Cretaceous Mammals of Southern Utah. II. Marsupials and Marsupial-Like Mammals from the Wahweap Formation (Early Campanian)

Cretaceous Mammals of Southern Utah. II. Marsupials and Marsupial-Like Mammals from theWahweap Formation (Early Campanian)Author(s): Richard L. CifelliSource: Journal of Vertebrate Paleontology, Vol. 10, No. 3 (Sep. 20, 1990), pp. 320-331Published by: Taylor & Francis, Ltd. on behalf of The Society of Vertebrate PaleontologyStable URL: http://www.jstor.org/stable/4523328 .

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CIFELLI-CRETACEOUS MARSUPIALS AND MARSUPIAL-LIKE MAMMALS 321

Canaan Peak Fm.

Salt Lake City .

Kaiparowits Formation

T E

Green River .

1

Wahweap * V8, V2 Kanab Formation * V49

vil Straight Cliffs Fm.

FIGURE 1. Map of Utah, showing location of Kaiparowits Plateau and relative stratigraphic position of localities men- tioned in the text.

list of mammalian taxa recovered from the Wahweap Formation was given by Eaton and Cifelli (1988).

Measurements and Abbreviations

Measurements, as defined and abbreviated by Lil- legraven and Bieber (1986) and Lillegraven and McKenna (1986), are in millimeters and are as follows:

AP--anteroposterior length, ANW - anterior width (greatest width of premolars; trigonid width of lower molars), POW -posterior width (talonid width of lower molars).

Institutional abbreviations are as follows: MNA, Museum of Northern Arizona, Flagstaff; OMNH, Oklahoma Museum of Natural History, University of Oklahoma, Norman; UALVP, University of Alberta Laboratory for Vertebrate Palaeontology, Edmonton; UCMP, University of California Museum of Paleon- tology, Berkeley.

SYSTEMATIC PALEONTOLOGY

Order MARSUPIALIA Illiger, 1811 Family PERADECTIDAE (Crochet, 1980)

Reig et al., 1987 Genus PROTALPHADON Cifelli, 1990a

PROTALPHADON CREBREFORME, sp. nov. (Fig. 2A)

Type Specimen-OMNH 20667, labial half of left M2.

Hypodigm - The type, and MNA V456 1, labial half of left M2 or 3, lacking the lingual part of the stylar shelf.

Horizons and Localities--The type was collected at OMNH locality V8, middle Wahweap Formation; MNA V4561 is from locality V11, lower Wahweap Formation; Garfield County, Utah (Fig. 1).

Diagnosis -Largest species of the genus, slightly ex- ceeding P. creber in size. Molar cusps somewhat inflated relative to the condition seen in P. lulli, P. creber, and P. wahweapensis, with paracone and metacone slightly shorter and not as tightly appressed as in those species.

Etymology--Named in reference to its similarity to

Protalphadon creber from the upper Milk River For- mation.

Discussion and Description--The genus Protalpha- don was established by Cifelli (1990a) to contain two species previously referred to Alphadon and a then- new species from the Kaiparowits Formation. All of these species appear to be advanced relative to Iqual- adelphis lactea, a primitive species from the upper Milk River Formation (Fox, 1987a), in such features as the more broadly developed protocone of the upper molars and the reduced trigonid height on lower molars. Nonetheless, they lack the consistently present and well- developed stylar cusp C seen in species of Alphadon, Turgidodon, and Albertatherium. Although currently represented only by two fragmentary specimens, Pro- talphadon crebreforme is clearly distinct from other species of the genus, as set forth in the diagnosis. Lower molars of this species may be present in the existing collection, but are not formally recognized as such.

No conventional measurements are available for either specimen of Protalphadon crebreforme owing to breakage. Some appreciation of the size of the species may be gained by comparison of the distance from paracone to metacone apices. OMNH 20667 measures 1.10; comparable measurements for M2 of other species are: P. wahweapensis (OMNH 20109, 0.75; 20587, 0.86); P. lulli (UCMP 47047 [type specimen], 0.82, measured from Clemens, 1966:fig. 7); and P. creber (UALVP 5541, 0.98, measured from Fox, 1971 :pl. IC).

The paracone is very slightly larger than the metacone; the former cusp is subjected to more apical wear, however, so that the cusps are approximately equal in height when the tooth is even lightly worn. The cusps are somewhat inflated, lacking the gracile appearance seen in Protalphadon wahweapensis, P. creber, and P. lulli, and are more widely divergent than in those species. The centrocrista is linear; the labial faces of paracone and metacone are gently convex. Cusp B, known only in OMNH 20667, is nearly as tall as the paracone and has a similarly inflated appearance. The preparacrista joining these cusps is sharp and bears a distinct V-shaped notch, whose trough is at the level of the stylar shelf. In the type, no cusp is present in the C position. Cusp D in that specimen, although much smaller than the stylocone, is strong, anteroposteriorly developed, and placed immediately posterior to the ectoflexus. Crests descend from the apex of cusp D anteriorly to the ectoflexus and posteriorly





CIFELLI- CRETACEOUS MARSUPIALS AND MARSUPIAL-LIKE MAMMALS 323

Diagnosis--Species slightly smaller than Turgido- don russelli. Molar cusps low and greatly inflated. Up- per molars differing from those of Turgidodon species in having a more broadly developed, centrally placed protocone; MI?, at least, unlike that of undoubted Cre- taceous marsupials in approximating equilateral triangle in occlusal outline. Upper molar conules poorly developed; metaconule weak or lacking, unlike the condition seen in comparable species of Turgidodon and Alphadon. Stylar cusp C broad-based but ex- tremely low; differs from Turgidodon species in lacking cusps D and A. Preparacrista and postmetacrista with wear surfaces developed, but low and blunt.

Etymology - Named for Richard W. Thorington, Jr., in recognition of his contributions to knowledge of living and fossil mammals.

Description--Many of the features listed above in the diagnosis are apparently correlative and are ex- pressed, to varying degrees, in various North American Cretaceous marsupials, notably certain species of Tur- gidodon and Glasbius. lugomortiferum thoringtoni is, however, unique among these and other taxa in com- pletely lacking a stylar cusp D, which is present in all Cretaceous marsupials.

The proportions of the only upper molariform tooth known, OMNH 20936 (Fig. 3A), are most similar to those of a marsupial M'. The tooth crown is almost perfectly symmetrical in occlusal view, with its outline thus approximating an equilateral triangle. The protocone is broadly developed anteroposteriorly, but not lingually, so that the tooth is actually somewhat longer anteroposteriorly than transversely (Table 1), an unusual condition among Cretaceous marsupials. Al- though the specimen is lightly worn, the tip of the protocone and adjacent parts of the pre- and postpro- tocristae are abraded through the enamel to the dentine. Protoconal cingula are lacking. Weak internal cristae run from the regions of the protoconule and the metaconule to the bases of paracone and metacone, respectively. The protoconule is faint; a metaconule as such cannot be discerned. The metaconule may have been present but became obliterated by the light wear present on the postprotocrista; if so, the cusp was very weakly developed. The paracone and metacone are inflated, broadly spaced, and of equal height, although the paracone is somewhat more robust than the metacone. Both cusps are equally worn on their apices. The stylocone is somewhat more broadly separated from the paracone than is generally the case for marsupial M's; if the tooth represents a more posterior locus, it differs even more greatly from those of other known taxa in its proportions. The stylocone is strongly inflated and is nearly as tall as the paracone, but because of the great swelling of the anterior part of the stylar shelf, it is poorly differentiated and has the appearance of being low and blunt. The preparacrista descends gently from the apex of the paracone along the anterior face of that cusp, effectively terminating in a slight notch about two-thirds of the distance toward the sty-

locone. A weak keel is present on the adjacent face of the stylocone but, judged by its poor development and lack of wear, it was not involved to any appreciable extent in shearing. A keel descends the anterior face of the stylocone; the preprotocrista is slightly expanded at its labial terminus but a cusp A, as far as can be determined (some of the enamel is missing from this part of the tooth), was lacking. There is no ectoflexus as such, although the anterior margin of the stylar shelf bulges somewhat labially at the stylocone. A broad- based but very low, conical cusp is present in the C position at the labial margin of the stylar shelf. Wear is present on the apex of cusp C and along the labial margin of the posterior stylar shelf, which is developed as a slight ridge. There is no cusp in the D position. A cusp E cannot be discerned either, although slight abrasion of the enamel at the posterolabial corner of the specimen leaves the presence or absence of that cusp open to question.

Three lower molars, each representing a different tooth locus, are provisionally referred to lugomortiferum thoringtoni on the basis of size, provenance, and similarity to the type upper molar in inflation of cusps. Although they are morphologically distinctive, their characteristics have been excluded from the diagnosis pending additional evidence of association. Further discussion of lower molar associations is given below.

The lower molars are distinctive in the relatively low trigonids and inflation of cusps. They further differ from those of typical Alphadon-like construction (Clemens, 1966; Lillegraven, 1969; Fox, 1971, 1979; Lillegraven and McKenna, 1986) in having a more poorly developed labial postcingulid and a less angular labial face on the hypoconid. M, (OMNH 20928; Fig. 3B-D) has a broadly open trigonid, with the paraconid anteriorly placed and the metaconid set posterior to the protoconid. The protoconid is the tallest cusp of those on the trigonid, with the metaconid being somewhat shorter and the paraconid (the tip of which is missing) being much shorter than either of the other cusps. The cristid obliqua joins the back of the trigonid below the notch in the metacristid, as it does in lower molars of Alphadon-like construction (Clemens, 1966; Lillegraven, 1969). A hypoconulid is not present on the single M, available; this may be due to specimen breakage or abrasion. OMNH 20705 (Fig. 3E-G) is probably M2, or, perhaps, M3, if the species had a marsupial dental formula. It is considerably larger than M,, but is of similar construction, with rather low, robust, inflated cusps. The talonid is noticeably broader than the trigonid. As on M,, the paraconid is lower than the metaconid, although the differential is less than in the preceding tooth. Apical wear is well-developed on the protoconid; well-marked shearing fac- ets, with large striations, are present on the anterior face of the paracristid and the labial face of the cristid obliqua. Wear is also strong on the postcristid, which has been abraded down to the dentine. The cristid obliqua joins the trigonid somewhat labial to the notch



324 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 10, NO. 3, 1990

. .. .. .L

ol,

Ar

FIGURE 3. lugomortiferum thoringtoni, gen. et sp. nov. (A-J), and cf. lugomortiferum, unnamed species (K-M). A, Ml? (OMNH 20936, type specimen) in occlusal view; B, C, D, M, (OMNH 20928) in labial, occlusal, and lingual views, respectively; E, F, G, Mx (OMNH 20705) in labial, occlusal, lingual views, respectively; H, I, J, M4? (OMNH 20136) in labial, occlusal, and lingual views, respectively; K, L, M, M4? (MNA V4539) in labial, occlusal, and lingual views, respectively. Tooth roots have been eliminated from the figure where necessary; scale bar = 1 mm.

in the metacristid, unlike the condition observed for

M1. The hypoconulid is lingually placed and "twinned" with the entoconid. The trigonid of the ultimate lower molar, M4? (OMNH 20136; Fig. 3H-J), is broader than the talonid, as is generally true of marsupial last lower

molars. The height of the protoconid cannot be determined because of specimen breakage, but the paraconid and metaconid are both low and subequal in height. The cristid obliqua is as described for M,. The labial face of the hypoconid is gently rounded, lacking




TABLE 1. Measurements of Iugomortiferum thoringtoni, gen. et sp. nov., from OMNH localities V2 and V8, middle Wahweap Formation, Utah. Abbreviations: AP, anteroposterior length; ANW, anterior width; POW, posterior width.

Tooth Specimen no. AP ANW POW

Ml? OMNH 20936 2.34 2.26 2.28 M, OMNH 20928 1.88 1.00 1.04 M2 or 3 OMNH 20705 2.51 1.31 1.66 M4? OMNH 20136 2.78 1.66 1.40

the angular or folded appearance ofAlphadon-like lower molars. The hypoconulid is in a lingual position, close to the entoconid, and is poorly differentiated from that cusp, which it slightly exceeds in height. Mea- surements are given in Table 1.

cf. IUGOMORTIFERUM, unnamed species (Fig. 3K, L, M)

MNA V4539 is a left ultimate molar (M4 if the mammal that bore it had a marsupial dental formula) from OMNH locality V 11, lower Wahweap Formation (Fig. 1). It is, in several respects, atypical of Cretaceous marsupial lower molars, and greatly resembles the much larger specimens provisionally referred to lugomortiferum thoringtoni, above. However, it is much smaller than those specimens (AP = 1.85; ANW = 1.11; POW = 0.89) and would seem to be too small to belong to the species represented by them. The molar cusps are rather low and are somewhat inflated. The protoconid is the tallest trigonid cusp; the paraconid is noticeably smaller than the metaconid and is placed in a more median position than is typical of most Cretaceous marsupials, in which it is generally in a line with the entoconid and metaconid (Clemens, 1979; Clemens and Lillegraven, 1986). The cristid obliqua meets the back of the trigonid at a point somewhat labial to the notch in the metacristid. The labial face of the hypoconid is gently rounded, not sharply angular as is generally true of small species with Alphadon-like lower molars. The talonid basin is also gently rounded, slop- ing gradually to the median faces of the talonid cusps, so that none of them is trenchant. The postcingulid is weak and does not extend anteriorly around the base of the hypoconid. The hypoconulid is closely approximated to the entoconid, and only a small notch sep- arates their apices.

ANCHISTODELPHYS, gen. nov.

Type Species--Anchistodelphys

archibaldi, sp. nov. Included Species-The type only. Distribution--As for the type and only species. Diagnosis-As for the type and only species. Etymology - Anchistos (Gr.), nearest; delphys (Gr.),

womb, a commonly-used suffix for New World opossums and related marsupials. Allusion is to the uncertain relationships of the genus, whose closest compar- isons are with marsupials.

ANCHISTODELPHYS ARCHIBALDI, sp. nov. (Figs. 4, 5)

Type Specimen - OMNH 20133, left M3 lacking stylar cusp A.

Hypodigm -The type, and the following specimens: MNA V4598, left M' lacking the posterior part of the stylar shelf; V4558, left M' with paracone broken; OMNH 20968, left M2 lacking stylar cusp A; 20132, right M3 lacking anterior part of stylar shelf; MNA V4532, partial right M3; V4592, right M,?; V4545, left M2; V4557, right M2; OMNH 20706, right M2 with trigonid cusps broken; 20137, broken left M3?; MNA V4623, left M3? with trigonid cusps broken; V4520, left M4; and V4563, right M4 with trigonid cusps broken.

Horizons and Localities- The type, MNA V4532, V4545, V4557, V4558, and V4563 are from OMNH locality V11; MNA V4520, V4592, V4598, V4623, and OMNH 20706 are frorri OMNH locality V8; OMNH 20132 and 20137 are from OMNH locality V2; and OMNH 20968 is from OMNH locality V49. These sites are distributed through the lower and middle Wahweap Formation (Fig. 1), Garfield and Kane counties, Utah.

Diagnosis -Diminutive species the size of, or slightly smaller than, Alphadon attaragos. Upper molars with relatively narrow protocone, as in species of Protal- phadon and Iqualadelphis, but differing from those taxa and all other Cretaceous marsupials in the slight development and variable absence of both stylar cusps C and D.

Etymology -Named for J. David Archibald in recognition of his contributions to knowledge of Creta- ceous mammals.

Discussion and Description--Individual upper molars of Anchistodelphys archibaldi compare well with those of the larger species Protalphadon creber and P. wahweapensis in most details. However, as noted in the diagnosis, stylar cusp D, always prominent in Prot- alphadon and all other Cretaceous marsupials, is either small and variable in its development or lacking entirely in A. archibaldi; cusp C is variable in its presence and expression, as it is in known species of Protalpha- don. This might indicate that the hypodigm represents a heterogeneous assemblage. However, the included upper molars are otherwise highly similar in size and morphology, and in these respects are entirely unlike those of other mammals known from the Wahweap Formation. Furthermore, cusps C and D of Anchis- todelphys archibaldi molars never achieve the size they do in taxa in which their presence is consistent. Judged by variability in development ofstylar cusps in modern species of the murine opossum Marmosa (pers. obs.), the variable presence and expression of cusps C and D in Anchistodelphys archibaldi may be accounted for by intraspecific variability.

Lower molars are referred to the species on the basis of relative size and abundance: Anchistodelphys archibaldi is the most abundant tribosphenic therian








cusps, which are of equal height, although the metaconid is somewhat more robust than the paraconid. As in small species of Alphadon and Protalphadon, the cristid obliqua and postcristid join each other at an acute angle, and the labial face of the hypoconid is sharply angular. The entoconid is tall and trenchant; the hypoconulid is closely approximated to the entoconid but nonetheless protrudes dorsally as a distinctive, thumblike projection. The succeeding teeth (Fig. 5G-L) are similar to M2, differing in ways that would be predicted according to locus (successively more anteroposteriorly compressed, taller, broader trigonids). Measurements are given in Table 2.

DISCUSSION

The only previously described mammal from the Wahweap Formation is Symmetrodontoides foxi Ci- felli and Madsen, 1986, whose closest relative is S. canadensis, from the upper Milk River Formation (Fox, 1976, 1985), of early Campanian age (Fox, 1978). Based on a preliminary faunal list from the Wahweap For- mation, Eaton and Cifelli (1988) indicated that the mammalian fauna of that unit is most similar to that of the upper Milk River Formation, although it differs significantly from that and all other described North American Cretaceous local faunas. The Wahweap For- mation obviously was deposited before local extinction of spalacotheriid symmetrodonts, which thus far have not been reported from post-Aquilan assemblages. The Kaiparowits Formation, which overlies the Wahweap Formation, has produced a mammalian fauna that also differs strongly from other known Cretaceous local faunas, but is most similar to those of Judithian age (Eaton and Cifelli, 1988). The diverse assemblage of marsupials from the lower Kaiparowits Formation includes a Milk River species (Iqualadelphis lactea Fox, 1987a); other taxa, including two species of Turgidodon (which is not known from the Aquilan), are more similar to those from Judithian local faunas (Cifelli, 1990a).

With the exception of Protalphadon crebreforme, none of the taxa described herein compares closely with those previously known from elsewhere. lugomortiferum thoringtoni somewhat resembles Judithian to Lancian species of Turgidodon in its cusp inflation, but it lacks stylar cusp D and is autapomorphous in certain respects, such as the central placement of the protocone on upper molars and the lower, more centrally-placed paraconid on lower molars. I. thoringtoni differs from Bistius bondi, described as a tribothere (Clemens and Lillegraven, 1986) from the San Juan Basin of New Mexico (but referred to the Marsupialia by Aplin and Archer, 1987; see also Cifelli, 1990a), in lacking protoconal cingula and stylar cusps A and D, among other features. The low, bulbous cusps of I. thoringtoni are also similar to those of Glasbius (see Clemens, 1966) and Ectocentrocristus (see Rigby and Wolberg, 1987) species, which do not otherwise invite close comparison. The smaller cf. lugomortiferum sp., represented by MNA V4539, somewhat resembles

TABLE 2. Measurements ofAnchistodelphys archibaldi, gen. et sp. nov., from OMNH localities V2, V8, V11, and V49, Wahweap Formation, Utah. Abbreviations as in Table 1.

Tooth Specimen no. AP ANW POW

M' MNA V4598 - 1.79 - MNA V4558 1.58 1.78 1.90

M2 OMNH 20968 1.60 1.92 2.05 M3 OMNH 20132 - - 2.18 Ml? MNA V4592 1.35 0.59 0.66 M2 MNA V4545 1.66 0.89 0.84

MNA V4557 1.57 0.95 0.91 OMNH 20706 1.78 0.94 0.99

M3or 2 OMNH 20137 1.95 1.10 1.15 MNA V4623 1.88 1.17 1.10

M4 MNA V4563 1.76 1.11 0.92 MNA V4520 1.83 1.08 0.85

Gallolestes pachymandibularis, a ?eutherian from the Late Cretaceous of Baja California (Lillegraven, 1972, 1976). However, the Utah species differs in having a lesser height differential between the trigonid and talonid, a paraconid that is larger and placed well anterior to the metaconid, and a less posteriorly projecting, more medially placed hypoconulid. In all of these features, cf. lugomortiferum sp. more closely resembles marsupials than does Gallolestes pachymandibularis.

Generally primitive North American Cretaceous marsupials were referred to the Peradectidae by Reig et al. (1987), who restricted usage of the Didelphidae (in which such taxa had conventionally been placed; e.g., Simpson, 1945) without specifying morphological criteria for this distinction. Marshall and de Muizon (1988) recognized the Didelphidae (contents not spec- ified) on the basis of presumed synapomorphies of the upper molars, including a V-shaped centrocrista and metacone larger than paracone. These characters appear to have evolved independently more than once among South American marsupials, however, and use of them as taxonomic criteria poses some taxonomic problems among North American Cretaceous taxa (Ci- felli, 1990a). Among the somewhat heterogeneous assemblage referred to the Peradectidae, Iqualadelphis lactea, known from the upper Milk River Formation of Alberta and the Kaiparowits Formation of southern Utah, appears to be the most primitive (Fox, 1987a; Cifelli, 1990a). Protalphadon species are only slightly more advanced, differing from I. lactea in the occasional presence of a stylar cusp C, in the somewhat reduced height differential between trigonid and talonid on lower molars, and in the slightly broader protoconal region and relatively larger metacone on upper molars. These differences, though presumably advanced, are fairly non-specific, and may represent gen- eralized trends among Late Cretaceous marsupial clades rather than synapomorphies linking Protalphadon to Alphadon, Turgidodon, and other more specialized taxa, to the exclusion of Iqualadelphis. Among the species of Protalphadon, P. crebreforme from the Wahweap




Formation compares favorably with P. creber from the upper Milk River Formation, except that the Utah species is autapomorphous with respect to this and all other members of the genus in its large size and somewhat inflated molar cusps. The available materials of P. crebreforme are few and fragmentary, and little more can be said about the relationships of this enigmatic, generally primitive marsupial.

The affinities and taxonomic placement of the remaining taxa described herein are problematic, insofar as characterization of the Marsupialia on the basis of shared, derived characters of the dentition (which is all that is available for Cretaceous species referred to the group) is open to question. These features have been recently reviewed by Clemens and Lillegraven (1986). Most workers agree that marsupials are derived in their extreme suppression of diphyodonty in post- canine teeth; however, the homology of the single tooth replaced in Recent marsupials has been variously in- terpreted. Archer (1978) argued that the last premolar, P3, displaces Ml (which develops on the dental lamina between P3 and the first permanent molar), so that the remaining four molars of marsupials should be con- sidered M2-5. This terminology has been adopted by some workers (e.g., Marshall, 1987, Reig et al., 1987; Aplin and Archer, 1987) but ignored by others in favor of a more traditional numbering scheme for marsupial cheek teeth, in which the premolars are numbered P1- 3 and the molars M1-4 (e.g., Clemens, 1979; Fox, 1979, 1987a; Cifelli, 1990a). Because uncertainty re- mains as to whether the marsupial deciduous molar represents a separate tooth locus between the last premolar and the first permanent molar of marsupials, the traditional numbering scheme is employed herein. Dental formulae are, in any case, of limited utility in determining the affinities of most Mesozoic mammals because of lack of knowledge of the full dentition and of the ontogenetic processes resulting in the adult dentition (see, e.g., Luckett and Maier, 1982). As discussed by Clemens and Lillegraven (1986), the major morphological features upon which marsupials may be characterized (relative to a primitive, metatherian-eutherian grade condition) involve a presumed functional complex associated with increase in anteroposterior expansion of the protocone and increase in height of the metacone relative to the paracone, with concomitant changes in the lower molars, including increase in the size of the embrasure between adjacent molars, strong development of a posterolabial cingulid, and a lingual shift of the hypoconulid, so that it is "twinned" with the entoconid. Unfortunately, these features in some respects seem to represent trends general to many tribosphenic therian groups of the Cretaceous, and have not been sufficiently circumscribed in order to consider them to be diagnostic apomorphies of the Marsupialia. Thus, some of the aforementioned characters are apparently lacking in taxa presumed on other grounds to be marsupials (Albertatherium primus, for instance, has a paracone larger than metacone, although more pre- cise definition of the character might reveal that this

species is advanced in this respect compared to a primitive tribosphenic condition), and some are present in presumed eutherians or "metatherian-eutherian grade" taxa (Paranyctoides has a strong posterolabial cingulid, Gallolestes has somewhat "twinned" hypoconulid and entoconid, and various Late Cretaceous eutherians and metatherian-eutherian grade taxa have subequal metacone and paracone).

The relative development of the stylar shelf and its cusps has figured prominently in discussions of marsupial evolution. While the full complement of five stylar cusps as seen in Alphadon has been hypothesized to be an appropriate model for later radiations of South America, European, and Australasian marsupial taxa (Clemens, 1966, 1979; Marshall, 1987), recognition of the earliest marsupials based on distinct, apomorphic characters involving the configuration and development of cusps on the stylar shelf of upper molars is problematic, particularly as regards the C and D positions. Although Slaughter (e.g., 1971) has argued that the earliest marsupials were characterized by a large cusp in the C position (see also Fox, 1971), recent evidence suggests that cusp D was acquired first (Clem- ens, 1979; Fox, 1987b). Fox (1987a) has suggested that the appearance of cusp C in marsupial evolution be used to characterize the Didelphidae as distinct from more primitive forms. Both cusps C and D evidently evolved independently in mammals that on other grounds do not appear to be referable to the Marsu- pialia, among both eutherians and "metatherian-eutherian grade" taxa. Thus, cusp D is present and C lacking in Pappotherium while the reverse is true of the closely similar Holoclemensia; cusp D may be found in some eutherians, such as Paranyctoides (cf. Fox, 1984; Cifelli, 1990b), and both cusps are present in Picopsis and Bistius (Clemens and Lillegraven, 1986), taxa described as tribotheres. Thus, caution seems to be called for in placing phylogenetic currency on cusps of the stylar shelf, which are not directly involved in occlusion, at least for primitive therians of which little other than isolated teeth are known.

Iugomortiferum thoringtoni of the Wahweap For- mation, which resembles marsupials to the extent that the paracone and metacone are equally developed on upper molars, is obviously advanced in its possession of bulbous, inflated cusps, a reduced preparacrista, and a broadly developed, centrally-placed protocone on upper molars. A low, bulbous cusp is present in the C position, but not in the D position, an unusual condition for marsupials. The referred lower molars, also bearing low, inflated cusps, are marsupial-like in the incipient "twinning" of hypoconulid with entoconid. Nonetheless, they are atypical of Cretaceous marsupials in several respects, including the relatively smaller, more centrally-placed paraconid and the lesser development of the postcingulid. The taxonomic position of this species is thus highly uncertain. Assuming that the lower molars are correctly referred, lugomortiferum thoringtoni could represent: 1) an advanced, somewhat aberrant marsupial, perhaps a didelphid (sensu




Fox, 1987a) that has lost stylar cusp D and has a mod- ified lower molar morphology; 2) a representative of a metatherian-eutherian grade lineage that acquired stylar cusp C independently from marsupials; or 3) a di- vergently specialized representative of a primitive marsupial or marsupial sister taxon, depending on the morphological criteria used to define the Marsupialia, which had acquired some marsupial dental characteristics but not others. This last interpretation is sup- ported, but in light of the foregoing far from verified, by the aforementioned early tendency in marsupial evolution toward the acquisition of apparently correlative features including enlarged metacone with respect to paracone, anteroposterior expansion of the protocone, and enlarged embrasures between lower molars, with concomitant development of a labial postcingulid and a lingual shift in the hypoconulid and paraconid (Clemens and Lillegraven, 1986). However, because marsupials apparently acquired stylar cusp D before cusp C (Clemens, 1979; Fox, 1987a, b; Cifelli, 1990a), independent acquisition of cusp C in Iugo- mortiferum thoringtoni would still be required to ex- plain its presence in that taxon.

The situation is similar for the Wahweap species Anchistodelphys archibaldi, which variably lacks or has small, irregular cuspules in both the C and D positions. When compared with Trinity therians, with other metatherian-eutherian grade taxa, and with various Cretaceous Eutheria, A. archibaldi more closely resembles marsupials in the equal development of paracone and metacone, although this is true of certain eutherians, such as Paranyctoides, which, however, has a more reduced stylar shelf. The lower molars, for which referral appears probable, are of typical Alphadon-like construction, strongly resembling those of early marsupials in the lingual position of the paraconid (in line with the metaconid and entoconid; Clemens, 1979), the distinctly "twinned" hypoconulid and entoconid, and the strongly developed labial postcingulid. An- chistodelphys archibaldi could represent a member of a marsupial clade that developed an early tendency to reduction and loss of the posterior stylar cusps (as in early Tertiary peradectids, such as Peradectes elegans), or simply a primitive marsupial or marsupial sister taxon (depending, again, on criteria used to define the Marsupialia) in which a synapomorphy of the group (stylar cusp D) had not attained full expression. This latter hypothesis is favored at present because it has the virtue of being the simpler of the two explanations. If cusp D appeared prior to cusp C in marsupial evolution, as discussed above, then the variable presence of a cusp C in A. archibaldi would be anomalous under this interpretation.

ACKNOWLEDGMENTS

Support for this research was provided by the Office of Research Administration and Department of Zo- ology, University of Oklahoma, and by grants from the National Geographic Society (2881-84), National

Science Foundation (BSR 8507598, 8796225, 8906992), and Petroleum Research Fund of the Amer- ican Chemical Society (20311 -G8). Student partici- pation in the project was made possible by an REU supplement to the NSF award. The SEM photos were prepared by Eric Sherburn at the Samuel Roberts No- ble Electron Microscopy Laboratory, University of Oklahoma, access to which was kindly provided by Dr. Scott Russell, Director. I thank Scott Madsen, Jeff Eaton, Ken Thiessen, Winford Sterling, Beth Larson, Dave Schmidt, and Mary C. Cifelli for assistance in field and laboratory activities, and Drs. Eaton, Mal- colm C. McKenna, William A. Clemens, Jr., and Rich- ard C. Fox for various comments and information. I am grateful to Fox for permitting and facilitating my study of specimens at the University of Alberta. Partial logistic support for some of the field work was provided by the Museum of Northern Arizona, and the help of that institution is warmly acknowledged.

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Received 24 May 1989; accepted 27 October 1989.



Documents

Cretaceous Mammals of Southern Utah. II. Marsupials and Marsupial-Like Mammals from the Wahweap Formation (Early Campanian)