upper temporal fossa correlatedwith the development of a dual jaw adductor system (Rieppel, 1989, 1994a), closureof the occiput, the loss of the impedance-matching middle ear (as inferred from theloss of the posterior concavity of the quadrate: Rieppel, 1989), the enforcement ofthe mandibular symphysis, and the development of a piscivorous dentition.

The study of character evolution will, of course, change with the application ofdifferent character optimization techniques. Implementing both ACCTRAN andDELTRAN character optimization results in the same interpretation of the followingfunctionally important characters: rostral constriction (3[1]) is a synapomorphy ofthe Nothosauridae, convergent in Placodus and Cymatosaurus; elongation of thepostorbital region of the skull (12[2]) is a synapomorphy of the Nothosauria,convergent in Cymatosaurus; enlargement of the upper temporal fossa (13[2]) is asynapomorphy of the Sauropterygia, reversed in Corosaurus—the small upper temporalfossa (13[3]) is an autapomorphy of pachypleurosaurs; closure of the occiput andreduction of the posttemporal fossae (31[2]) is a synapomorphy of Nothosauroidea;reduction of the ectopterygoid-pterygoid flange (44[1]) likewise is a synapomorphyof Nothosauroidea; the presence of anterior dentary and premaxillary fangs (55[1])is a synapomorphy of the Nothosauridae convergent in the Pistosauroidea ; thepresence of maxillary fangs (56[0]) is a synapomorphy of the Nothosaurinae con-vergent in the Pistosauroidea. The only reversal occurring in pachypleurosaurs usingboth ACCTRAN and DELTRAN character optimization concerns the stronglyprocumbent anterior dentary and premaxillary teeth (54[1]) which are synapo-morphic at the level of the Sauropterygia. Using DELTRAN character optimization,the strongly enforced and ‘scoop’-shaped mandibular symphysis (51[2]) is sy-napomorphic in the Nothosaurinae (unknown in Germanosaurus), convergent inCymatosaurus (or Pistosauria respectively). Using ACCTRAN optimization, the same

O. RIEPPEL36

character (51[2]) is synapomorphic at the level of Sauropterygia, reversed at thelevel of the Nothosauroidea, and re-developed again in the Nothosauridae (alsoreversed in Corosaurus). Again using DELTRAN character optimization, the straightposterior border of the quadrate (37[0], assumed to indicate the loss of the impedancematching middle ear) is synapomorphic at the level of the Nothosauria, andconvergent in the Pistosauroidea. Using ACCTRAN character optimization, thesame character (37[0]) is synapomorphic at the level of the Eosauropterygia, andreversed in pachypleurosaurs. In summary, DELTRAN character optimizationcontinues to allow the earlier conclusion (Sues, 1987; see also Rieppel, 1989, 1994a;Tschanz, 1989) that pachypleurosaurs and Simosaurus retain a plesiomorphic anatomyrelative to other Eosauropterygia. It does so, however, at the cost of extensiveconvergence in the Nothosauria (and subclades) on the one hand, and in thePistosauroidea (and subclades) on the other.

At a more general level, the invasion of the Mesozoic seas by the Sauropterygiabecomes a more complex phenomenon than had previously been assumed. Onemajor clade of the Eosauropterygia, the Nothosauroidea (including pachypleurosaursand Nothosauria) seems to have remained restricted to shallow warm-water epi-continental seas or near shore habitats, whereas the other major clade, the Pisto-sauroidea (including Corosaurus and Pistosauria) appears to have rapidly invaded theopen sea (plesiosaurs and pliosaurs).

Placodonts are restricted to the western Tethyan province (Europe andMediterranean), where they first occur at the transition from the Upper Buntsandstein(Rot, Scythian) to the Muschelkalk (Anisian) (Pinna, 1990; Rieppel, 1995). Thegroup diversified throughout the Anisian and Ladinian in the epicontinental Ger-manic Basin, and in peritethyan intraplatform basins (see Pinna, 1990, and Rieppel,1995, for a review and further references), and as a whole seems to have shared thesame paleoecological constraints as are evident in the pachypleurosaur–nothosaurianclade within the Eosauropterygia.

Pachypleurosaurs, simosaurids and nothosaurs occur both in the western Pacific(China: Young, 1958, 1959, 1960, 1965, 1978) and western Tethyan (Europe andMediterranean: Brotzen, 1957; Gorce, 1960; Haas, 1959, 1963, 1969, 1975, 1980,1981; Halstead & Stewart, 1970; Lehman, 1965; Rieppel & Lin, 1995; Rieppel &Wild, 1996) provinces. The earliest occurrence of the Nothosauroidea in China isthe lower Middle Triassic (Young, 1965). In the western Tethyan province the cladeappears first at the transition from the Upper Buntsandstein (Rot, Scythian) to theMuschelkalk (Anisian) in the Germanic Triassic (Rieppel & Lin, 1995; Rieppel &Wild, 1996), as well as in the basal Muschelkalk layers of Makhtesh Ramon in theNegev (Brotzen, 1957; Parnes, 1975), and of Djebel Rehach in southern Tunisia(Gorce, 1960). The clade diversified throughout Anisian and Ladinian times withinthe epicontinental Germanic Basin, the Alpine intraplatform basin facies (which itinvaded at the Anisian-Ladinian boundary: Rieppel & Hagdorn, 1997), and theshallow carbonate platform extending along the northern Gondwanan shelf. Thefirst appearance of the clade in the western Tethyan faunal province coincides withthe onset of cyclic marine transgressions lasting from the early Anisian through theearly Carnian. All occurrences of the Nothosauroidea are in shallow epicontinentalwarm-water sea deposits, or in the near shore lagoonal setting of a carbonateplatform. Even within the Germanic Muschelkalk sea, the Nothosauroidea are foundpredominantly in nearshore areas (Hagdorn, 1993), indicating a distinct faciesinterdependence which is in accordance with their overall patchy geographical


distribution. The high degree of taxonomic, and morphological, diversification wouldappear to have reduced ecological competition between the representatives of thisclade within each basin. At the same time, the clade preserves some of the progressivemorphological trends identified to characterize sauropterygian evolution (Sues, 1987),such as: loss of the tympanic membrane (Nothosauria as opposed to pachy-pleurosaurs), development of an elongated snout and constricted rostrum furnishedwith procumbent premaxillary fangs (Nothosauridae as opposed to Simosaurus),development of maxillary fangs (Nothosaurinae as opposed to Germanosaurus), and aprogressive depression of the skull correlated with a progressive elongation of thepostorbital skull region (upper temporal fenestra) to accommodate an increasinglydifferentiated dual jaw adductor muscle complex (Rieppel, 1994a). The clade goesextinct during the Upper Triassic. With its diversity peaking during Ladinian times,the evolution of the Nothosauroidea appears to be directly correlated with “theprogressive spread of sea from the early Triassic to a Ladinian–early Carnianmaximum, followed by a retreat . . . and corresponding shallowing of the sea in theTethyan zone” which has been attributed to a “fundamental eustatic effect” (Hallam,1981: 33; see also Haq, Hardenbol & Vail, 1987).

Until recently, Corosaurus was a singular sauropterygian occurrence in the up-permost Lower Triassic (early Middle Triassic?) Alcova Limestone of the westernUnited States. The genus documents an early widespread geographic distributionof the Pistosauroidea, further supported by the recent finding of a new pistosaur inthe Western United States, i.e. Augustasaurus from the Upper Anisian of northwesternNevada (Sander, Rieppel & Bucher, 1997). Corosaurus in particular shares withthe Chinese representatives of the pistosaur clade (Chinchenia sungi Young, 1965;Sanchiaosaurus dengi Young, 1965; Kwangsisaurus orientalis Young, 1959) a derivedcharacter which is absent in the western Tethyan representatives of that clade, i.e.a very distinct groove on the posterior surface of the proximal region of the dorsalribs. Although the analysis of phylogenetic interrelationships among all knownTriassic Pistosauroidea lacks detail, and is seriously hampered by the incompletenessof the Chinese material, this character provides at least some indication that thephylogenetic relationship of Corosaurus from the eastern Pacific province might bewith pistosaurs from the western Pacific province, rather than with the westernTethyan taxa. This makes sense in view of the fact that there were no direct marineconnections or seaways between the western Tethyan province and the easternPacific province during the Triassic (Ricou, 1996; Dore, 1991; Ziegler, 1988). Trans-Pacific relationships of east Asiatic and western North American floral and faunalelements have been recognized for several groups of organisms from late Paleozoicthrough Tertiary times (MacGinitie, 1969; Ross and Ross, 1981, 1985; Grande,1994), and pistosaurs might provide just another example.

Among western Tethyan stem-group pistosauroids, Cymatosaurus is only knownfrom the Lower Muschelkalk of the Germanic Triassic (Lower Anisian), andfrom a singular occurrence in the Alpine Triassic (Rieppel & Hagdorn, 1997).Analysis of detailed geographic and stratigraphic distribution shows an almostcomplete mutual exclusion of the morphologically very similar genera Cymatosaurusand Nothosaurus in the Lower Muschelkalk (Rieppel & Werneburg, 1997). Pistosaurusappeared in the Germanic Basin with the Upper Muschelkalk transgression (firstappearance: atavus biozone, Upper Illyrian), and persisted until the beginning ofthe regression (last appearance: postspinosus biozone). Bones of Pistosaurus are foundmost frequently—but still very rarely—in the Tonplatten facies of the spinosus to

O. RIEPPEL38

postspinosus biozones which were deposited during the maximum flooding periodin earliest Ladinian times (H. Hagdorn, pers. comm.). By the Jurassic, pelagicplesiosaurs and their fossil relatives had become more frequent and diverse, andachieved a global distribution. The clade persisted to the Upper Cretaceous. Insummary, the evolutionary history of the two clades of Eosauropterygia reflectsthe two major extinction events affecting marine reptiles that were identified byBardet (1994).

ACKNOWLEDGEMENTS

I thank Brent H. Breithaupt and Jean-Pierre Cavigelli from the University ofWyoming Geological Museum for the permission to further prepare the holotypeof Corosaurus. Robert Masek, Paul Brinkman, Jessica Scott, and my family, weregood company in the field. Laurie Bryant from the Casper District Office of theBureau of Land management was very helpful and receptive to our needs. Johnand Jo-Ann Milnes generously granted access to their ranch land on the south slopeof Muddy Mountain. The photographic work for this paper was done by John S.Weinstein, the careful preparation of Corosaurus was performed by Kathryn Passaglia,both on staff at the Field Museum. I am particularly greatful to John Merck andHans Hagdorn for the many opportunities they provided to discuss the materialpresented in this paper, and to Hans-Dieter Sues and Glenn Storrs, who read thepaper, offering much helpful advice and criticism. This study was supported byNSF-grants DEB-9220540 and DEB-9419675.

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Documents

Rieppel, 1998