Enantiornithine (Aves) Tarsometatarsi and the Avian Affinities of the Late Cretaceous Avisauridae

Enantiornithine (Aves) Tarsometatarsi and the Avian Affinities of the Late CretaceousAvisauridaeAuthor(s): Luis M. ChiappeSource: Journal of Vertebrate Paleontology, Vol. 12, No. 3 (Sep. 3, 1992), pp. 344-350Published by: Taylor & Francis, Ltd. on behalf of The Society of Vertebrate PaleontologyStable URL: http://www.jstor.org/stable/4523457 .

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Journal of Vertebrate Paleontology 12(3):344-350, September 1992 @ 1992 by the Society of Vertebrate Paleontology

ENANTIORNITHINE (AVES) TARSOMETATARSI AND THE AVIAN AFFINITIES OF THE LATE CRETACEOUS AVISAURIDAE

LUIS M. CHIAPPE Museo Argentino de Ciencias Naturales, CONICET, Av. Angel Gallardo 470,

1405 Buenos Aires, Argentina

ABSTRACT--Several characters of the tarsometatarsus of the Cretaceous enantiornithine birds are

discussed, with emphasis on the genus Avisaurus, which was considered a non-avian theropod taxon by its authors. Two synapomorphies (metatarsal IV reduced with respect to metatarsals II and III; well-developed knob on the anterior face of metatarsal II) relate Avisaurus to the remaining enantiornithine tarsometatarsi types and support reference of this taxon within the Enantiornithes. Three other synapomorphies (laterally compressed, J-shaped metatarsal I; anterior surface of the mid-shaft of metatarsal III strongly convex transversely; strong posterior projection of the internal rim of trochlea on metatarsal III) shared by Avisaurus and a Late Cretaceous enantiornithine from northwestern Patagonia further support its avian affinities. Avisaurus and the family Avisauridae are assigned to the avian subclass Enantiornithes.

INTRODUCTION

Enantiornithines are peculiar Cretaceous flying birds, recognized by Walker (1981) on the basis of an abun- dant assortment of mostly unassociated bones from the Maastrichtian Lecho Formation of El Brete, Prov- ince of Salta, northwestern Argentina (Bonaparte et al., 1977). Previously recorded from the Upper Cretaceous of South America (Walker, 1981; Chiappe, 1991 a, b), North America (Martin, 1983), and Asia (Nessov and Borkin, 1983; Nessov, 1984; Nessov and Jarkov, 1989), and the Lower Cretaceous of Australia (Molnar, 1986), enantiornithines may represent the sister-taxon of ornithurine birds (the common ancestor of Hesperor- nithiformes and Neornithes plus all its descendants) (Walker, 1981; Chiappe, 1991b).

Among the enantiornithine material of El Brete, there are three different types of tarsometatarsi (Figs. 1A- C), representing three distinct unnamed taxa (Walker, 1981; Martin, 1983). In 1985, Brett-Surman and Paul considered one of these taxa (Fig. 1 C) congeneric with Avisaurus archibaldi Brett-Surman and Paul, 1985, which they based on an isolated tarsometatarsus (Fig. 1 D) from the Upper Cretaceous Hell Creek Formation of Montana. They also erected the family Avisauridae to include Avisaurus, although they considered this clade a non-avian taxon of theropods. Although most later authors rejected this phylogenetic interpretation (Bo- naparte, 1984, 1986a; Olson, 1985; Cracraft, 1986), it was never challenged on the basis of morphological characters.

In this paper, I will try to demonstrate the enantiornithine affinities of avisaurids by reassessing the characters used by Brett-Surman and Paul (1985) and using new morphological information available with

the discovery of an articulated enantiornithine skeleton (Chiappe, 199 lb; Fig. 2).

Abbreviations - MUCPv, Museo de Ciencias Na- turales, Universidad Nacional del Comahue, Neu- quen, Argentina; PVL, Instituto-Fundaci6n "Miguel Lillo," Tucumain, Argentina; UCMP, University of California Museum of Paleontology, Berkeley, Cali- fornia.

SYSTEMATIC PALEONTOLOGY

The present taxonomy of the subclass Enantior- nithes is unclear. Several names, applied to different taxonomic categories (e.g., order Enantiornithiformes, family Enantiornithidae), have been used, but they have never been properly diagnosed nor their composition defined (Olson, 1985). Walker (1981) erected the subclass Enantiornithes on the basis of a single genus and species, Enantiornis leali, which in fact was not de- scribed but just listed in the legend to a table. He pointed out the existence of three distinct evolutionary lines based on the different morphology of the tarsometatarsi (types A and B, and Avisaurus; Fig. 1). An ordinal status was suggested by Walker (1981) for each of these lines, although none of these were named. Subsequently, Martin (1983, 1987) used the name Enantiornithes as an infraclass, alleged to be the sister- taxon of Archaeopteryx, including the orders Enan- tiornithiformes, Alexornithiformes, and Gobipterygi- formes, for E. leali (and apparently all the other unnamed taxa of El Brete), Alexornis antecedens Brod- korb, 1976 and Gobipteryx minuta Elzanowski, 1976 respectively. Nessov and Borkin (1983) and Nessov (1984, 1986) used the terms Enantiornithidae and En- antiornithiformes while discussing the affinities of certain Late Cretaceous avian bones of Central Asia, al-

344



CHIAPPE- ENANTIORNITHINE BIRDS 345

though they did not mention which taxa were included in that order and family.

In the present paper, I will use the term Enantior- nithes as the name of a subclass of Aves, as diagnosed below. Enantiornithines is used in the same sense. Un- til a comprehensive study of this group of birds is completed, we will hardly be able to recognize the status of all the above mentioned taxa, and to order their diversity hierarchically.

Class AVES Linnaeus, 1758

Subclass ENANTIORNITHES Walker, 1981

Known Distribution - Toolebuc Formation (Albian) of Queensland, Australia (Nanantius eos Molnar, 1986), Turonian-Coniacian of Kizylkum Desert, Uzbekistan, USSR (Nessov and Borkin, 1983:fig. 4a, b; Kizylku- mavis cretacea Nessov, 1984; Sazavis prisca (Nessov in Nessov and Jarkov, 1989), Rio Colorado Formation (Coniacian-Santonian) of Patagonia, southern Argen- tina (new genus and species, Chiappe and Calvo, in preparation; Fig. 2), Bocana Roja Formation (Cam- panian) of Baja California, Mexico (Alexornis antecedens Brodkorb, 1976; see Martin, 1983), Lecho For- mation (Maastrichtian) of El Brete, northwestern Argentina (Avisaurus sp. Brett-Surman and Paul, 1985; Enantiornis leali Walker, 1981 and other unnamed avian taxa (Walker, 1981; Martin, 1983; Chiappe, 1991 a), Hell Creek Formation (Maastrichtian) of Mon- tana (Avisaurus archibaldi Brett-Surman and Paul, 1985).

Diagnosis -Ornithopectine birds (Chiappe, 1991 b) with sister-group relationships to Ornithurae. Humeral head strongly concave palmarily and convex anconal- ly. In palmar view, the superior margin of the humeral head is concave in its central portion, rising ventrally and dorsally. Bicipital crest prominent, palmo-ventrally projecting. Distal end of humerus very compressed; external condyle almost transversely orien- tated. External cotyle of ulna convex, being separated from the olecranon by a deep groove. Shaft of radius with a long longitudinal groove on its ventro-anconal face. Coracoid with convex lateral margin and broad and deep fossa on its dorsal surface. Coracoidal foramen opening into an elongate furrow medially. Process on coracoid articulating with a deep scapular fossa. Dorsal vertebrae with strong lateral grooves on centra. Proximal end of femur with very well developed posterior trochanter. Distally, the lateral border of the femur projects posteriorly. Tibiotarsus with a single, smooth cnemial crest on antero-medial margin. Inter- nal condyle of tibiotarsus wide and bulbous; intercon- dylar groove narrow. Metatarsal IV reduced with respect to metatarsals II and III. Anterior face of metatarsal II with distinct knob for muscle attachment.

Family AVISAURIDAE Brett-Surman and Paul, 1985

Type Genus--Avisaurus Brett-Surman and Paul, 1985.

A B C K

K-

Type-A Type-B Avisaurus sp,

D F

E G

Avisaurus archibaldi MUCPv-142

FIGURE 1. Left enantiornithine tarsometatarsi. A-C, El Brete tarsometatarsi in anterior view. A, Type A, PVL-4053 (reversed from right); B, type B, PVL-402 1; C, Avisaurus sp., PVL-4690; D, E, anterior view and mid-shaft section of Avisaurus archibaldi, based on the right tarsometatarsus UCMP 117600 (after Brett-Surman and Paul, 1985). F, G, Anterior view and mid-shaft section of the Patagonian specimen MUCPv-142, reconstructed on the basis of both tarsometatarsi. Note the prominent knob (k) on metatarsal II and the reduction of metatarsal IV. Figures not to scale.

Referred Genera--Avisaurus Brett-Surman and Paul, 1985; unnamed genus (Chiappe and Calvo, in prep.; Fig. 2).

Known Distribution - Hell Creek Formation (Maas- trichtian) of Montana (A. archibaldi Brett-Surman and Paul, 1985); Lecho Formation (Maastrichtian) of northwestern Argentina (Avisaurus sp. Brett-Surman and Paul, 1985); Rio Colorado Formation (Coniacian- Santonian) of northwestern Patagonia, Argentina (new genus and species Chiappe and Calvo, in prep.; Fig. 2).

Emended Diagnosis--Laterally compressed metatarsal I, with a J-like shape; distinct backward projection of the internal rim of trochlea of metatarsal III; strongly transversely convex anterior surface of metatarsal III (see Discussion for comparisons).

DISCUSSION

Brett-Surman and Paul (1985) recognized three morphological features supporting their hypothesis of non- avian theropod affinities of Avisaurus:

(Al) Metatarsals fused only proximally in adults. Although present in ceratosaurians (Gilmore, 1920; Gauthier, 1986; Rowe, 1989), Elmisaurus (Osmolska,



346 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 12, NO. 3, 1992

'IAN 1, C 0ll-i :::, ::I':,I:

FUR

ITLSL

RHUFFU

RCAI

LTIT

lcm

RTIM'Af

~~:::2

FIGURE 2. Specimen MUCPv-142, which represents a new genus and species of Avisauridae (Chiappe and Calvo, in preparation), from the Upper Cretaceous Rio Colorado Formation of northwestern Patagonia, Argentina (Chiappe, 1991 b). Abbreviations: FUR, furcula; LCO, left coracoid; LHU, left humerus; LSC, left scapula; LTI, left tibiotarsus; LTM, left tarsometatarsus; PEL?, fragment of pelvis?; RCM, right carpometacarpus; RCO, right coracoid; RFE, right femur; RHU, right humerus; RTI, right tibiotarsus; RTM, right tarsometatarsus.

1981), and Avimimus (Kurzanov, 1987), the metatarsals are fused proximally and unfused distally in Ar- chaeopteryx (Ostrom, 1976; Martin, 1983; Wellnhofer, 1988), enantiornithines (Martin, 1983; Chiappe, 1991 b; Fig. 1), and other Cretaceous birds (Rao and Sereno, 1990). The distribution of this feature is clearly not restricted to non-avian theropod clades.

(A2) Proximally robust metatarsal III. Brett-Surman and Paul (1985:135) considered this character distinct from the condition shared by several non-avian theropod clades (e.g., Tyrannosauridae, Ornithomimidae, Troodontidae) and, to a lesser extent, most omithurine birds, where the proximal end of metatarsal III is reduced with respect to the proximal ends of metatarsals II and IV. A "robust" (or unreduced) proximal end of metatarsal III is primitive for theropods (Gauthier, 1986). Varying degrees of "robustness" occurred in ceratosaurians (Welles, 1984; Rowe, 1989), Piatnitz-

kysaurus (Bonaparte, 1986b), Allosaurus (Madsen, 1976), Compsognathus (Ostrom, 1978), and Deinony- chus (Ostrom, 1969). This condition is still retained by basal birds such as Archaeopteryx (Ostrom, 1976; Wellnhofer, 1988), the Early Cretaceous bird from Las Hoyas, Spain (Sanz et al., 1988; Sanz and Bonaparte, in press), and other non-ornithurine groups such as enantiornithines (Chiappe, 1991 b) and certain new Late Cretaceous flightless birds from Argentina (Chiappe, 1990; Bonaparte, 1991; Alvarenga & Bonaparte, in press). Thus, there is no reason to consider a proximally "robust" metatarsal III as a non-avian theropod trait. The proximally wedged metatarsal III of several non-avian theropods is almost certainly convergent with the slight reduction present in ornithurine birds (Gauthier, 1986).

(A3) Hypotarsus slightly developed and medially displaced. The hypotarsus is defined as a process on



CHIAPPE-ENANTIORNITHINE BIRDS 347

4 4'

II IIIr Iv

B a a

hy

FIGURE 3. Proximal view of right metatarsals II-IV (A) and distal view of the trochlea of right metatarsal III (B) of Deinonychus antirrhopus (after Ostrom, 1969), Avisaurus archibaldi (after Brett-Surman and Paul, 1985), Hesperornis regalis and Ichthyornis victor (after Marsh, 1880), and the red-fronted coot, Fulica rufifrons. Hy, Hypotarsus. Figures not to scale.

the plantar aspect of the tarsometatarsus formed mostly by distal tarsal elements capping the proximal end of metatarsal III (Baumel, 1979:106). This feature is absent in non-avian theropods (Gauthier, 1986), Ar- chaeopteryx (Ostrom, 1976), and, in my opinion (see also Walker, 1981), in Avisaurus (Fig. 3A). The hypotarsus probably appears more than once during the evolutionary history of birds. More derived enantiornithines, as exemplified by tarsometatarsus type B from El Brete, probably developed this feature indepen- dently of all living birds.

Thus, the characters used by Brett-Surman and Paul (1985) to relate avisaurids with non-avian theropods were either misinterpreted or are common to both a variety of taxa of birds as well as non-avian theropods. None of them are adequate characters to establish avisaurid relationships.

On the other hand, the recent discovery of an articulated skeleton of a Late Cretaceous enantiornithine in Patagonia (Fig. 2) sheds new light on the avian nature ofAvisaurus (Chiappe, 1991 b). This new specimen (MUCPv-142) is closely related to Avisaurus and af- fords the first true association of the primitive structure of the enantiornithine foot with undoubtedly avian features such as a strut-like coracoid, U-shaped furcula with hypocleideum, and a large carinate sternum (Fig. 2). It also confirms Walker's tentative association of the forelimb and hind-limb elements from El Brete (Walker, 1981).

Characters supporting the enantiornithine nature of Avisaurus are:

(B 1) Metatarsal IV reduced with respect to metatarsals II and III. In Avisaurus and MUCPv-142, metatarsal IV is very thin anteroposteriorly and much less prominent than metatarsals II and III (Fig. 1 E, G). As was pointed out by Walker (1981), metatarsal IV is much more reduced in type A and type B tarsometatarsi from El Brete (Fig. 1). An unreduced metatarsal IV is present in most non-avian theropod taxa (Lambe, 1917; Gilmore, 1920; Ostrom, 1969; Osmolska et al., 1972; Madsen, 1976; Osmolska, 1981, 1987; Welles, 1984) as well as in Archaeopteryx (Ostrom, 1976; Wellnhofer, 1988), the Las Hoyas bird (Sanz and Bona-

parte, in press), hesperornithiforms and Ichthyornis (Marsh, 1880), and neornithines. It is clear that an unreduced metatarsal IV represents the plesiomorphic condition for birds, and the reduction of this metatarsal is exclusive to enantiornithines. The reduction of metatarsal IV is certainly an enantiornithine synapomorphy.

(B2) Well-developed knob on the anterior face of metatarsal II. Avisaurus exhibits a fairly prominent knob [perhaps the area of insertion of M. tibialis an- ticus, the main flexor of the ankle (Brett-Surman and Paul, 1985)] on the anterior surface of the proximal half of metatarsal II (Fig. 1C, D). Although the condition of this feature remains uncertain in the articulated enantiornithine specimen, due to the poor preservation of the proximal half of metatarsal II (Fig. IF), it is shared by the tarsometatarsi types A and B from El Brete (Fig. 1A, B). A prominent knob on the anterior face of metatarsal II is apparently absent-or slightly developed as in Syntarsus (Rowe, 1989)- in non-avian theropods, Archaeopteryx (Ostrom, 1976), the Las Hoyas bird (Sanz and Bonaparte, in press), and ornithurine birds. This feature is very probably another enantiornithine synapomorphy. The presence of a poorly developed knob on the anterior face of metatarsal II in the ceratosaurian Syntarsus (Rowe, 1989) is here considered convergent, and not as an indication of close relationships with Avisaurus as was claimed by Brett-Surman and Paul (1985).

(B3) Laterally compressed, J-shaped metatarsal I. Metatarsal I of the right tarsometatarsus of specimen PVL-4048 (Avisaurus sp.) is very compressed laterally, and at its distal end, sharply curves backwards (Fig. 4). The J-like shape of metatarsal I of Avisaurus is remarkably similar to that of MUCPv-142 (Fig. 4). The unique structure of metatarsal I of Avisaurus and MUCPv- 142 is absent in non-avian theropods and the remaining bird clades. In the tarsometatarsi types A and B from El Brete (Fig. 1 A, B), metatarsal I is not preserved. Consequently, until new information is available, the peculiar structure of metatarsal I of Avi- saurus and MUCPv-142 is considered a synapomorphy of the Avisauridae.




A B

IV

III III

Avisaurus sp. MUCPv-142

FIGURE 4. Avisaurid metatarsals I-IV. A, Left metatarsal I of Avisaurus sp. (PVL-4048) in medial (left) and anterior view of metatarsals I-IV (right). B, Right metatarsal I of specimen MUCPv-142 in medial view (right) and anterior view of metatarsals I-IV (left). Scale bars equal 2 mm.

(B4) Anterior surface of the mid-shaft of metatarsal III strongly convex transversely. Avisaurus shares with MUCPv- 142 a remarkable transverse convexity of the anterior face of metatarsal III, especially pronounced along the mid-shaft, which makes it stand out with respect to metatarsals II and IV (Fig. 1 E, G). Otherwise, this condition is absent in the tarsometatarsi types A and B from El Brete. This distinctive convexity of the anterior mid-shaft is also absent in most non-avian theropods, in which the mid-shaft is instead slightly convex or nearly flat, as in ceratosaurians (Welles, 1984; Rowe, 1989), carnosaurs (Lambe, 1917; Madsen, 1976), ornithomimids (Osmolska et al., 1972; Smith and Gal- ton, 1990), and dromaeosaurids (Ostrom, 1969). Like- wise, in Archaeopteryx and the Las Hoyas bird, the anterior surface of metatarsal III is apparently not as convex as in Avisaurus, although the two-dimensional preservation of specimens of these taxa makes the comparisons more difficult. In most ornithurines, the anterior surface of metatarsal III forms a shallow de- pression for the digital extensor musculature. The prominent transverse convexity of the anterior face of metatarsal III very probably represents another synapomorphy indicating the monophyly of the clade formed by Avisaurus and MUCPv- 142.

(B5) Strong posterior projection of the internal rim of trochlea metatarsal III. Another character shared by Avisaurus and MUCPv-142 is the strong backward projection of the internal rim of the central trochlea exhibited by the tarsometatarsi of these two taxa (Fig. 3B). Both trochlear rims are subequal in the tarsometatarsus type A from El Brete, and no distal end is known for the type B (Fig. I1B). Among non-avian theropods this projection is absent or only slightly developed (Lambe, 1917; Ostrom, 1969; Madsen, 1976; Welles, 1984; Currie and Russell, 1988; Smith and Galton, 1990). Otherwise, in many ornithurines, it is

the external rim that projects backwards, although this is not very pronounced and in many other forms the rims are subequal (Fig. 3B).

CONCLUSIONS

Reappraisal of the characters used by Brett-Surman and Paul (1985) and newly available specimens indi- cate that Avisaurus was a bird, not a non-avian theropod. Avisaurus possesses two synapomorphies that clearly place it in the Enantiornithes: (B 1) reduction of metatarsal IV, and (B2) presence of a well-developed knob on the anterior face of metatarsal II. Further- more, Avisaurus shares with the Patagonian enantiornithine specimen (MUCPv-142) three other synapomorphies: (B3) laterally compressed, J-shaped metatarsal I, (B4) strongly transversally convex anterior surface of metatarsal III, and (B5) unique backward projection of the internal trochlear rim of metatarsal III. The last three characters diagnose the enantiornithine family Avisauridae, which include Avisaurus and a second genus represented by MUCPv- 142.

Apparently no clear synapomorphies diagnosing Aves are present in the tarsometatarsus (Aves is here considered to include the common ancestor of Ar- chaeopteryx and modern birds plus all its descendants). The absence of complete fusion of metatarsals in adults, the unwedged proximal end of metatarsal III, the absence of a distal foramen between metatarsals III and IV, and the absence of a true hypotarsus, are primitive features retained by Archaeopteryx, enantiornithines, and other basal birds. The modern aspects of the hind- limb were not developed until the rise of the Omithur- ae (Chiappe, 1991b). The retention of ancestral traits reflects the mosaic pattern of evolution of early birds, where characters related with the acquisition of sus- tained flight preceded the remodelling of typical bird pelves and hind-limb (Sanz et al., 1988; Chiappe, 1991b; Sanz and Bonaparte, in press). As a conse- quence of this evolutionary pattern, interpretations of small isolated "non-avian theropod-like" tarsometatarsi, without diagnostic features of a particular lineage, as non-avian theropod remains, are premature.

In the case of the isolated tarsometatarsus of Avi- saurus archibaldi, recognition of synapomorphic traits shared by this species, the El Brete tarsometatarsi, and particularly the articulated enantiornithine specimen MUCPv- 142, lead to the conclusion that Avisaurus and the remaining El Brete specimens represent enantiornithine birds.

ACKNOWLEDGMENTS

I am especially grateful to Jose F. Bonaparte (Museo Argentino de Ciencias Naturales, Buenos Aires), Mi- chael Brett-Surman (Smithsonian Institution, Wash- ington), Oscar de Ferraris (Universidad Nacional del Comahue, Neuquen), Larry D. Martin (University of Kansas, Lawrence), Mark Norell (American Museum of Natural History, New York), Storrs L. Olson (Smith-



CHIAPPE -ENANTIORNITHINE BIRDS 349

sonian Institution, Washington), John H. Ostrom (Yale Peabody Museum, New Haven), and Jaime E. Powell (Fundaci6n-Instituto Miguel Lillo, Tucumain) for let- ting me study material under their care. I am also grateful to Jose F. Bonaparte, Thomas Bown (U.S. Geological Survey), Michael Brett-Surman, and Fer- nando E. Novas (Museo Argentino de Ciencias Na- turales, Buenos Aires) for useful comments on the manuscript and to Larry D. Martin, Storrs L. Olson, and John H. Ostrom for their critical reviews. The Consejo Nacional de Investigaciones Cientificas y Tec- nicas, the Museo Argentino de Ciencias Naturales, and the American Museum of Natural History gave partial support to this research.

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Received 24 April 1991; accepted 1 October 1991.

