Journal of Mammalian Evolution, Vol. 6, No. 3, 1999
A New Marsupial from the Early Eocene Tingamarra Local Fauna of Murgon, Southeastern Queensland: A Prototypical Australian Marsupial?
Henk Godthelp,l Stephen Wroe,1,2 and Michael Archerl
Djarthia murgonensis, a new genus and species of marsupial from the early Eocene Tingamarra Local Fauna of Murgon in southeastern Queensland, is described on the basis of dental material. The combination of marsupial synapomorphies and symplesiomorphies present in D. murgonensis suggests phylogenetic placement within either Didelphidae or Australidelphia. Tarsal morphology, fundamental to the concepts of Ameridelphia and Australidelphia respectively, is not yet known for this taxon. Consequently, it cannot be assigned to either clade with confidence. If this taxon is australidelphian, it constitutes support for the hypothesis that the common ancestor of the Australian marsupial radiation was didelphoid-like in dental features. Some previous authors have contended that marsupial faunas of South America and Australia are manifestly distinct, excepting for the australidelphian affinity of South American microbiotheres. However, because tarsal anatomy is unknown in some generalized Australian fossil taxa, including D. murgonensis, and character analysis reveals that no synapomorphies of the dentition unequivocally define either Ameridelphia or Australidelphia to the exclusion of the other, we consider this interpretation to be premature. In short, available evidence neither supports nor refutes the argument of distinct South American and Australasian marsupial faunas. A further ramification is the need to reconsider the phylogenetic position of Ankotarinja tirarensis and Keeuna woodburnei. These central Australian fossil taxa might be referred to either Australidelphia or Ameridelphia, and it is recommended that both be treated as Marsupialia incertae sedis until further material comes to light. KEY WORDS.. Djarthia murgonensis; Eocene; marsupial; Tingamarra; Australia.
INTRODUCTION Only in Australia have marsupials diversified to fill a breadth of niches comparable to those of the placental mammal radiations which currently dominate other continents. Discoveries in Riversleigh, northwestern Queensland, have shown that the diversity of Australia's marsupial fauna was even greater from the late Oligocene to middle Miocene (Archer et al., 1994). Most investigators now accept that the ancestor of this most sue1
Vertebrate Palaeontology Laboratory, School of Biological Sciences, University of New South Wales, N.S.W., Sydney, Australia, 2052. 2 To whom correspondence should be addressed. e-mail: [email protected]
unsw.edu.au 289 1064-7554/99/0900-0289$16.00/0 @ 1999 Plenum Publishing Corporation
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cessful marsupial radiation was an unspecialized australidelphian. Unfortunately, reconstruction of the prototypical Australian marsupial has been constrained by the complete absence of early Tertiary mammal-bearing deposits in Australia. Historically, because generalized dasyurids have been considered to exhibit many plesiomorphic features, relative to other Australian marsupials, dasyuromorphians have been treated as structurally ancestral within the Australian marsupial radiation (Bensley, 1903; Ride, 1964; Szalay, 1993, 1994). However, while the interpretation of plesiomorphy may be defensible for dental features, Archer (1976a) has shown that dasyurids possess numerous derived features in the basicranium, and Wroe (1996, 1997a, 1999) has identified well-supported synapomorphies for the family based on the analysis of basicranial data. Among Australian clades Peramelemorphia is another possible sister taxon to all remaining taxa. The phylogenetic position of bandicoots within the Australian radiation is enigmatic and the focus of perennial debate (Abbie, 1937; Kirsch, 1977; Hall, 1987; Kirsch et al., 1991). Some authors have proposed, on the basis of molecular data, that peramelemorphians may represent a taxon basal to all other Australian marsupials or, possibly, the entire marsupial radiation (Kirsch et al., 199 1; Springer et al., 1994, 1996; Retief et al., 1995). Several American taxa have been advanced as ancestral or structurally ancestral to the Australian marsupial radiation, including Peratherium (Bensley, 1903), Mirandatherium [specifically compared with Ankotarinja tirarensis by Marshall (1987)1, and Andinodelphys (Marshall et al., 1990). Szalay's (1982) grouping of microbiotheriids within Australidelphia, based on tarsal bone morphology, has impacted strongly on most subsequent interpretations, although some are diametrically opposed to Szalay's hypothesis (Reig et al., 1987; Hershkovitz, 1992, 1995). Recent attempts to synthesize data from various character systems into formalized higher-level systematic classifications which include the Australian marsupials are those by Aplin and Archer (1987), Marshall et al. (1990), Hershkovitz (1992), and Szalay (1993, 1994). Aplin and Archer (1987), although accepting inclusion of microbiotheriids within Australidelphia, were noncommittal regarding the affinities of the clade to any particular Australian taxa. They based their position on irresolvable conflict between various character systems. Development of a large rostral tympanic process of the petrosal and loss of the intestinal cecum (Hume, 1982) support a relationship between microbiotherians and a dasyuromorphian-notoryctid clade, while an investigation of spermatozoan morphology (Temple-Smith, 1987) found potential synapomorphies between Dromiciops australis and some diprotodontians (phalangerids, burramyids, petaurids). Serological data then available provided conflicting evidence, with Kirsch (1977) failing to find evidence for a special relationship between D. australis and the Australian marsupial radiation and Sarich (personal communication to Archer) suggesting the opposite. Marshall et al. (1990) treated Microbiotheria as the sister taxon to Andinodelphys, pwith the latter being the sister [email protected]
the Australian marsupial radiation. They interpret 13 putative dental and tarsal features and the results of molecular studies as supportive of a special relationship between microbiotherians and all other australidelphians. A further four putative dental synapomorphies were advanced as evidence supporting an affinity between Andinodelphys and Australian marsupials. Hershkovitz (1992) considered dasyuromorphians to be basal within the Australian clade and derived from colonizing didelphimorphians. Fundamental to Hershkovitz's view is his rejection of the concept of Australidelphia and a special relationship between Aus-
tralian marsupials and microbiotheriids. Within his classification, dasyuromorphians are allied with didelphimorphians by synapomorphies that unite them to the exclusion of microbiotheriids and stem marsupials. These synapomorphies include contraction of the incisor field with the 13 and its alveolus crowded into a staggered position and loss of the entotympanic bone. Hershkovitz (1 992, p. 21 1) dismissed the growing battery of data uniting Australidelphia (Szalay, 1982; Sharman, 1982; Temple-Smith, 1987; Gallardo and Patterson, 1987; Marshall et al., 1990; Kirsch et al., 1991) as "erroneously founded phylogenies, shared primitive characters, or parallelisms." Szalay (1993, p. 237) considers microbiotheriids and dasyuromorphians as the sister taxon to all remaining australidelphians and that the "protodasyuromorphian" would have exhibited unspecialized ameridelphian dental morphology. This interpretation provides little hope of identifying the ancestral Australian marsupial on dental features alone but is consistent with Szalay's opinion that the relatively narrow spectrum of dental morphology shown by unspecialized omnivorous-insectivorous marsupials is an unreliable indicator of higher level phylogeny. Szalay (1994, p. 348) reinforces this position and considers that Microbiotheria-Dasyuromorphia (his Gondwanadelphia) originated "from an animal which, prior to its acquisition of its apomorphies, any modem taxonomist would not hesitate to allocate to the Didelphidae." The diversity of interpretations offered among predominantly morphology-based attempts to unravel higher level australidelphian phylogenetics is mirrored by the incongruence between various phylogenies proposed on the basis of biochemical data. Controversy exists regarding the phylogenetic placement of microbiotheriids, peramelemorphians, and dasyuromorphians, although some broad areas of agreement are apparent. Most molecular-based studies have concurred about the monophyly of Dromiciops australis, the only extant microbiotheriid, with other australidelphians. But consensus regarding the position of D. australis within this clade has not been achieved. Westerman and Edwards (1 99 1), using DNA hybridization, found no special relationship between D. australis and Australian marsupials, with an unresolved trichotomy between microbiotheres, didelphoids, and the Australian clade. Gemmell and Westerman (1994), using 12S RNA sequence data, found evidence allying D. australis with the Australian marsupial radiation but considered that their data set was too small to clarify interordinal relationships. Dyzenchauz et al. (1993), in a comparison of Australian and South American species, found that autosomic G-banding patterns allied D. australis more closely with diprotodontians than any other clade. However, the two didelphids included in this study (Thylamys and Micoureus) were found to be closer to D. australis than the single dasyurid examined, S. crassicaudata. Analysis of 12S ribosomal DNA sequences (Springer et al., 1994) placed D. australis within Australidelphia but could not determine whether or not this taxon represented the sister taxon to Diprotodontia or Dasyuridae. Regarding the phylogenetic placement of peramelemorphians, Baverstock et al. (1990), Kirsch (1977), and Maxon et al. (1975), based on studies of albumin serology, interpreted a dasyuromorphian-peramelemorphian clade to be the sister taxon to all other Australian marsupials. In contrast, Kirsch et al. (1991), Springer et al. (1994), and Retief et al. (1995) suggest that peramelemorphians may be distinct from all other australidelphians and possibly all other extant marsupials. Kirsch et al. (1997, p. 255), in a review of past DNA hybridization studies, conclude that bandicoots "form part of an unresolved quadrichotomy with didelphids, caenolestids and the remaining [marsupial] taxa." The
Godthelp, Wroe, and Archer
position of microbiotheriids and dasyuromorphians within the phylogenies produced by these authors varies. Kirsch et al. (1991) advocated placement of Dasyuromorphia as the sister taxon of Microbiotheriidae-Diprotodontia (DNA hybridization). Retief et al. (1995), based on a study of protamine P1 genes, suggested that D. australis may be the sister clade to all Australian marsupials (within which dasyurids form the basal taxon) or allied with dasyurids and notoryctids. In short, although considerable headway has been achieved over recent years, consensus has not been attained regarding either the systematic identity or structural detail of the prototypical Australian marsupial. On the basis of molecular evidence, the stem Australian marsupial might be found among unspecialized ameridelphians, microbiotherians, or peramelemorphians. Most attempts to reconstruct the Australian marsupial morphotype on the basis of synapomorphies have relied on dental attributes and are confounded by the conservative morphology of unspecialized dasyuromorphians and their close phenetic similarity to a variety of generalized ameridelphians. This point was formally recognized by Ride (1964) who placed all didelphoids, borhyaenoids, and dasyuroids in a single paraphyletic order, Marsupicamivora. The timing and mode of Australian marsupial origins also remain contentious. Although most authors accept that Australian marsupials are descendant from a South American taxon that entered via Antarctica [Kirsch (1984) formerly argued the case for an Australian origin of marsupials], the period over which dispersal remained feasible is uncertain. Certainly it would appear that access was barred by around 35 million years ago (mya) (Veevers, 1991; Archer et al., 1995), and Woodbume and Case (1996) argue that the dispersal of marsupials to Australia occurred prior to 64 mya. Estimates based on molecular studies for the separation of Australian marsupials from ancestral South American taxa vary from between 103 and 128 mya (Richardson, 1988) to 61.7 mya (Springer et al., 1996). Most recently Janke et al. (1997) reported an estimated divergence for American and Australian marsupials at 75 mya. For different reasons, Godthelp et al. (1992), Marenssi et al. (1994), Archer et al. (1993), and Woodbume and Case (1996) have argued that filter barriers to dispersal were in effect even in the earliest Tertiary. The new genus and species of didelphoid-like marsupial, from deposits of early Tertiary age in southeastern Queensland, described below, simultaneously shed new light on, and raises new questions about, the origins of Australian marsupials. Dental nomenclature follows Flower (1867) and Luckett (1993) regarding the molar-premolar boundary, such that the adult (unreduced) postcanine cheektooth formula of marsupials is PI-3 and MI-4. Dental terminology follows Wroe (1999). Systematic terminology follows that used by Wroe (1996, 1997a). Specimens are currently held at the University of New South Wales, where they are on loan from the Queensland Museum. Specimen numbers are denoted by the prefix QMF. SYSTEMATIC PALEONTOLOGY Supercohort MARSUPIALIA Illiger, 1811 Djarthia, New Genus Type Species. Djarthia murgonensis, new genus and species. Diagnosis. A small, probably insectivorous marsupial characterized by the follow-
ing combination of features: stylar cusps A, B, C, and D well developed; stylar cusp B higher than subequal cusps C and D, which are higher than A; parastyle projects anteriorly to overlie posterobuccal portion of preceding tooth; paracone smaller and lower than metacone for M1-3; metacone well developed on M4 but slightly smaller than paracone; postparacrista-premetacrista form V-shaped centrocrista; "central cusp" positioned at buccal extremity of centrocrista; complete anterior cingulum; protoconule and metaconule large; metacristid oriented transversely to long axis of dentary; anterior point of termination for cristid obliqua is buccal to the "carnassial notch" of metacristid; entoconid tall and conical, higher than hypoconulid which extends posteriorly; and distinct anterior and posterior cingulids with notch in anterior cingulid for reception of hypoconulid. The presence of a "central cusp" clearly distinguishes Djarthia murgonensis from all other marsupials, excepting Ankotarinja tirarensis and Keeuna woodburnei. Djarthia murgonensis can otherwise be distinguished from these two central Australian taxa by the presence of better-developed stylar cusps, protoconules, and metaconules. Etymology. Djarthia is the Koolaburra Aboriginal word for elder sister (Shirley, 1897). Djarthia murgonensis, New Species
Diagnosis. As for the genus. Etymology. The species is named after the closest town to the type locality, Murgon. Holotype. QM F31458. Left dentary fragment with M1-3; isolated partial right and left C1; isolated P3 and P2 or Pl; isolated right M1-4; left maxillary fragment with P3 and M1; and left maxillary fragment with M3. Paratypes. Specimens QM F31459 (left dentary fragment with P2-3, M1, alveoli for M2, M3, and the trigonid of M4 and QM F31460 (right dentary fragment containing P2-3, M1-3, and the trigonid of M4). Age and Locality of Material Tingamarra Local Fauna. Latitude, 26oS; longitude, o 152 E. Main Quarry, property J. and M. Porter, boat mountain area, Murgon, southeastern Queensland, Australia. The early Eocene age deduced for this site is based on the results of radiometric dating, which has provided a minimal age estimate of 54.6 ± 0.05 x 106 million years (Godthelp et al. 1992), as well as biocorrelative data. Woodburne and Case (1996) have disputed this interpretation, suggesting an Oligocene age for the deposit, but failed to address the well-documented support provided by biocorrelation. Taxa common to the Tingamarra Local Fauna and Late Cretaceous or Early Tertiary sites of undisputed age include madtsoiid snakes (Scanlon, 1993), archaeonyctoid bats (Hand et al., 1994), meckosuchine crocodylimorphs (Willis et al., 1993; Salisbury and Willis, 1996), and graculavid birds (Boles, 1997). In our view, this evidence constitutes overwhelming support for the minimum age suggested by K-Ar dating. Description Upper Dentition. No incisors are preserved. Cl is markedly recurved and compressed on the transverse axis (Fig. IG). A well-developed wear facet is evident along the anterior margin of the tip. P3 has two roots (Fig. ID). Its dominant cusp forms a near-equilateral triangle in
Godthelp, Wroe, and Archer
Fig. 1. A, D-1, Djarthia murgonensis gen. et sp. nov. Holotype, QM F31458. (A) Left dentary fragment with M1-3, occlusal view; x 13. (B, C) Djarthia murgonensis gen. et sp. nov. Paratype, QM F31460, right dentary fragment containing P3, M1-3, and M4 trigonid in buccal (B) and occlusal (C) views; x 13. (D) Right P3; x 15. (E) P2 or P3, buccal view; x 1 5. (F, F') Right M1-4, stereo occlusal view; x 13.7..(G) Partial right C, buccal view; x 14.8. (H) Left maxillary fragment with P3 and M1, occlusal view; x 8.9. (1) Right M1-4, lingual view; x 14.
lateral view and is symmetrical along the longitudinal axis in occlusal view (Fig. 1H). Both the L and the R P3 of the holotype are unworn. Well-defined blades descend from the principal cusp both anteriorly and posteriorly. The tooth only slightly exceeds the height of the highest cusp of M1 (metacone). A distinct but small cusp is present anterobasally and a larger cusp is present posteriorly. The apex of the posterior cusp is continuous with buccal and lingual cingula, which circumscribe the base of the crown to a point level with the posterior root. An isolated premolar of the holotype (Fig. 1E), which is smaller than the P3, may represent P1 or P2. This tooth is otherwise similar to P1 except in showing a slightly recurved lateral profile. M1 has three roots. In order of decreasing height, the principal cusps are metacone, stylar cusp B, stylar cusp C, stylar cusp D, stylar cusp A, and paracone (the latter two being approximately equal in height), protocone, metaconule, and protoconule (Figs. 1F, H, I). The protocone is dorsoventrally elongate and broadest ventrally and shows some anteriorly directed torsion. In anterior view the cusp is oriented buccally. The pre and postprotocrista meet at an angle that is less than 450. Both the metaconule and the protoconule are well developed but the metaconule is slightly larger. In occlusal view these cusps contribute significantly to the talon's total surface area. The metacone is about twice the height of the paracone. The latter cusp shows considerable wear on both the R and the L M1 of the holotype. Stylar cusp A is distinct, but smaller and lower crowned than M other stylar cusps and connected to stylar cusp B by a weakly developed blade. A parastyle projects anteriorly (Fig. 1 H), overlying the posterobuccal portion of P3 . A notch in the anterior cingulum is present anteroventral to stylar cusp A. Stylar cusp B is both the largest stylar cusp in occlusal view and the tallest in lateral view. A weakly developed posteriorly directed blade connects this cusp with stylar cusp C, the next tallest cusp. Stylar cusp C shows slight transverse compression. From the occlusal tip of this cusp a well-developed blade runs posteriorly to a notch that separates C and D. Stylar cusp D is slightly lower crowned than C, shows further compression on the transverse axis, and is the longest stylar cusp anteroposteriorly. A continuous blade runs from the notch dividing cusps C and D to the occlusal tip of the latter cusp and then on to the metastylar corner of the tooth. The ectoflexus is very weak, with the deepest lingual incursion evident anterior to the anteroposterior midpoint of the tooth. An additional cusp is present immediately lingual and basal to stylar cusp C. Blades emanate anterolingually and posterolingually from the tip of this "central cusp," which are continuous with the postparacrista and premetacrista (i.e., the centrocrista). Together these blades form a distinct "V" shape in occlusal view. The lingual face of the central cusp shows wear, presumably the product of occlusion with the M1, hypoconid. The postmetacrista is approximately three times the length of the preparacrista and represents the dominant vertical shearing crest of M1. The preparacrista runs to the base of stylar cusp B. A continuous anterior cingulum descends the anterior face of the tooth, connecting stylar cusp A with the preprotocrista. No posterior cingulum is evident. On both M2 and M3 the protocone is broken away, with only the metaconules remaining. M2 differs from M1 in the following ways: the paracone is larger but it is still smaller than the metacone; stylar cusp B is much larger (about twice as large); the ectoflexus is better developed and positioned midway along the stylar shelf, the "central cusp" is larger and the blades show more wear; and the metaconule is about twice as large in occlusal view (Figs. 1F, 1).
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M3 can be distinguished from M2 as follows: stylar cusp D is shorter on the anteroposterior axis and the posterior blade is not continuous with the metastylar corner of the tooth; the ectoflexus is better developed; the paracone is slightly bigger; and the preparacrista is longer, approximately two-thirds the length of the postmetacrista. M4 differs from M3 as follows: the metacone is slightly smaller than the paracone; of the four stylar cusps, the two largest, positioned midway along the shelf, are probably C and D; stylar cusp B is tiny; the preparacrista is longer and continuous with a tiny cusp A; the metaconule is smaller than the protoconule; and the tooth is approximately two thirds as long. Meristic gradients from M1-4 include the following: preparacrista length increases from M1-4; postmetacrista length increases from M1-2, is about equal for M2 and M3, then decreases dramatically for M4; paracone height increases from M1-3, then decreases for M4; metacone height increases from M1-2, With M2 and M3 approximately equal, then decreases for M4; stylar cusp B height increases from M1-2 then decreases from M3-4; stylar cusp C size is roughly uniform for M1-3 but greatly reduced on M4; and the degree of ectoflexus increases from M1-3, then decreases for M4. Dentary. Description of the dentary is based on QM F31458 (holotype), QM F31459, and QM F31460. Its morphology is unknown anterior to the alveolus of the posterior root for P1, as preserved in QM F31460, or posterior to the basal portion of the ascending ramus of the coronoid process, as preserved in QM F31458 (holotype). This bone tapers anteroposteriorly, from a point ventral to M4 in both occlusal and lateral views (Fig. 1B). What remains of the ascending ramus of the coronoid process slopes anteroposteriorly at about 45o to the horizontal plane. A mental foramen is preserved in each of the three dentaries. However, its position is variable, being located beneath the rnidpoint of P3 in QM F31458 (holotype) and the anterior root of P2 in QM F31459 and QM F31460. Lower Dentition. The morphology of P1-3 and M1-4 is described on the basis of QM F31458 (holotype), QM F31459, and QM F31460 (Figs. IA-C). The morphology of the lower incisors, canine, and P1 is unknown. P2 has two roots. No substantial diastema separates the tooth from the alveolus of the posterior root for P1. In lateral view the anterior face of P2 is strongly convex; posteriorly the crown is concave, the latter terminating posteriorly in a strongly developed heel. In occlusal view the tooth is symmetrical on the long axis and no distinct blades are apparent. P3 differs from P2 in being more robust, and slightly higher crowned but anteroposteriorly shorter, and in showing a distinct cristid which connects the principal cusp with the heel. No diastemata separate P3 from P2 or M1. In M1, principal cusps in order of decreasing height are the protoconid, metaconid, entoconid, paraconid, hypoconid, and hypoconulid. The trigonid basin is clearly higher than that of the talonid but the talonid is longer than the trigonid. The internal angle formed between the protocristid and the metacristid is less than 90o. In combination the latter two blades form the dominant vertical shearing system of the trigonid. The metacristid runs almost parallel to the transverse axis of the dentary and an acute interior angle is evident between it and the protocristid. The cristid obliqua contacts the posterior face of the trigonid at a point buccal to the "carnassial" notch of the metacristid. An angle of about 45o is formed between the hypocristid and the cristid obliqua. The entoconid is a tall, conical cusp with no blades. The low crowned, posteriorly projecting hypoconulid is received posteriorly by a distinct notch in the anterior cingulid of M2. In
occlusal view the hypocristid is transverse to the long axis of the dentary and runs to the base of the hypoconulid. A distinct posterior cingulid traverses the posterior face of the talonid from a point immediately basal to the hypoconulid to the distobuccal corner. An anterior cingulid is present but there is no buccal cingulid. M2 is similar to M1, except as follows: the trigonid is wider relative to the talonid and the internal angle formed by the protocristid and metacristid is more acute, the hypoconid and the three cusps of the trigonid are larger, and the metacristid is both longer and more closely aligned with the transverse axis of the dentary. M3 differs from M2 as follows: the trigonid is wider, being approximately equal to the talonid in the transverse dimension; the protoconid shows further hypertrophy relative to the paraconid and metaconid; and the hypoconid is slightly smaller. Complete M4 morphology is known only from QM F31458 (holotype) which shows relatively heavy wear, but in QM F31459 and QM F31460 the M4 trigonid is preserved. This tooth can be distinguished from M3 by the following differences: the talonid is reduced on both the long and the transverse axes; all three talonid cusps are smaller, but remain distinct; and no posterior cingulid is evident, but because a distinct wear facet is present on the posterior face, it is possible that this feature was lost through abrasion. Meristic gradients from M1-4 are that the interior angle formed between the protocristid and the metacristid becomes More acute from M1 to M4; trigonid width increases from M1 to M3, decreasing for M4; talonid width increases from M1 to M2, then decreases for M3 to M4; and protoconid, metaconid, and paraconid height increase from M1-3 then decrease on M4. Character Analysis Method. Outgroup data have been taken from all plausible potential sister groups. Some basal metatherian and ameridelphian taxa are known to us on the basis of published material only, including Asiatherium reshetovi, Aenigmadelphys archeri, and Kokopellia juddi. However, the excellent and detailed descriptions for these taxa provided by Szalay and Trofimov (1996), Cifelli and Johanson (1994), Cifelli (1993), and Cifelli and Muizon (1997), respectively, enable data for these taxa to be included in the following character analysis. Measurements were made using a Wild NIMS 235 Digital Length-Measuring Set attached to a Wild M5A Stereomicroscope. Postcanine Tooth Formulae and Replacement. The presence of three premolars and replacement of only one postcanine tooth are widely acknowledged as marsupial or metatherian synapomorphies (Marshall et al., 1990; Luckett, 1993; Muizon, 1994; Cifelli et al., 1996; Szalay and Trofimov, 1996; Rougier et al., 1998). Whether or not this postcanine tooth homology is treated as a synapomorphy for the taxon is dependent on the phylogenetic position accepted for deltatheroideans (see Kielan-Jaworowska and Nessov, 1990; Cifelli, 1993; Szalay and Trofimov 1996; Rougier et al., 1998). Among marsupials many specialized carnivorous and herbivorous taxa show further reduction of the premolar tooth row, but all peradectids, rnicrobiotheriids, and peramelemorphians, as well as generalized didelphimorphians and dasyuromorphians, retain three premolars. Djarthia murgonensis is plesiomorphic within Metatheria for this feature. Tooth replacement pattern is unknown for D. murgonensis. Relative Size of P2 and P3. The distribution of this feature among Marsupialia is considered in detail by Archer (1976b) and Wroe (1996, 1997a, b). Both authors con-
eluded that the presence of a P3 which is higher crowned than or very similar in height to P2 represents a probable plesiomorphy for the clade. Djarthia murgonensis shows the plesiomorphic state for marsupials, with P3 slightly higher crowned than P2. Size of M3 Relative to M4. Marshall et al. (1990, p. 442, node 39) treat reduction of M4 relative to M3 as an australidelphian synapornorphy, but it is unclear to which particular dimension(s) the authors refer. In their analysis of relationships among American "opossum-like" marsupials, Reig et al. (1987, p. 13) treat the relative Sizes Of M3-M4 and M3-M4 as separate characters. An M4 anteroposteriorly reduced relative to M3, but equal or subequal to M3 on the transverse dimension (width), was treated as plesiomorphic and an M4 reduced in size and width relative to M3 was treated as apomorphic for Marsupialia. In the lower dentition an M4 subequal in length to, or anteroposteriorly longer than M3 was considered plesiomorphic, while an M4 narrower and moderately shorter than M3 and an M4 markedly shorter and narrower than M3 were treated as successively more derived states. In the present study we follow the character state definitions of Reig et al. (1987), although we consider it likely that some features of the upper and lower dentitions form character complexes. Peradectids, most didelphoids, some dasyuromorphians (e.g., Neophascogale) and peramelemorphians (e.g., Yarala bunchfieldi), and Djarthia murgonensis show an M4 that is anteroposteriorly shorter than M3 but equal or subequal to M3 on the transverse dimension. In most dasyuromorphians and peramelemorphians, M4 size and width relative to M3 are clearly reduced from the plesiomorphic state. Microbiotheriids show a degree of reduction in M4 size and width relative to M3 which exceeds that of any dasyuromorphian or peramelemorphian. In the lower dentition an M4 subequal in length to, or anteroposteriorly longer than, M3 is present in peradectids, most didelphoids, some dasyuromorphians (e.g., species of Neophascogale, Thylacinus cynocephalus), and Djarthia murgonensis. An M4 moderately shorter and narrower than M3 is present in most dasyuromorphians and peramelemorphians. A marked reduction of M4 relative to M3 occurs in Microbiotheriidae. No dasyuromorphians or peramelemorphians show a reduction of these dimensions comparable to that seen in microbiotheriids. Djarthia murgonensis exhibits the plesiomorphic marsupial condition regarding both the length and width of M4 relative to M3 and the length and size of M4 relative to M3. We find no support for the application of reduction of M4 relative to M3 as an australidelphian synapomorphy as suggested by Marshall et al. (1990), because unspecialized dasyuromorphians are plesiomorphic within Marsupialia regarding gross dimensions of these teeth. Length of the M4 Preparacrista Relative to that of M3. M3 and M4 preparacristae are both well developed in generalized marsupials. In absolute terms the M3 and M4 preparacristae are of near-equal length in peradectids, some ameridelphians (e.g., Pucadelphys andinus), and some dasyurornorphians (e.g., species of Neophascogale). Peramelemorphians, Djarthia murgonensis, and most didelphoids and dasyuromorphians have an M4 preparacrista that is clearly longer than that of M3. In microbiotheriids the M4 preparacrista is much shorter than that of M3. Marshall et al. (1990, p. 438, node 44) treat elongation of the M4 preparacrista as a dasyuromorphian synapomorphy. We consider the outgroup data to be highly equivocal for this feature. Hypertrophy of this shearing crest is found in derived didelphoids, dasyurids, and thylacinids, but unspecialized extant dasyurids such as species of Neophascogale and Phascolosorex show an M4 preparacrista clearly shorter than that of M3. The character state shown by Ankotarinja tirarensis can be
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reasonably inferred from the relative lengths of the M3 and M4 rnetacristids, the occlusal partners of the M3 and M4 preparacristae in the lower dentition. In this unspecialized dasyuromorphian the M4 preparacrista is notably shorter than that of M3, Strongly suggesting the presence of a relatively short M4 preparacrista. An M4 preparacrista that is longer than that of M3 is a derived marsupial feature in Djarthia murgonensis, which could indicate affinity with any one of several clades, including specialized dasyurids, thylacinids, and didelphoids. Relative Size of the Paracone and Metacone. In some Cretaceous marsupials the paracone exceeds the metacone in size. In microbiotheriids and most peradectids the metacone is roughly equal to or only slightly larger than the paracone in size. Djarthia murgonensis, Andinodelphys cochabambensis, and all taxa referred to Peramelemorphia, Didelphidae, and Dasyuromorphia have a metacone which is significantly larger than the paracone. Within the latter two clades advanced specialization is apparent in many carnivorous taxa (e.g., species of Sparassocynus, Thylacinus, Sarcophilus), wherein the size differential between these two cusps is further accentuated. The size of the paracone relative to the metacone has been considered a phylogenetically indicative feature by many students of marsupial evolution. The paracone is slightly higher crowned than the metacone in Asiatherium reshetovi (Szalay and Trofimov, 1996), although these authors consider that at least some hypertophy of the metacone, relative to the condition present in potential eutherian outgroups, is ubiquitous for Cretaceous Metatheria. Cifelli and Johanson (1994) observe that the paracone is larger than the metacone in their description of Aenigmadelphis archeri material originally referred to Iqualadelphis lactea, by Cifelli (1990b), was also included in this new taxon. Aenigmadelphis archeri is considered to be a marsupial of uncertain suprageneric status by Cifelli and Johanson (1994). The paracone is larger than the metacone in Albertatherium primus (Cifelli, 1990b; Johanson, 1994) and Kokopellia juddi (Cifelli and Muizon, 1997). Cifelli (1993) considers hypertrophy of the metacone to be a possibly derived feature within Marsupialia and treats the development of a "metacone equal to or exceeding paracone in size" as a synapomorphy uniting Alphadon and Turgidodon. Interestingly, Marshall et al. (1990, p. 442) treat the presence of a metacone larger than the paracone as a synapomorphy uniting Andinodelphys cochabambensis with the Australian marsupial radiation to the exclusion of microbiotheriids. This same character state is also considered to be synapomorphic for Polydolopimorphia-Didelphimorphia and Sparassodonta, requiring acceptance of independent derivation for this feature at least three times within Marsupialia. The presence of a paracone that is higher crowned than the metacone is probably a marsupial plesiomorphy, with marked hypertrophy of the metacone relative to the paracone clearly derived within the clade. Djarthia murgonensis is derived relative to peradectids and microbiotheriids, showing a metacone distinctly larger than the paracone, but no more derived than unspecialized didelphoids,and dasyuromorphians. Dilambdodonty Versus Predilambdodonty. Various authors have assigned phylogenetic significance to the development of dilambdodonty among marsupials, following the suggestion of Crochet (1980; cited from Reig et al., 1987) that the presence of this feature distinguishes didelphids from peradectids. Crochet (1980), Reig et al. (1987), Marshall and Muizon (1988), and Marshall et al. (1990) have applied "linear and V shaped centrocrista" as synonyms of "pre-dilambdodonty and dilambdodonty," respectively. As
noted by Cifelli (1990a) this usage is not, strictly speaking, correct: "dilambdodonty" describes the condition of the preparacrista and postmetacrista in conjunction with the postparacrista and premetacrista (centrocrista), wherein these four crests produce a Wshaped sequence of blades. A predilambdodont condition is evident in most peradectids, Andinodelphys cochabambensis, and all Microbiotheriidae. Dilambdodonty is present in all didelphoids, peramelemorphians, dasyuromorphians, a specimen initially referred to Alphadon (Cifelli and Johanson, 1994) and later to Alphadon jasoni (Johanson, 1996a), and Djarthia murgonensis. Cifelli (1990a, b) observes that dilarnbdodonty appears to have evolved more than once among South Arnerican marsupials. Marshal] et al. (1 990, p. 438) treat acquisition of dilambdodonty as an independently derived synapomorphy for both Polydolopoidea-Didelphimorphia and the Australian marsupial radiation. In short, this feature has been independently derived at least three times within Marsupialia and its value in unifying higher marsupial taxa must be viewed with some skepticism, unless additional and less equivocal support is identified from examination of other character states. The degree of subjectivity and inconsistencies apparent in distinguishing between "predilambdodonty" and "dilambdodonty" are another cause for concern. At exactly which point within a transformation series "predilambdodonty" becomes "dilambdodonty" is open to interpretation. For example, we concur with Reig et al. (1987) in considering Caluromys as dilambdodont, but Szalay (1994) does not. Moreover, Marshall and Muizon (1995, p. 32) describe this feature as a "weak W-shaped structure in occlusal view" in Pucadelphys andinus and place the taxon in Didelphidae [this species has been recently placed in the new didelphimorphian family Pucadelphydae by Muizon (1998). Marshall (1987, p. 143) considers that Mirandatherium shows a "centrocrista nearly rectilinear," thereby supporting its placement within Microbiotheriidae, but Marshall et al. (1990, p. 461) reinterpret the same material to conclude that "... Mirandatherium has a weakly developed V-shaped centrocrista" and consequently treat it as a didelphid; Marshall and Muizon (1988, p. 35) describe Andinodelphys cochabambensis as showing a "weakly V-shaped" centrocrista and consider the species to be a didelphine, but Marshall et al. (1 990) and Woodbume and Case (1 996) observe that A. cochabambensis retains the plesiomorphic state for this feature, i.e., a "linear centrocrista," and treat the taxon as an australidelphian. Thus, in some cases, the same authors appear to have alternatively treated a "weakly V-shaped" centrocrista as representing either dilambdodonty or predilambdodonty, even for the same taxa, and based on the same specimens. Consequently, both the phylogenetic value of this character as used by these authors and their conclusions based on the distribution of this feature are compromised. We consider Djarthia murgonensis to be dilambdodont and derived within Marsupialia for this feature, which may constitute a synapomorphy with any one of at least three marsupial clades. Size of the Protoconule and Metaconule. Both the protoconule and the metaconule are distinct, well-developed cusps in most Cretaceous marsupials, unspecialized didelphoids (e.g., Pucadelphys andinus), Djarthia murgonensis, and many dasyuromorphians, including unspecialized dasyurids. Metaconules and/or protoconules are absent in some Peradectidae, many extant didelphids, microbiotheriids, peramelemorphians, and some dasyuromorphians. Archer (1976b) observed that, for dasyurids, loss of the protoconule is more common than loss of the rnetaconule and considered the presence of both conules to be a marsupial plesiomorphy. Marshall et al. (1990, p. 442) treat the
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condition of the metaconule being larger than the protoconule (their paraconule), with the protoconule "greatly reduced," as a synapomorphy uniting Andinodelphys cochabambensis and the Australian marsupial radiation. However, Marshall and Muizon (1988, p. 34) described A. cochabambensis as showing "paraconules and metaconules well developed." Outgroup data for D. murgonensis are equivocal for this feature. But, because both conules are well developed in unspecialized didelphoids and the closest likely ingroup taxon (Dasyuromorphia), we consider the presence of large protoconules and metaconules in D. murgonensis to be a retained marsupial plesiomorphy. We find no support for the contention that marked reduction of the protoconule is synapomorphic for a clade comprised of Andinodelphys cochabambensis and the Australian marsupial radiation. Relative Size of Stylar Cusps B and D. Distribution and form-function data for this feature among Marsupialia were reviewed by Wroe (1996, 1997a). The presence of stylar cusps B and D, with B the larger, is widely accepted as plesiomorphic for marsupials (Fox, 1987; Cifelli, 1993) and is common to peradectids, the late Cretaceous Asiatherium reshetovi (Szalay and Trofimov, 1996), Aenigmadelphys archeri [(considered to approximate the ancestral marsupial morphotype by Cifelli and Johanson (1994), Andinodelphys cochabambensis, most unspecialized didelphoids, and Djarthia murgonensis. Stylar cusp D is larger than B in some didelphoids, e.g., Lestodelphys halli and some specimens referred to Herpetotherium (Rotheeker and Storer, 1996), the basal peramelemorphian Yarala burchfieldi (Muirhead and Filan, 1995), unspecialized thylacinids (e.g., Muribacinus gadiyuli), all dasyurids, and Dasyuromorphia incertae sedis. Gross reduction of the stylar shelf characterizes a number of marsupial clades including Pediomyidae, Microbiotheriidae, Myrmecobiidae, and specialized Thylacinidae. The presence of a stylar cusp B that is larger than D probably represents a retained marsupial plesiomorphy in D. murgonensis. Stylar Cusp, or Cusps, Present or Absent in the C Position. Stylar cusp C is absent in a number of taxa considered to be prototypical marsupials (Fox, 1987; Marshall et al., 1990; Cifelli and Johanson, 1994). The consistent presence of stylar cusp C is common to most Alphadon, Djarthia murgonensis, some pediomyids, and unspecialized didelphoids and dasyurids. This cusp is absent in some Alphadon, most pediomyids, some specialized didelphoids, and all microbiotheriids, peramelemorphians, thylacinids, and carnivorous dasyurids. Twinned cusps in the C position are reported in some representatives of the didelphid taxon Herpetotherium from middle Eocene deposits of Saskatchewan (Rotheeker and Storer, 1996), Andinodelphys cochabambensis, undescribed Late Oligocene taxa referred to Peramelemorphia by Marshall et al. (1990), and some dasyuromorphians incertae sedis (e.g., Keeuna woodburnei). Archer (1975) reported the presence of twinned, as well as extra stylar cusps of uncertain homology, in a number of atypical dasyurid specimens. Recent authors have contended that stylar cusp C was absent in the ancestral marsupial (Fox, 1987; Marshall et al., 1990; Cifelli and Johanson, 1994; Szalay and Trofimov, 1996). Marshall et al. (1990) suggested that independent acquisition of a cusp in the C position has occurred at least five times within Marsupialia. Johanson (1996a, b) suggests that stylar cusp C has arisen independently within Alphadontinae. If so, then the minium estimate for independent evolution of this feature among marsupials is six times. We further note that independent reduction or loss of this feature characterizes many marsupial clades (e.g., Pediomyidae, Microbiotheriidae, Borhyaenidae Thylacinidae, and specialized Dasyuridae). The presence of twinned cus-
pules in the C position has been forwarded as a synapomorphy uniting A. cochabambensis with the Australian marsupial radiation by Marshall et al. (1990) or, more specifically, with undescribed fossil bandicoot taxa from central Australia. We are highly skeptical of this view. Marshall et al. (1990, p. 454), in the consideration of material referred to Pediomys, noted that a "preliminary examination of taxonomically diverse examples suggests that a proliferation of cuspules between stylar cusps B and D is a precursor condition to the development of a distinct, single stylar cusp C." In this context the character state evident in A. cochabambensis and some undescribed Peramelemorphia might be treated as symplesiomorphous. The presence of twinned cusps in the C position evident in Herpetotherium of middle Eocene age (Rotheeker and Storer, 1996) further complicates arguments for a special relationship between Andinodelphys and Australian marsupials. Moreover, Muizon (1992) considers the presence of this feature to be variable in A. cochabambensis and the specimen figured by Muizon et al. (1997) shows no twinned stylar cusps in the C position. Thus, the presence of this feature is polymorphic in A. cochabambensis and may represent a potential symplesiomorphy, synapomorphy, or homoplasy for both it and some late Oligocene peramelemorphians. The presence of a well-developed cusp C in Djarthia murgonensis represents a derived feature within Marsupialia, which may indicate affinity with any one of perhaps five marsupial taxa. Central Cusp Morphology. No cusp is present at the juncture of the postparacrista and premetacrista in the vast majority of marsupial taxa considered in the present study. In at least one peradectid (i.e., Alphadon rhaister) a small cusp is present at the base of and immediately posterior to the paracone and stylar cusp B. However, in this species the cusp does not abut the apex of the centrocrista. Djarthia murgonensis, Keeuna woodburnei, and Ankotarinja tirarensis may be unique among marsupials in showing a cusp in this position, lingual to stylar cusp C. This central cusp is particularly well developed in unworn specimens of Djarthia murgonensis. In Ankotarinja tirarensis and Keeuna woodburnei this cusp is less well developed. But to what degree this reflects wear or genetic factors cannot be determined at present because all known specimens of these central Australian taxa are moderately to heavily worn. This feature may serve an autocclusal function (sensu Mellet, 1984) in capturing the hypoconid of the corresponding lower molar or provide a point-cutting or puncturing device subtended by the postparacrista and premetacrista. Presence of this feature is regarded as a possible synapomorphy uniting these three taxa. Anterior Cingulum Morphology. A complete anterior cingulum on M1, connecting the preprotocrista with the anterobuccal cingulum, is common to peradectids (e.g., Turgidodon and Alphadon), some didelphoids, microbiotheriids, unspecialized peramelemorphians (e.g., Yarala burchfieldi and Echymiperra), Djarthia murgonensis, and generalized dasyurids and thylacinids. Reduction or loss of this feature is evident in some didelphoids (e.g., Didelphis marsupialis) and derived dasyurids and thylacinids (e.g., species of Dasyurus and Thylacinus). This feature is considered a marsupial plesiomorphy by Archer (1976b), a view which is accepted in the present study, with Djarthia murgonensis showing the plesiomorphic state. Relative Widths of Talonid and Trigonid on M2-3. In most peradectids for which quantitative data concerning this feature are available, talonid width exceeds that of the trigonid for M2 but not for M3 (e.g., Iqualadelphis lactea, Protoalphadon wahweapensis, Turgidodon lillegraveni, T. madseni, Alphadon sahnii, A. marshi). This observation also
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applies to some didelphids (e.g., Lestodelphys, Thylatheridium), Yarala burchfieldi, and most dasyuromorphians, including a single specimen of Ankotarinja tirarensis (F7331), as measured by Archer (1976b). In some peradectids (e.g., Alphadon attaragos) and didelphids (e.g., Metachirus nudicaudatus), niicrobiotheriids, most peramelemorphians, Djarthia murgonensis, and some dasyuromorphians, including a single specimen of Ankotarinja tirarensis (P18190), as measured by Archer (1976b), the talonid is wider than the trigonid on both M2 and M3. For most of these taxa the differential between M3 trigonid versus talonid width is slight, not readily apparent to the naked eye and is clearly evident to the authors only with the aid of an electronic measuring device (see above), but rnicrobiotheriids, most peramelemorphians, and Wakamatha tasselli show unmistakable hypertrophy of the M3 talonid on the transverse dimension. Considerable variation is apparent regarding these dimensions for M3 within Marsupialia, but the presence of an M2 in which the talonid width exceeds that of the trigonid is widespread. The presence of an M3 talonid which is slightly narrower than the trigonid appears to be plesiomorphic for marsupials. Slight expansion of the M3 talonid on the transverse dimension, relative to the trigonid, has probably developed within a number of clades. Djarthia murgonensis shows very slight modification of the hypothesized ancestral marsupial condition, which may indicate affinity with some didelphoids, microbiotheriids, peramelemorphians, or dasyuromorphians. Marshall et al. (1990, p. 438, node 39) treat the condition in which the talonid is wider than the trigonid on M2-3 as a synapomorphy for Australidelphia. We consider the evidence for this assertion to be weak. The degree of M3 talonid widening evident in microbiotheriids must be considered synapomorphic for the family, unless reversal is accepted for unspecialized dasyuromorphians and Yarala burchfieldi. It remains possible that the very slight transverse expansion of this feature in Djarthia murgonensis, the ancestral microbiotheriid, and Wakamatha tasselli could represent a synapomorphy uniting these taxa. However, we are reluctant to confer any great phylogenetic significance on this feature at such a high taxonomic level, given the variability shown within the clades considered, even within species (e.g., Ankotarinja tirarensis). Relative Lengths of Trigonids and Talonids. Some peradectids (e.g., Alphadon), didelphids (e.g., species of Caluromys) have talonids that are clearly longer than the trigonids for all lower molars. Many marsupial taxa have almost equal trigonid and talonid lengths, including some didelphids (e.g., species of Didelphis), some dasyuromorphians (e.g., Ankotarinja tirarensis) and peramelemorphians (e.g., Peroryctes), and Djarthia murgonensis. For these taxa slight variation in the subjective delimitation of the trigonid-talonid boundary may result in the species considered receiving either "trigonid longer than talonid" or "talonid shorter than trigonid" status. In some peradectids (e.g., Peradectes), most didelphids, Yarala burchfieldi, and all dasyuromorphians excepting A. tirarensis, the talonids of the lower molars are clearly shorter than the trigonids. Marshall et al. (1990) and Marshall and Muizon (1995) treat the presence of a "trigonid shorter than talonid" as a polydolopimorphian--didelphimorphian synapomorphy. Interestingly, Reig et al. (1987, p. 13) consider "talonid of Mx, longer than trigonid" to be the plesiomorphic state for marsupials, the opposite conclusion reached by Marshall et al. (1990). Following consideration of the above character state distribution and given the apparent lack of consensus among previous investigators concerning the plesiomor-
phic marsupial state for this feature, we consider the phylogenetic value of this feature to be doubtful at this taxonomic level. Djarthia murgonensis may be apomorphic or plesiomorphic among marsupials regarding the relative length of talonids and trigonids. Orientation of the Metacristid. In peradectids, most didelphoids, peramelemorphians, Ankotarinja tirarensis, Wakamatha tasselli, most sminthopsine dasyurids except some Planigale, and Djarthia murgonensis, the metacristids are transverse to the long axis of the dentary. Some didelphoids (e.g., Mirandatherium), microbiotheriids, and most dasyuromorphians show oblique alignment of the metacristids. Archer (1976b, c) considered transverse orientation of the metacristid to be plesiomorphic for Marsupialia. This interpretation is favored in the present study. Djarthia murgonensis shows the plesiomorphic state. For Australidelphia this interpretation requires acceptance of independent derivation of oblique metacristid alignment within Microbiotheriidae and Dasyuromorphia. Anterior Cingulid Morphology. An anterior cingulid is present on the lower molars of nearly all taxa which have been included in the present character analysis. However, this feature shows marked hypertrophy, producing a roughly "square" outline in occlusal view for the lower molars in some didelphids (e.g., Didelphis, Monodelphis, Lestodelphys), all peramelemorphians, and Wakamatha tasselli. Anterior cingulid hypertrophy is absent for all other taxa treated in this study, including Djarthia murgonensis. Because basal marsupials, didelphoids, and Andinodelphys cochabambensis do not show hypertrophy of this feature, it is concluded that this character state, evident in D. murgonensis, is plesiomorphic. Reig et al. (1987) also infer that this state is plesiomorphic for marsupials. However, the presence of a very welldeveloped anterior cingulid in all peramelemorphians and Wakamatha tasselli casts doubt on this conclusion. Nevertheless, we consider it most parsimonious to regard development of anterior cingulid hypertrophy as a peramelemorphian synapomorphy. Wroe (1996) discusses the possibility that W tasselli may represent an unspecialized peramelemorphian. Anterior Point of Terinination of the Cristid Obliqua. In Kokopellia juddi, considered a basal marsupial by Cifelli (1993), and alphadontines, the cristid obliqua contacts the posterior face of the trigonid beneath the "carnassial notch" of the metacristid from M1-3 In didelphoids, microbiotheriids, Djarthia murgonensis, most dasyuromorphians in which the metaconid is retained (i.e., the M1 or M1-4 metaconids are lost in some derived dasyurines and thylacinids), and the unspecialized peramelemorphian Yarala burchfieldi, the cristid obliqua contacts the posterior face of the trigonid at a point buccal to this notch for M1-3. In some dasyurids a buccal position is evident for M1-2, but placement beneath the carnassial notch is evident in M3 (e.g., Sminthopsis granulipes). Peramelemorphians other than Y. burchfieldi show an anterior point of termination for the cristid obliqua which is lingual to the carnassial notch (Muirhead and Filan, 1995). Marshall et al. (1990, p. 438, node 39) regarded the condition in which the cristid obliqua meets the "rear of the trigonid labial to protocristid notch" as synapomorphic for Australidelphia and Polydolopimorphia-Didelphimorphia, respectively. Archer (1976b) considered buccal placement to be a marsupial plesiomorphy. Following consideration of the above character distribution, it is likely that Marshall et al. (1990) are correct in considering a buccal placement to be apomorphic within Marsupialia. However, we consider the proposal that this character state necessarily represents an independently derived feature for Australidelphia and Polydolopimorphia-Didelphimorphia to be poorly founded. The lin-
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gual position of this feature in most peramelemorphians is also derived among Marsupialia. Djarthia murgonensis is apomorphic for this feature relative to prototypical marsupials but this derivation may indicate a special relationship with either Didelphimorphia, Australidelphia, or a single taxon inclusive of both clades. Twinning of the Hypoconulid and Entoconid. The vast majority of marsupials, including Djarthia murgonensis, shows twinning of the entoconid and hypoconulid. Historically, this feature has been regarded as a marsupial synapomorphy. Recently it has been suggested that close approximation of these cusps was not present in the prototypical marsupial (Cifelli, 1993; Szalay and Trofimov, 1996; Cifelli and Muizon, 1997). his interpretation is dependent on whether or not Kokopellia juddi from the early Cretaceous of North America is considered a marsupial. Because all undoubted Marsupialia show this feature, its presence in D. murgonensis is treated as plesiomorphic within the clade. Relative Height of the Hypoconulid and Entoconid. In some peradectids (e.g., Turgidodon masdeni) and didelphoids (e.g., Pucadelphys andinus), the hypoconulid closely approximates the height of the entoconid. In some peradectids (e.g., Alphadon marshi) and didelphids (e.g., Didelphis marsupialis) and all microbiotheriids, peramelemorphians, dasyuromorphians (excepting some specialized taxa in which the entoconid is lost or greatly reduced), and Djarthia murgonensis, the hypoconulid is significantly tower crowned than the entoconid. Marshall et al. (1990, p. 438, node 5) treat "hypoconulid lower than entoconid" as a synapomorphy for PolydolopimorphiaDidelphimorphia. However, Marshall and Muizon (1995, p. 35) refer Pucadelphys andinus to Didelphidae and observe that "in lateral view of mi-3 all three talonid cusps are subequal in height." In light of the variability of this feature at a basal level within Marsupialia, we consider the presence of a hypoconulid lower than the entoconid to be of dubious phylogenetic significance at the interordinal level. If, however, the polarity decision of Marshall et al. (1990) is assumed to be correct and the relatively low height of the hypoconulid present in D. murgonensis is considered to be a metatherian apomorphy, it could represent a potential synapomorphy allying D. murgonensis with some didelphoids, australidelphians, or some peradectids. DISCUSSION Of the 18 characters considered above, Djarthia murgonensis is probably plesiomorphic among metatherians for at least 8: retention of three premolars; a P3 higher crowned than P2; M4 anteroposteriorly shorter than M3, but equal or subequal to M3 on the transverse dimension, and an M4 subequal in length to, or anteroposteriorly longer than, M3; well-developed protoconules and metaconules; well-developed stylar cusps B and D, with B the larger of the two; continuous anterior cingula; metacristids oriented at an angle transverse to the long axis of the dentary; and moderately welldeveloped anterior cingulids. A ninth character state, twinned hypoconid and entoconid, may represent a derived feature within Marsupialia. But if accepted as such, the presence of this feature in D. murgonensis only unites the species within a taxon which includes all marsupials to the exclusion of Kokopellia juddi. Outgroup data are too equivocal to permit confident assessment of polarity for two characters, the length of the trigonids compared to that of the talonids and the relative heights of the entoconids and hypoconulids. Seven remaining features evident in D. murgonensis appear to be derived within Marsupialia. Of these, six are
present in at least some representatives of both Ameridelphia and Australidelphia: hypertrophy of the M4 preparacrista relative to that of M3, marked reduction of the paracones relative to the metacones, dilambdodonty, stylar cusp C present, slight expansion of the M3 talonid relative to that of the trigonid, and termination of the cristid obliqua at a point buccal to the metacristid notch. Two of these features, the presence of dilambdodonty and stylar cusp C, are also shared with at least some peradectids. One apomorphic character state, the presence of a "central cusp" at the apex of the centrocrista, is shared only with Keeuna woodburnei and Ankotarinja tirarensis among marsupial taxa known to the authors. The results of character analysis indicate that Djarthia murgonensis represents the least derived Australian marsupial known. On the basis of dental evidence, this taxon could be considered an unspecialized representative of either Ameridelphia or Australidelphia. There can be little doubt that most investigators would treat this taxon as a didelphoid were it found in South America. A single derived feature, the presence of a "central cusp," might be construed as indicative of a special relationship between D. murgonensis and two Australian taxa (Keeuna woodburnei and Ankotarinja tirarensis). However, even if this character state is considered to be a reasonable basis for the inference of monophyly for D. murgonensis, K. woodburnei, and A. tirarensis, the arguments both for and against australidelphian affinity of such a clade are weak. Wroe (1996) allocated K. woodburnei and A. tirarensis to Australidelphia and Dasyuromorphia because they shared character states identified by Marshall et al. (1990) as australidelphian or dasyuromorphian. These were twinned cusps in the C position and Vshaped centrocrista, and talonids reduced relative to trigonids. But the results of the character analysis in the present study engender little confidence in the value of these features at high taxonomic levels. Consequently, the position of these two species, which have been formally or informally allied with various Australian and American taxa by previous authors, has become still less certain. It is suggested that both be considered as Marsupialia incertae sedis until less equivocal evidence comes to hand. These problems in more precisely classifying D. murgonensis draw attention to the difficulties which confront students of Australian marsupial origins. At present, no synapomorphies of the cranium or dentition (possibly excepting reduction of lower incisor number for an Australian clade) clearly define either the Australidelphian or the Australian marsupial radiations to the exclusion of Ameridelphia, contra Marshall et al. (1990). In addition to other features considered above, the presence of upper incisors that are spatulate and lower incisors that are reduced in number have been forwarded as synapomorphies for Australidelphia and an Australasian clade, respectively, by Marshall et al. (1990). Neither of these features is known for D. murgonensis. Furthermore, while spatulate upper incisors are present in D. australis and many dasyuromorphians, peg-like morphology is present in some dasyurids (Archer, 1976b), and Reig et al. (1987) suggested a high likelihood of homoplasy for this feature. As cautioned by Archer (1984), Aplin and Archer (1987), Reig et al. (1987), and Luckett (1994), the highly derived nature of known microbiotheriids precludes confident reconstruction of the plesiomorphic state for the family. Reduction of the lower incisor number, from four to three or fewer, is common to all Australian marsupials and constitutes the best morphology-based support for monophyly of an Australian clade. But even this specialized feature is present in some derived borhyaenoids, caenolestoids, and polydolopoids among ameridelphians.
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Woodbume and Case (1996, p. 121) observe that "... except for South American microbiotheres being australidelphians, marsupial faunas of South America and Australia still are fundamentally disjunct" and both Woodburne and Case (1996) and Marshall et al. (1990) maintain that the prototypical australidelphian possessed recognizably microbiotherian rather than didelphimorphian dental features. This view is not supported by the results of the present analysis. Many of the putative synapomorphies for Australidelphia, the Australian marsupial radiation, or Dasyuromorphia forwarded by Marshall et al. (1990) are undoubtedly derived within Marsupialia. However, of the seven synapomorphies defining Polydolopimorphia-Didelphimorphia proposed by Marshall et al. (1990, p. 438), at least four also apply at basal nodes for Australidelphia: V-shaped (dilambdodont) centrocrista, paracone smaller and lower than metacone, cristid obliqua meets posterior wall of trigonid buccal to protocristid notch, and hypoconulid lower than entoconid. Of the remaining three synapomorphies applied at this node, we consider outgroup data too equivocal to permit reliable polarization for one, "trigonid shorter than talonid." The remaining two, "entoconid tall, spire-like" and "posteriorly expanded protoconal base," are too subjective to be of value. Further, we find no support for the suggestion by Marshall et al. (1990, p. 442) that the presence of "reduction of M4 relative to M5" (i.e., M3 and M4 here) and "talonid wider than trigonid on M3-4" (M2-3 here) represent australidelphian synapomorphies or that the presence of a "Metaconule > paraconule, which is highly reduced" (paraconule = protoconule in the present study) represents a synapomorphy uniting Andinodelphys with the Australian marsupial radiation. Another suggested synapomorphy for the Australian marsupial radiation forwarded by Marshall et al. (1990, p. 442), "well-developed squamosal epitympanic sinus covered by pars flaccida of tympanic membrane," cannot be applied at this phylogenetic level, unless its complete absence in the following taxa are treated as a reversals to a plesiomorphic state: Yalkaparidon coheni (Archer et al., 1988), basal extant peramelemorphians such as Echymiperra kalubu and Peroryctes longicauda, and a single dasyuromorphian, Badjcinus turnbulli (Muirhead and Wroe, 1998; Wroe et al., i998). Similarly, M5 preparacristae elongated" (M4 in the present study) cannot be considered as a dasyuromorphian synapomorphy as suggested by Marshall et al. (1990) because it is not a feature of unspecialized dasyurids. Marshall (1987), Marshall et al. (1990), and Woodburne and Case (1996) have flagged dasyuromorphian and undescribed peramelemorphians from late Oligocene (Woodbume et al., 1993) deposits of central Australia as possible structural intermediates between specific South American and Australian clades. Marshall (1987, p. 143) postulates a special relationship between Ankotarinja tirarensis and Mirandatherium based on the shared presence of the following features: trigonids moderately elevated above talonids; M2-4 (M1-3 in the present study) talonids distinctly basined and wider than trigonids; M5 reduced compared to M4 (M4 and M3 in the present study); stylar shelf moderately developed, centrocrista nearly rectilinear, stylar cusps B and D moderately well developed and subequal in size; cusp C present but smaller than cusps B and D; and ectoflexus on M5 (M4 here) moderately deep and metacrista only moderately developed. Which, if any, of these features represent derived character states is not indicated and it is unclear whether or not this hypothesized special relationship is founded on overall similarity or synapomorphy. Possible synapomorphies of A. tirarensis and Mirandatherium, among those listed above, are effectively relegated to homoplasious status by Marshall
et al. (1990), wherein Mirandatherium is reinterpreted as a didelphine. Mirandatherium is considered a didelphid by Reig et al. (1987) and a microbiotheriid by Marshall (1987). Marshall et al. (1990) forward the Paleocene Andinodelphys cochabambensis, from the Tiupampa local fauna of Bolivia, as a potential sister taxon to the Australian marsupial radiation, comparing it with undescribed bandicoot taxa from the Etadunna Formation of central Australia, which are Late Oligocene in age. Proposed synapomorphies of A. cochabambensis and the Australian marsupial clade are given by Marshall et al. (1990, p. 460) as follows: "... metacone > paracone; metaconule > paraconule, which is highly reduced in size; closely oppressed twinned stylar cuspules in the C position, with the posterior cuspule > than the anterior cuspule in size." Andinodelphys cochabambensis was placed in Didelphinae by Marshall and Muizon (1988) and Muizon (1992). But, as shown above, the first two of these features represent equivocal bases for the postulation of special relationships. The presence of a "metacone > paracone" is also common to unspecialized didelphoids, while a "metaconule > paraconule, which is highly reduced in size" is not present in unspecialized dasyurids. As observed above, Marshall et al. (1990) propose that acquisition of a stylar cusp in the C position has occurred independently at least five times within Metatheria and further suggest that proliferation of cuspules between stylar cusps B and D represents an intermediate stage in the acquisition of stylar cusp C. Given the extent of both temporal and geographic separation, the presence of twinned cusps in the C position in some didelphoids of Eocene age (Rothecker and Storer, 1996), and the demonstrably labile nature of stylar cusp morphology, even at the generic level within some marsupial clades, the likelihood of homoplasy for this feature must be treated as high. Consequently, the postulation of a special relationship between A. cochabambensis and the Australian marsupial radiation is very difficult to justify without special pleading. On the basis of complete new cranial and dental material, Muizon et al. (1997) consider A. cochabambensis to be didelphoid, and Muizon (1998) places it in the new family Pucadelphydae, along with Pucadelphys andinus. Woodburne and Case (1996, p. 144) cite the presence of a continuous ankle joint as the only reliable synapomorphy uniting Australidelphia, but Marshall et al. (1990) propose 10 dental synapomorphies for the clade, in addition to tarsal evidence. These interpretations necessitate acceptance of independent derivation for many dental features at basal nodes within Ameridelphia and Australidelphia. Central to this argument is the concept that the linear centrocrista (predilambdodonty) of microbiotheriids is a retained plesiomorphy for the family (Woodburne and Case, 1996). In the light of difficulties with the subjective interpretation of this feature aside (see above), it is equally, if not more, parsimonious to postulate reversal to a plesiomorphic state for this feature in undoubted microbiotherians, i.e., Dromiciops and Microbiotherium, particularly given general acceptance that these taxa are highly derived for many other dental and cranial characters. It is conceded that the weight of morphological and molecular evidence, currently supports the contention of monophyly of non-microbiotheriids with Australian marsupials; however, the position of this taxon within Australidelphia is far from resolved. Woodburne and Case (1996) propose a phylogenetic scenario in which Peramelemorphia represents the sister taxon to Microbiotheriidae-Dasyuromorphia-Notoryctidae-Diprotodontia. A consequence would have to be homoplasious development of dilambdodonty at least twice within Australidelphia, i.e., for both Peramelemorphia and Dasyuromorphia-NotoryctesDiprotodontia.
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CONCLUSIONS Dental apomorphies in Djarthia murgonensis allow for its referral to either Ameridelphia or Australidelphia. If D. murgonensis is ultimately shown to be australidelphian, it could represent a structural ancestor for the rest of the Australian marsupial radiation and evidence that Australian marsupials were descendant from a taxon that was very didelphoid-like for dental features. However, on the basis of known material, there is, at present, no reason to dismiss the possibility that both ameridelphians and australidelphians were present in the early Tertiary of Australia. The suggestion by Woodburne and Case (1996) that marsupial faunas of South America and Australia are manifestly distinct with the exception of an austmlidelphian affinity for South American microbiotheriids is treated as purely speculative.
ACKNOWLEDGMENTS We would like to thank R. L. Cifelli, W. P. Luckett, and an anonymous reviewer for their constructive criticism and comment on drafts of this article. Work at Murgon has been supported by the Australian Research Council, the Department of Arts, Sport, Environment, Tourism and Territories, National Estate Programme Grants (Queensland), the University of New South Wales, and the Queensland Museum. Funding was also provided by grants to S. Wroe from the following institutions: French Ministry of Foreign Affairs, Linnean Society of New South Wales, Australian Geographic Society, and the Institute of Wildlife Research. Special thanks are extended to C. de Muizon for inviting S. Wroe to study the highly significant fossil material under his care, as well as for his hospitality. Many thanks also to J. Brammall for taking the photographs presented, M.r. and Mrs. Porter for permission to excavate their property, and many volunteers and students including E. Clarke, A. White, J. Muirhead, P. Willis, J. Scanlon, A. Gillespie, and S. Williams for help in the field. The Murgon clays which produced the described material have been processed by S. Williams, V. O'Donoghue, and A. Gillespie. LITERATURE CITED Abbie, A. A. (1937). Some observations on the major subdivisions within the Marsupialia with special reference to the position of the Peramelidae and Caenolestidae. J. Anal. 71: 429-435. Aplin, K., and Archer, M. (1987). Recent advances in marsupial systematics, with a new, syncretic classification. In: Possums and Opossums: Studies in Evolution, M. Archer, ed., pp. xv-lxxii, Surrey Beatty and Sons, Sydney. Archer, M. (1975). Abnormal dental development and its significance in dasyurids and other marsupials. Mem Qld. Mus. 17: 251-265. Archer, M. (1976a). lie basicranial region of marsupicarnivores (Marsupialia), interrelationships of carnivorous marsupials, and affinities of the insectivorous marsupial peramelids. Zool. J. Lin. Soc. 59: 217-322. Archer, M. (1976b). The dasyurid dentition and its relationship to that of didelphids, thylacinids, borhyaenids (Marsupicamivora) and peramelids (Peramelina: Marsupialia). Aust. J. Zool. Suppl. Ser. 39: 1-34. Archer, M. (1976c). Miocene marsupicamivores (Marsupialia) from central South Australia, Ankotarinja tirarensis gen. et. sp. nov., Keeuna woodburnei gen. et. sp. nov., and their significance in terms of early marsupial radiations. Trans. R. Soc. South Aust. 100: 53-73. Archer, M. (1984). Origins and early radiations of marsupials. In: Vertebrate Zoogeography and Evolution in Australasia, M. Archer and G. Clayton, eds., pp. 585625, Hesperian Press, Carlisle. Archer, M., Hand, S., and Godthelp, H. (1988). A new order of zalambdodont marsupials. Science 239: 1528-1531.
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