Journal of Mammalian Evolution, Vol. 4, No. 4, 1997

Dentition and Jaw of Kokopellia juddi, a PrimitiveMarsupial or Near-Marsupial from the Medial Cretaceousof Utah

Richard L. Cifelli1,3 and Christian de Muizon2

We add to the knowledge of the dentition and lower jaw of the primitive marsupial or nearmarsupial, Kokopelliajuddi, based on newly collected materials from the medial Cretaceous(Albian-Cenomanian) of central Utah. The dental formula, i4 cl p3 m4, is primitive for (orwith respect to) Marsupialia, as are a number of features of the dentary and dentition: presenceof a labial mandibular foramen, ?an inflected angle, ?and a trace of the meckelian groove; lackof "staggering" of the lower incisor series; lack of "twinning" between entoconid and hypo-conulid on lower molars; incompletely lingual position of lower molar paraconid; upper molarprotocone relatively small and mesiodistally narrow; and conules placed about halfway betweenthe protocone and the paracone-metacone. Other than the stylocone, cusps are lacking fromthe stylar shelf; we argue that this represents the primitive marsupial condition based on theeconomy of character change and the stratigraphic record of marsupials in the Cretaceous ofNorth America. Recent discoveries of early marsupials, eutherians, and therians of metather-ian-eutherian grade provide data indicating that some derived features of the dentary and den-tition (e.g., loss of coronoid, meckelian groove, and labial mandibular foramen; acquisition ofstrong, "winged" conules, double rank postvallum/prevallid shearing, and stylar cusp D) prob-ably arose independently, in some cases more than once, among the major groups of tribo-sphenic mammals. In turn, this suggests that a common ancestor for marsupials and piacentaiswas more primitive than has generally been appreciated.

INTRODUCTION

Marsupials have long held the interest of evolutionary biologists for a number of reasons,including their biogeographic distribution (e.g., Clemens, 1968; Lillegraven, 1974;Marshall, 1980) and a combination of generally primitive morphology (e.g., Gregory,1947) with peculiarly specialized developmental and reproductive biology (e.g., Tyn-dale-Biscoe and Renfree, 1987; Renfree, 1993; Luckett, 1993; Luckett and Woolley,1996; see also Lillegraven, 1975). Owing to the shared presence of tribosphenic molars

1 Oklahoma Museum of Natural History and Department of Zoology, University of Oklahoma, Norman, Okla-homa 73019.

2URA 12 CNRS, Laboratoire de Paleontologie, Museum National d'Histoire Naturelle, 8 rue Buffon, 75005Paris, France.

3To whom correspondence should be addressed at Oklahoma Museum of Natural History, University of Okla-homa, 1335 Asp Avenue, Norman, Oklahoma 73019. e-mail: ric@ou.edu

KEY WORDS: Marsupialia; dentition; stylar cusps, tooth formula.

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(see Simpson, 1936; Crompton, 1971), marsupials are presumed to have had an EarlyCretaceous common ancestor both with eutherians and with a number of suspected fossilrelatives [termed Theria of metatherian-eutherian grade or tribotheres (Patterson, 1956;Butler, 1978; Clemens and Lillegraven, 1986)] that cannot be reliably placed with eitherliving group—though there is not universal agreement on this point (e.g., Lillegraven elal., 1987; Kielan-Jaworowska, 1994). Indeed, the early fossil record of marsupials andpotential allies, described by one leading researcher as "an odontologist's delight" (Cle-mens, 1979, p. 192) because it consists largely of isolated teeth, is permissive of manyinterpretations—as controversy over the Aptian-Albian genus Holoclemensia attests(e.g., Slaughter, 1971; Butler, 1978; Jacobs etal., 1989; Cifelli, 1990a).

The earliest generally recognized Marsupialia, from the Cenomanian [about 95 Ma(see Gradstein et al., 1995)] of western North America, include six taxa described fromthe Dakota Formation of Utah (Cifelli and Eaton, 1987; Eaton, 1993; see also Fox andNaylor, 1995). Succeeding assemblages are known from the Turanian (Cifelli, 1990b)and early Campanian (Fox, 1971; Cifelli, 1990c), but it is not until nearly 20 millionyears later, in the late Campanian (ca. 75 Ma; cf. Goodwin and Deino, 1989) that taxabecame reasonably well known (e.g., Sahni, 1972; Fox, 1979a, b, 1981; Montellano,1988, 1992).

Outside of North America, putative marsupials or marsupial relatives have beendescribed from the Cretaceous of Asia (Kielan-Jaworowska and Nessov, 1990; Szalayand Trofimov, 1996), and it has even been suggested that one Asian group (Deltather-oida) bears a special relationship to the North American Stagodontidae and the SouthAmerican Borhyaenoidea (Marshall and Kielan-Jaworowska, 1992). In this context, andconsidering the generally poor representation of marsupials or potentially related taxa inNorth America prior to the Campanian, new fossils offer the potential to provide addi-tional insight.

Kokopellia juddi was described as a marsupial or marsupial-like mammal from themedial Cretaceous of Utah, based on a mandible preserving most of the postcanine den-tition (Cifelli, 1993a). Herein we describe newly collected specimens of this species,including hitherto unrepresented parts of the dentition, and discuss their implications forthe acquisition of dental features during the origin and early evolution of Marsupialia.

MATERIALS AND METHODS

All specimens discussed and figured herein are from the same site [OMNH (Okla-homa Museum of Natural History) V695] as the type, though work in progress indicatesthat the species is rather abundant in the upper part of the Cedar Mountain Formation,Emery County, Utah. The more complete specimens were obtained through quarrying;isolated teeth were also recovered by screenwashing and associated concentration tech-niques (see Cifelli et al., 1996a). Because of the abundance of Kokopellia juddi, itsdissimilarity in size and morphology from other tribosphenic mammals in the upper partof the Cedar Mountain Formation, and the fact that the vertebrate assemblage from thispart of the unit is well represented [being based on more than 5000 catalogued specimens(Cifelli et al., 1997)], referral of upper molars to this species is relatively straightfor-ward. Our basis for relative terms in the descriptions is comparison with primitive didel-phimorphians (in the sense of Marshall et al., 1990) from the early Paleocene of Bolivia,

242 Cifelli and de Muizon

represented by complete skulls and jaws (Marshall and Muizon, 1995; Muizon andCifelli, unpublished data), together with specified taxa from the North American Cre-taceous. Dental terminology follows that of Bown and Kraus (1979) and is illustrated inFig. 1.

The upper part of the Cedar Mountain Formation has produced a diverse vertebrateassemblage, including several other species of tribosphenic mammals, as well as tricon-odonts (Cifelli and Madsen, 1998), symmetrodonts, and multituberculates (Cifelli etal.,1997). Multiple radiometric dates, three of which are based on samples from a horizondirectly overlying the bone level at OMNH V695, establish the age of the fauna as 98.39± 0.07 Ma (Cifelli et al., 1998a), and thereby place Kokopellia juddi on the Albian-Cenomanian (Early-Late Cretaceous) boundary (e.g., Obradovich, 1993; Gradstein etal., 1995).

RESULTS

Mandibular and Dental Morphology of Kokopellia juddi

Dentary. In addition to the type, OMNH 26361 (Figs. 2A and B), the lower jawof K. juddi is also known by OMNH 34200, which consists of a fragment of the rightdentary, broken posterior to m2, and including alveoli for four incisors, pi, p3, and ml,and with c, p2, and m2 in place (Figs. 2C and D). The symphysis is relatively elongate,with a significant component anterior to the canine, and extends posteriorly to the pos-terior root of p2; it is also rather low-angled, being nearly horizontal with respect to thelong axis of the jaw. In both respects, Kokopellia differs from small Recent didelphidsas well as Paleocene Pucadelphys, which is similar in size. The symphysis is rugose andthere is no sign of fusion. Both OMNH 34200 and OMNH 26361 have two mentalforamina, one just anterior to the anterior root of p2 and the other situated below ml.The dentary increases gradually in depth to the level of the posterior root of p2 and thenis relatively constant in depth through the remaining posterior part of the tooth row.

Fig. 1. Left upper (OMNH 33248) and lower (OMNH 33243) molars of Kokopellia juddi, illustrating dentalterminology.

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Fig. 2. Dentary and lower dentition of Kokopellia juddi (stereopairs). OMNH 26361 (holotype) in labial (A)and lingual (B) views. OMNH 34200 (right dentary, with c, p2, and m2 in place) in labial (C) and lingual(D) views. Scale bar = 5 mm.

Only the anteroinferior part of the masseteric fossa is preserved; it is deep and wellmarked, as it is in both Paleocene and Recent didelphimorphians, and a small labialmandibular foramen, obscure in lateral view (and not visible in Fig. 2), is present at thebase of the masseteric fossa. OMNH 26361 is somewhat distorted by mediolateral crush-ing, but a lingual flaring at the inferior margin of the dentary, just posterior to the baseof the ascending ramus, suggests the anterior margin of the inflected angle, a featureinterpreted alternatively as a synapomorphy (Sanchez-Villagra and Smith, 1997) or ple-siomorphy (Clemens, 1979; Marshall and Muizon, 1995) of marsupials. Anterior to this,there appears to be a remnant of the meckelian groove, extending anteriorly along theinferolingual surface of the dentary. The lingual surface of the lower jaw bears no evi-dence of separate coronoid or other postdentary bones.

Lower Dentition. The lower postcanine dentition of Kokopellia was briefly describedpreviously (Cifelli, 1993a). In many respects, it is similar to Cretaceous marsupials suchas Alphadon (see Clemens, 1966, 1979), so that a complete description is unnecessary.

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Here we focus mainly on morphology that has not previously been known or that maybe of most interest to systematics.

There are four incisor alveoli. The first three of these are subequal in size, oval incross section, and narrower mesiodistally than buccolingually (Fig. 3). The alveoli areseparated by thin, bony septa; the labial margin of the dentary dips between these septa,corresponding to the midpoints of the alveoli, as is typical of most mammals. The fourthalveolus, which lies directly mesial to the anterior base of the canine and is not obviouslyseparated by a bony septum from the canine alveolus, is smaller and more mesiodistallycompressed than the first three. There is no evidence for mesial displacement of thesecond incisor, or for the development of a bony labial buttress, characteristic of themarsupial "staggered" incisor pattern described by Hershkovitz (1982, 1995).

The canine has a single, robust root and tapers rapidly crownward; the crown bearsweak mesial and distal keels, as seen in both Paleocene and Recent didelphimorphians.The three premolars are each two-rooted; p1 is not known, but judged by the alveoli thepremolars increased in size from first to third, the usual or primitive marsupial condition(Figs. 2A and B). The premolars resemble those of Alphadon and practically any otherprimitive marsupial: the protoconid is tall, lanceolate, and anteriorly recumbent, withkeels descending mesially and distally. There is no anterobasal cuspule on either p2 orp3. A weak lingual cingulum and a faint posterolabial, basal bulge are present; theseconverge posteriorly and merge with the crest descending the posterior face of the pro-toconid to form a trenchant heel cusp.

The lower molars (Figs. 2, 4A, and 5A-C) also strongly resemble those of LateCretaceous Alphadon. The first molar is much the smallest in the series and has a lower,more open trigonid (as a result of a more anteriorly placed paraconid) and a relativelybroader, more mesiodistally compressed talonid than the succeeding teeth. Posteriorlyin the series, through the third molar at least, the trigonid increases in height and the

Fig. 3. Anterior part of the dentary of Kokopellia juddi, OMNH 34200 (ste-reopair), in occlusal view, showing alveoli for four incisors, anterior to thecanine. Scale bar = 1 mm.

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Fig. 4. Molar structure in Kokopellia juddi (stereopairs). (A) OMNH 26361(holotype), showing left ml-4 in occlusal view. (B) OMNH 33248; left M3 inocclusal view. Scale bar: A = 2 mm; B = 1.5 mm.

Primitive Marsupial from the Medial Cretaceous

Fig. 5. Molar structure in Kokopelliajuddi. OMNH 33243; left ml in lingual (A), occlusal(B), and labial (C) views. (D) OMNH 27639; fragment of M3 in occlusal-posterior view.Scale bar: A-C = 1.5 mm; D = 0.66 mm.

talonid in length. The paraconid is more lingually placed than in Cretaceous Eutheriabut does not achieve the extreme lingual position (see Clemens, 1979; Clemens andLillegraven, 1986) typically encountered in Late Cretaceous and Cenozoic marsupials,being more comparable to Anchistodelphys in this regard (Cifelli, 1990b, c). The pro-toconid is much taller than the paraconid and metaconid, which are subequal in size.The cristid obliqua attaches to the base of the trigonid below the notch in the metacristid,as is characteristic ofAlphadon and other primitive marsupials (Clemens, 1979; Clemensand Lillegraven, 1986). The three talonid cusps are salient and subequal in development.

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The hypoconulid forms a finger-like projection at the distal margin of the molar; althoughthere is some variability among available specimens, it does not achieve a position inwhich it can be said to be lingually placed, or "twinned" with the entoconid, as isgenerally characteristic of marsupials (Clemens, 1979). However, a strong labialpostcingulid is present, giving the appearance that the entoconid and hypoconulid are,in some fashion, integrated.

Upper Dentition. Upper premolars ofKokopellia are, without doubt, represented inthe existing collection but remain to be identified as such. Upper molars of this genus(Figs. 4B and 5D) appear relatively transverse, with a mesiodistally narrow, small pro-tocone that is somewhat anteriorly recumbent compared with those of Alphadon; in theserespects, they resemble presumably more primitive taxa such as Iqualadelphis andAenigmadelphys (see Fox, 1987a; Cifelli and Johanson, 1994). The protocone is trench-ant and lacks basal cingula; the paracone and metacone are also sharp, tall cusps, withthe paracone being slightly taller than the metacone. The centrocrista joining the para-cone and metacone is straight. The conules are placed about halfway from the apex ofthe protocone to the paracone and metacone, respectively, differing from most Late Cre-taceous marsupials, such as Alphadon and Iqualadelphis, where they tend to be locatedcloser to the bases of these main buccal cusps. Despite this apparently primitive position,the conules are strongly developed, project from their respective mesial and distal mar-gins of the tooth, and bear salient internal cristae which terminate at the respective basesof the paracone and metacone. The preparaconular crista joins the preprotocrista andextends labially to the base of the stylocone (stylar cusp B); this development, and espe-cially the presence of a well-developed, vertically oriented wear surface on this crest(labial to the paracone), shows that en echelon shear (Hiiemae and Kay, 1973) waspresent on the prevallum surface of the tooth (see Fox, 1975). The stylar shelf is broad,both mesially and distally, bears a deep ectoflexus, and is more symmetrical in outlinethan it is, for example, in Iqualadelphis, Alphadon, or Pucadelphys. The stylocone islocated at the margin of the stylar shelf labial to the paracone and is connected to thatcusp by a sharp preparacrista which attaches on the mesial side of the stylocone. Thestylocone is the only cusp on the stylar shelf, although the latter bears a slight rim withfour minute cuspules, one posterior to the stylocone, one at the deepest part of theectoflexus, and two more on the distal margin of the ectoflexus. The postmetacrista is asharp, well-developed crest with a facet showing that postvallum/prevallid shearing wasimportant; the crest descends steeply from the apex of the metacone and is weakly notchedlabial to the base of that cusp. The postprotocrista extends labially around the base ofthe metacone, as is typical of North American Late Cretaceous marsupials.

Comparative Mandibular and Dental Morphology

Clemens and Lillegraven (1986) have provided an extensive treatment of dentalcharacter states among Theria, with special reference to Tribosphenida, including pos-tulation of primitive conditions. The following discussion uses the framework from theircomprehensive review, together with data made available since that time and other rel-evant characters that have appeared in the literature, in evaluating the significance ofjaw and tooth structure in Kokopellia.

Dentary. The labial mandibular foramen, placed at the base of the masseteric fossa,was first identified as a character of potential systematic utility by Dashzeveg and Kielan-

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Jaworowska (1984), who noted its presence in the primitive deltatheroidan, Kielan-therium. The labial mandibular foramen is generally lacking in later Deltatheroida (Mar-shall and Kielan-Jaworowska, 1992) and in marsupials (see Marshall and Muizon, 1995),though it is reported to be present in Miocene Microbiotherium (Sinclair, 1906; Mar-shall, 1982) and in at least one specimen of the North American Late Cretaceous Alpha-don (R.C. and C.M., unpublished data). This distribution led Marshall and Kielan-Jaworowska (1992) to suggest that the absence of the labial mandibular foramen mightbe regarded as a synapomorphy for Metatheria. Unfortunately, the distribution and sig-nificance of this enigmatic character are not well understood. The labial mandibularforamen is lacking in Kuehneotherium (Kermack et al., 1968) and in a Symmetrodon-toides-like symmetrodont from the Mussentuchit local fauna (R.C., unpublished data).In addition to Kokopellia and Kielantherium, the foramen is present in a variety of taxa,including Vincelestes (Rougier, 1993), Triconodontidae in which that part of the jaw isknown (Cifelli et al., 1998b), and some early eutherians, such as Prokennalestes andZalambdalestes (Kielan-Jaworowska and Dashzeveg, 1989). The presence of the fora-men (the presumed primitive condition) in Kokopellia could imply independent loss inDeltatheroida and marsupials, reversal in the taxon from Utah, or phylogenetically distalplacement of Kokopellia with respect to Deltatheroida and marsupials. Given the poorunderstanding of the distribution of this feature, together with evidence suggesting fre-quent loss in mammals, we find no basis to interpret its presence in Kokopellia, exceptthat it is almost surely primitive with respect to Marsupialia.

A scar for the coronoid bone on the lingual side of the dentary, at the junction ofthe horizontal and ascending rami, is a primitive feature retained by a number of Meso-zoic mammals, including Kuehneotherium (Kermack et al., 1968), Henkelotherium(Krebs, 1991), Kielantherium (Dashzeveg and Kielan-Jaworowska, 1984), the dodo-donts Docodon (see Kermack and Mussett, 1958) and Haldanodon (see Lillegraven andKrusat, 1991), and the eutherians Prokennalestes (Kielan-Jaworowska and Dashzeveg,1989) and Otlestes (Nessov et al., 1994: pi. 3, Fig. 1C). The coronoid scar is lackingin Kokopellia, all undoubted early marsupials for which the lower jaw is known (see,e.g., figures in Clemens, 1966; Marshall and de Muzion, 1995), Asiatherium [conditionuncertain (see Szalay and Trofimov, 1996, Fig. 7)], North American Late Jurassicdryolestoids (Prothero, 1981), and an unnamed spalacotheriid symmetrodont (R.C.,unpublished data). The condition is unknown or remains to be described for most Euro-pean eupantotheres and deltatheroidans; it seems clear, however, that the scar for thecoronoid was lost several times during mammalian history.

We tentatively identify a small remnant of the meckelian groove on the dentary ofKokopellia. The meckelian groove has a broad distribution among Mesozoic mammals(see, e.g., figures in Simpson, 1928, 1929); it is strongly developed, for example, inHenkelotherium (Krebs, 1991), docodonts (Kermack and Mussett, 1958; Lillegravenand Krusat, 1991), and Kielantherium (Dashzeveg and Kielan-Jaworowska, 1984). Aweak meckelian groove is seen in the Paleocene marsupials Pucadelphys (Marshall andMuizon, 1995), Mayulestes (Muizon, 1998), Andinodelphys, and Incadelphys (C.M.,unpublished data) and in some early eutherians, such as Prokennalestes (Kielan-Jaworowska and Dashzeveg, 1989) and Kennalestes (Kielan-Jaworowska, 1981). Thegroove may be absent in Deltatheroida (Marshall and Kielan-Jaworowska, 1992), as itis in most marsupials, eutherians, at least one triconodontid (Cifelli et al., 1998b), anda Cretaceous spalacotheriid symmetrodont (R.C., unpublished data). Clearly, the feature

Primitive Marsupial from the Medial Cretaceous 249

has been lost a number of times among mammals and it is seen in several uncontestedearly marsupials; hence we are unable to evaluate the significance of its presence (if infact it is present) in Kokopellia, other than observing that it is almost surely a retainedmammalian plesiomorphy.

The presence of an inflected angular process in Kokopellia juddi is suggested by alingual flaring at the posterior margin of OMNti 26361, where the specimen is broken.This feature has been generally regarded as a plesiomorphy of marsupials (e.g., Cle-mens, 1979), but Sanchez-Villagra and Smith (1997) suggest that it is a synapomorphyof the group. The angular process of Kokopellia is too incompletely known to warrantcomment with respect to this issue.

Dental Formula. The lower dentition of Kokopellia includes i4 cl p3 m4; nothingis known of the upper dental formula, but four upper molar loci are represented byexisting specimens, and based on the development and occlusal relationships of the lowermolars, it is a reasonable supposition that four upper molars were present as well. Acount of four lower incisors has long been hypothesized to be primitive for Marsupialia,at least, and existing data suggest that the same may be true for Eutheria (e.g., Kielan-Jaworowska, 1981; see discussion by Clemens and Lillegraven, 1986). In deltathe-roidans, four or more upper incisors are present, whereas the lower dentition includedonly one or two (Kielan-Jaworowska et al., 1979). Hershkovitz (1982) has provided aplausible argument suggesting that the primitive number of incisors in the lower jaw ofmarsupials was five, as are present in the upper jaw, that marsupials reduced the incisorcount to four in the lower jaw through loss of il, and that crowding of the remainingfour left in the jaw resulted in a labial buttress and lingual "staggering" at the second[identified as i3 by Hershkovitz (1982)] of the four remaining tooth positions. Earlyfossils bearing on this problem are not, unfortunately, very common. Hershkovitz (1995)identified the condition in a specimen previously described by Patterson (1956) from theEarly Cretaceous of Texas (and placed by Hershkovitz in a newly created genus andspecies, "Adinodon pattersoni," which we view as a nomen dubium) and in the LateCretaceous marsupial Eodelphis browni, based on illustrations published by Matthew(1916). We are unable to verify the staggered condition in either of these cases basedon our own examination of published illustrations and a cast of the type of E. browni.{We applaud Hershkovitz's (1995) exploration of this character as an important contri-bution to marsupial systematics. However, this paper is so heavily laden with misstate-ments of fact [e.g., description of Eodelphis browni as the "only known species" of thegenus and as being "marmosine-like in size" (Hershkovitz, 1995, p. 165)] and idiosyn-cratic interpretation [e.g., "family Marmosidae" (Hershkovitz, 1995, p. 163)] that weurge caution in its use.} To our knowledge, the oldest evidence for "staggering" in thelower incisor series is from the early Paleocene of Tiupampa, Bolivia, as represented byPucadelphys, Mayulestes (Muizon, 1998), and Andinodelphys (C.M., unpublished data).Hence, in the configuration of the lower incisor series (which includes four teeth withno indication of staggering), Kokopellia is primitive with respect to known Marsupialia.In this context, it is interesting to note that the symphyseal region, particularly that partanterior to the canine, is somewhat more elongate in Kokopellia than is generally seenin primitive didelphimorphians such as Pucadelphys. This suggests less crowding in theincisor region, perhaps explaining the lack of staggering.

With respect to the postcanine teeth, the presence of three premolar and four molar

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loci, found in both Deltatheroida and Marsupialia, is believed to be a derived conditionwithin Tribosphenida (Clemens and Lillegraven, 1986; Kielan-Jaworowska and Nessov,1990). In marsupials, ontogenetic data suggest that the adult dental formula results froma unique developmental pattern, involving delayed eruption and nonreplacement at thefirst two premolar loci (which thus are, technically speaking, dpl-2) and normal butdelayed replacement at the third premolar locus (Luckett, 1993; Luckett and Woolley,1996). The antiquity of this pattern is not well understood, but available data suggestthat it is present in the Late Cretaceous North American marsupial Alphadon, at least(Cifelli etal., 1996b). The ontogeny of the deltatheroidan dentition unfortunately is notknown; the issue is of some importance, as it would provide a robust test of the hypoth-esis that the presence of a postcanine tooth formula of p3 m4 in marsupials and delta-theroidans is an unambiguous synapomorphy uniting the two groups. Based on currentevidence, we can say only that both Kokopellia and Deltatheroida share the marsupialpostcanine dental formula, which is generally believed to be a derived condition withinTribosphenida.

Upper Molars. The paracone is slightly taller than the metacone in Kokopellia, aprimitive condition (Clemens and Lillegraven, 1986); this contrasts with the usual con-dition obtaining in marsupials, where the metacone is the taller of the two (e.g.,Clemens, 1979), although a taller paracone is seen in some Late Cretaceous taxa suchas Iqualadelphis, Aenigmadelphys, and Anchistodelphys (Fox, 1987a; Cifelli, 1990b, c;Cifelli and Johanson, 1994) and also in the Asian taxon Asiatherium (Szalay and Tro-fimov, 1996). The condition of the upper molar conules in Kokopellia is somewhatsurprising: they are well developed and bear strong cristae ("wings"), yet they are morelabially placed than is typical in Late Cretaceous marsupials such as Alphadon, Pediomys(Clemens, 1966; Lillegraven, 1969), Iqualadelphis (Fox, 1987a), and Aenigmadelphys(Cifelli and Johanson, 1994). In the latter respect, they are presumably somewhat prim-itive, judged by the condition in Pappotheriidae (Butler, 1978; Cifelli, 1997), Potamo-telses (Fox, 1972), and Deltatheridiidae (e.g., Marshall and Kielan-Jaworowska, 1992,Fig. 2), although well-developed conules do apparently appear independently within thelatter family (see Cifelli, 1993b). Nonetheless, the strong development of the conulesand their associated cristae in Kokopellia contrasts strongly with the condition in thesetaxa and, notably, with the Early Cretaceous eutherian Prokennalestes (Kielan-Jaworowska and Dashzeveg, 1989). The conules of Asiatherium are similar in both posi-tion and relative development to those of Kokopellia.

The most noteworthy aspects of upper molar morphology in Kokopellia concernpostvallum shearing and configuration of the stylar shelf. The stylar shelf of Kokopelliais devoid of cusps other than the stylocone: Is this the primitive marsupial condition(e.g., Cifelli, 1993b), or is this secondarily derived from a condition in which a stylarcusp D was present (Fox, 1987a, b; Fox and Naylor, 1995)? While several Cretaceouseutherians and "therians of metatherian-eutherian grade" are known in which a stylarcusp is present in the D position, a number of other taxa, including otherwise primitivemarsupials as well as tribotheres, do not have this cusp (see reviews by Clemens andLillegraven, 1986; Cifelli, 1993b). Given the facts that homology of a cusp in this posi-tion remains to be established between major groups, such as Marsupialia and Eutheria,and that so little is known about the early radiations of tribosphenic mammals, this issuecannot be settled at present. (Asiatherium, described from the Late Cretaceous of Asia,

Primitive Marsupial from the Medial Cretaceous 251

presents a good example: Is the small, posteriorly placed stylar cusp homologous withthat in advanced North American Cretaceous marsupials, with that in certain CretaceousEutheria, with both, or with neither?) However, we favor the interpretation that a stylarcusp D was lacking in ancestral marsupials for the following reasons. First, lacking anyreasonable basis for homology outside of undoubted Late Cretaceous (and later) mar-supials, we do not consider the sporadic presence of a cusp in the D position amongsome early eutherians [e.g., Paranyctoides (Fox, 1984)] and tribotheres [e.g., Pappo-therium (see Slaughter, 1971)] to be relevant. It is clear that, at some point in the ances-try of marsupials, cusp D was lacking. The hypothesis that it was so in early marsupials,or in a proximate sister taxon of Marsupialia, has the advantage of simplicity; fewersteps are involved, if the cusp is developed early in the history of the group, to explainits presence in (most) Late Cretaceous and later taxa, rather than gained and then lost(to explain its absence in Kokopellia, some Anchistodelphys, and unnamed taxa fromthe Cedar Mountain Formation; see below). A second reason involves geological age[an argument, we note, that was invoked in interpreting the acquisition of stylar cusp Dprior to cusp C in marsupials (see Fox, 1987b)]. Kokopellia is significantly older thanall uncontested North American Marsupialia. In addition, the vertebrate fauna of theCedar Mountain Formation includes at least two dentally more advanced taxa that wouldgenerally be placed within Marsupialia; both of these lack stylar cusps C and D as well(R.C., unpublished data). We point out that Anchistodelphys, which variably lacks a Dcusp as well, also predates the marsupials of the Aquilan-Lancian, on which the hypoth-esis of a primitively present D cusp was based.

The other significant feature of upper molars referred to Kokopellia is the fact thatthe postprotocingulum extends labially past the base of the metacone, so that double-rank postvallum/prevallid shearing had been achieved (see Fox, 1975). This feature isseen in North American Late Cretaceous marsupials that are not secondarily specialized,in Asiatherium, and in Eutheria, including the otherwise rather primitive Prokennalestes(see Kielan-Jaworowska and Dashzeveg, 1989, p. 8). The labially extended postproto-crista was previously suggested as a synapomorphy uniting the two major groups ofTribosphenida (Cifelli, 1993b). The derived condition is lacking in extremely primitivetribosphenidans (e.g., Potamotelses), pappotheriids, and Deltatheroida.

Lower Dentition. In light of uncertainties regarding the primitive morphologicalcondition for lower premolars (Clemens and Lillegraven, 1986), those known for Koko-pellia (p2-3) deserve only passing mention; they are rather simple and lack the antero-basal cuspule sporadically encountered among primitive Theria, Triconodontidae, andprimitive Eutheria. The significance of this cannot be determined with available data.Molars of Kokopellia depart from those of primitive tribotheres (e.g., Kermackia, Pota-motelses), pappotheriids (e.g., Holoclemensia), Deltatheroida, and Eutheria in having areduced height differential between the trigonid and the talonid, a broader talonid, a greatsize discrepancy between ml and the remainder of the series [which, in marsupials, maybe due to the retention of a deciduous tooth in this position (Luckett, 1993)], and in thepresence of a labial postcingulid; all of these are characters shared by marsupials (Cifelli,1993a). The relative proportions of trigonid and talonid are rather generalized features,but the labial postcingulid is a more striking character [again, we refer readers to thediscussion by Clemens and Lillegraven (1986)]. As noted in the description, lower molarsof Kokopellia lack the distinctive "twinning" seen in marsupials, where the hypoconulid

252 Cifelli and de Muizon

is lingually displaced; nonetheless, the labial postcingulid, and the attachment of thehypoconulid to the entoconid via a connecting crest, gives the appearance that the twoare integrated, suggesting a possibly protomarsupial condition.

DISCUSSION

Evidence presented elsewhere (Cifelli, 1993a) and summarized above, includingpostcanine tooth formula and morphology of the lower molars, suggests placement ofKokopellia near or within Marsupialia, depending on how that group is defined. Thesedata, coupled with features of the upper molars (relative development of the protocone,conules, and their associated cristae), also suggest that Kokopellia is closer to conven-tional or generally accepted marsupials than are Deltatheroida, regardless of whether ornot that group is regarded as a sister taxon of Marsupialia within the Metatheria. On thispoint, existing data have already been presented and variously argued, as have the def-inition and diagnosis of Marsupialia (Kielan-Jaworowska and Nessov, 1990; Marshalland Kielan-Jaworowska, 1992; Cifelli, 1993a, b; Muizon, 1994, 1998; Cifelli et al.,1996b). Further analysis and, hopefully, resolution await new information from the fos-sil record.

With respect to mammals generally recognized as belonging to Marsupialia (e.g.,common ancestor of Iqualadelphis, Stagodontidae, remaining Cretaceous and all Ter-tiary taxa, and all of its descendants), Kokopellia appears to retain a number of primitivefeatures. These include the absence of several traits present in many advanced marsu-pials, such as the lack of staggering in the lower incisor series, twinning of the entoconidand hypoconulid on the lower molars, and stylar cusps other than the stylocone [alsoseen in Anchistodelphys (variably) and several unnamed taxa from the Cedar MountainFormation]. Other primitive features include the retention of a larger paracone thanmetacone (also seen in Iqualadelphis, Anchistodelphys, and Aenigmadelphys), a rathersmall protocone (also seen in the same three taxa), median placement of the conules, alabial mandibular foramen, and possibly the retention of a meckelian groove (also seenin Pucadelphys). Summarizing data now available, Kokopellia appears to be more prim-itive than any described marsupial and bears no known autapomorphies that would barit from ancestry of all other Marsupialia.

We defer judgment on the affinities of Asiatherium, recently described from theLate Cretaceous of Mongolia (Szalay and Trofimov, 1996); such an evaluation must bebased on all available information, and our present concern is the dentition and dentaryonly. However, in view of the biogeographic interest raised by Asiatherium, we offerthe following comments in the context of our interpretation of dental and mandibularcharacters in tribosphenic mammals. The postcanine dental formula (p3 m4) is the derivedcondition found in marsupials and deltatheroidans. The molars bear a number ofmarsupial-like features that vary in degree of ambiguity, the most significant of whichwe believe to be the presence of a labial postcingulid (also seen in some Eutheria butassumed herein to represent a synapomorphy of Marsupialia); strong, winged conules(also developed within Eutheria and Deltatheroida); small Ml (though the difference insize is less than generally encountered in marsupial taxa from the North American Cre-taceous); proportions of the trigonid and talonid; and twinnning of the hypoconulid tothe entoconid.

Primitive Marsupial from the Medial Cretaceous 253

In a number of other features, the dentition of Asiatherium seems to be autapo-morphic: the protocone is strongly trenchant (though it is not particularly short, mesio-distally) and bears strong pre- and postcingula (not known in deltatheroidans and presentin some Cretaceous marsupials, though generally not so well developed; present in manyEutheria, if not characteristic of that group); and the stylar shelf is reduced, especiallyits anterior part, where the stylocone and preparacrista are lacking altogether, althoughthe parastyle is well developed (not seen in Deltatheroida, encountered in pediomyidmarsupials, and common in Eutheria—although unusual in combination with a retainedposterior part of the stylar shelf). The paraconid is shorter than the metaconid (the oppo-site of the situation in Deltatheroida; a short paraconid is generally the condition inEutheria, though it is not lingually placed and is generally placed much closer to themetaconid; North American Cretaceous marsupials generally have the two cusps of moreor less equivalent height, though the range of variation includes the condition seen inAsiatherium). Within the context of the North American radiation of Cretaceous mar-supials, Asiatherium is relatively primitive in retaining a paracone taller than metacone(similar to the condition encountered in Kokopellia, Aenigmadelphys, and Iqualadelphis,for example), in the somewhat labial placement of the conules, and in the apparentabsence of a stylar cusp in the D position (the condition in Kokopellia). In the presenceof the twinned hypoconulid and entoconid, Asiatherium is similar to uncontested mar-supials of the North American Late Cretaceous and is more advanced than Kokopellia.

To sum up, we can identify no derived features of Asiatherium shared with Del-tatheroida, except the postcanine dental formula (also seen in Marsupialia); indeed,available evidence (e.g., relative heights of paraconid and metaconid) suggests entirelydifferent specialization between the two, and in many respects (e.g., development ofconules and protocone, trigonid to talonid proportions) Asiatherium is distinctly moreadvanced, even disregarding its apparent autapomorphies. Although Asiatherium is quiteunlike marsupials known from the North American Cretaceous (as recognized by Szalayand Trofimov, 1996), we know of no features of the dentition that would preclude arelationship to or within Marsupialia, which in fact is corroborated by several derivedcharacters (e.g., dental formula, labial postcingulid, twinning of hypoconulid to ento-conid). Within the context of the North American Mesozoic radiation, Asiatherium isclearly rather primitive, though it bears at least one derived feature not seen in Koko-pellia.

The fundamental character complex uniting Tribosphenida is an upper molar pro-tocone that occludes with a multicusped, basined lower molar talonid (McKenna, 1975).Yet compared with the many groups of Mammalia (herein arbitrarily defined as the lastcommon ancestor of Haldanodon, Morganucodon, and Mus, plus all of its descendants)known from the Mesozoic—e.g., triconodonts, docodonts, symmetrodonts, eupanto-theres, multituberculates, and tribotheres—marsupials and eutherians share many otherresemblances in the dentition. One challenge presented by the growing number of newdiscoveries in the Cretaceous System is the evaluation of the extent to which these resem-blances either bear evidence of recency in common ancestry or were acquired indepen-dently in Eutheria and Marsupialia. Several examples are illustrated by comparison ofProkennalestes and Kokopellia. The former taxon has poorly developed, unwinged con-ules that appear to be more primitive than in certain tribotheres (e.g., Cifelli, 1997) anda scar for the coronoid bone; Kokopellia, on the other hand, retains a rather small pro-

254 Cifelli and de Muizon

tocone and lacks stylar cusps other than the stylocone. Both taxa are presumably prim-itive in retaining a labial mandibular foramen and, possibly, a trace of the meckeliangroove (uncertain in Kokopellid). If these taxa are assumed to be members of (or closerelatives to) Eutheria and Metatheria, respectively, then it would appear that someresemblances, at least, were acquired independently between the two groups (e.g.,Cifelli, 1993b). Possibilities along these lines become staggering if one accepts unionof Deltatheroida and Marsupialia into a monophyletic Metatheria (Kielan-Jaworowskaand Nessov, 1990). Late Cretaceous deltatheroidans such as Deltatheridium are char-acterized by a remarkable number of primitive features, including the small protoconeand talonid, presence of a distal metacristid, and lack of double-rank postvallum/pre-vallid shearing. Early Cretaceous Kielantherium, also generally referred to the Delta-theroida, is far more primitive still. Indeed, in certain illustrations (e.g., Kielan-Jaworowska et al., 1979, Fig. 10.1), its status as a tribosphenidan appears debatable;this may challenge the concept of a monophyletic Tribosphenida (see also Kielan-Jaworowska, 1994). As the cases of Kuehneotherlum (see Rougier et al., 1996) andHaldanodon (see Lillegraven and Krusat, 1991) illustrate, homoplasy evidently playeda significant role in the evolution of Mesozoic mammals, serving to underscore the needfor additional basic data from the fossil record.

ACKNOWLEDGMENTS

This collaborative research was made possible by a "professeur associe"' appoint-ment for R.L.C. at the Laboratoire de Pal6ontologie, Museum National d'Histoire Natu-relle, Paris, and we gratefully acknowledge Dr. P. Taquet for facilitating our work.Partial funding for support of the project was provided by the National Geographic Soci-ety (Grants 4761-91 and 5021-92) and the National Science Foundation (Grants BSR8906992 and DEB 9401094). We thank S. K. Madsen for his remarkable work in pre-paring the specimens; Madsen, E. M. Cifelli, K. S. Smith, R. L. Nydam, and E. Millerfor their efforts in field- and lab work; and N. Czaplewski for helping to draft Fig. 1.We are especially grateful to J. Alexander for providing us with a cast of Eodelphisbrowni. Our continuing thanks go to the Judd family of Castle Dale, Utah, for logisticsupport in the field area and T. Rasmussen, San Rafael Resource Area, for the continuingcooperation and good will of the U.S. Bureau of Land Management.

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