Basal Metatherians

Infraclass METATHERIA

fHoloclemensia, f Sinodelphys Order fDELTATHEROIDA Order fASIADELPHIA Cohort MARSUPIALIA

fKokopellia,1 flugomortiferum,1 fAnchistodelphys1 Magnorder AMERIDELPHIA

Order DIDELPHIMORPHIA fPeradectidae2 Didelphidae fSparassocynidae ?Order DIDELPHIMORPHIA fPediomyidae2 fStagodontidae2 fProtodidelphinae Order PAUCITUBERCULATA

Superfamily fCaroloameghinioidea fGlasbiidae fCaroloameghiniidae Superfamily Caenolestoidea fSternbergiidae Caenolestidae fPalaeothentidae fAbderitidae Superfamily fPolydolopoidea fSillustaniidae fPolydolopidae fPrepidolopidae fBonapartheriidae Superfamily fArgyrolagoidea fArgyrolagidae fGroeberiidae fPatagoniidae Order fSPARASSODONTA fMayulestidae fHathliacynidae fBorhyaenidae fProborhyaenidae fHondadelphidae Magnorder AUSTRALIDELPHIA

Djarthia Superorder MICROBIOTHERIA

Microbiotheriidae

Superorder EOMETATHERIA

Order NOTORYCTEMORPHIA Grandorder DASYUROMORPHIA Grandorder SYNDACTYLI Order PERAMELIA Order DIPROTODONTIA

Notes: Modified after McKenna and Bell (1997). The dagger (f) denotes extinct taxa. Families and genera in boldface in this table are known from the Paleocene or Eocene.

1 Could lie just outside Marsupialia.

2May lie outside Ameridelphia; Case et al. (2004) separated these families together with Caroloameghiniidae in the Alphadelphia, comparable in rank to the magnorders used here. McKenna and Bell (1997) included Peradectidae within Didelphidae.

the extinct carnivorous borhyaenids and their relatives; Aus-tralidelphia comprises South American microbiotheres and all Australian marsupials. Case et al. (2004), mostly following Marshall et al. (1990), restricted Paucituberculata to the caenolestoids, assigning all the other paucituberculate taxa in Table 5.1 to a separate order Polydolopimorphia.

Little over a decade ago, almost nothing was known of the earliest metatherians. Since then, a flurry of new fossil discoveries in North and South America and Asia has expanded our knowledge so rapidly that our notions of primitive metatherians and their phylogeny have been in a state of flux. Several of the recently found Cretaceous forms seem to occupy important roles in the evolution of Metatheria.

One of the most intriguing new fossils is mouse-sized Sinodelphys (Fig. 5.3), the oldest known mammal that is probably closely related to Metatheria. It is based on a skeleton recently found in the 125-million-year-old Lower Cretaceous Yixian Formation of Liaoning Province, China (Luo et al., 2003), the same deposits that produced skeletons of Jeholodens and Zhangheotherium. Sinodelphys differs from other metatherians in dental formula (4.1.4.4/4.1.4.3)—having an equal number of upper and lower incisors, one more premolar, and one less lower molar—and in lacking an inflected mandibular angle. But it has several derived features characteristic of marsupials, including the shape of the upper incisors, closely approximated (but not twinned) hypo-conulid and entoconid, and modifications of the wrist (enlarged scaphoid, hamate, and triquetrum) and ankle (broad navicular, oblique calcaneocuboid joint) that are indicative of arboreal or scansorial habits. Based on this combination of features, Luo et al. (2003) considered Sinodelphys to be a basal metatherian more primitive than Deltatheridium; but its precise phylogenetic position relative to other metathe-rians is uncertain.

Also of note (though not new) is Holoclemensia (see Fig. 4.20), which was established on isolated teeth from the Lower Cretaceous (Albian) Trinity Formation of Texas. Although initially considered a basal metatherian, it was subsequently classified as a "tribothere" (Butler, 1978; McKenna and Bell, 1997), but was once again placed at the base of Metatheria by Luo et al. (2003). Its upper molars have a larger paracone than metacone and a wide stylar shelf with several stylar cusps, including an enlarged cusp C. The lower molars have a tall protoconid, somewhat reduced paraconid, and closely approximated hypoconulid and entoconid (Slaughter, 1971). Cifelli (1993b) considered Holoclemen-sia to represent a structural stage leading to marsupials (see Fig. 5.7).

Deltatheroida

Deltatheroidans were primitive, tribosphenic marsupiallike therians, mainly from the Late Cretaceous of central Asia. There are two families, Deltatheridiidae and Delta-theroididae. Besides having a marsupial-like postcanine formula (three premolars and four molars), Deltatheridium (Fig. 5.4), the best-known deltatheroidan, resembles marsupials in replacing only the last premolar (P3), and in having an inflected mandibular angle (Rougier et al., 1998). The upper molars have a very wide stylar shelf with several cusps. Unlike primitive marsupials, however, different stylar cusps are

Fig. 5.2. Cladogram of metatherian and marsupial relationships, modified after Marshall et al. (1990) and Luo et al. (2002). This arrangement differs in several ways from the one used in this chapter. In particular, in this chapter, Peradectidae and Pediomyidae are included in Ameridelphia and Stagodontidae is not considered to be closely related to borhyaenoids (see Table 5.1).

Fig. 5.2. Cladogram of metatherian and marsupial relationships, modified after Marshall et al. (1990) and Luo et al. (2002). This arrangement differs in several ways from the one used in this chapter. In particular, in this chapter, Peradectidae and Pediomyidae are included in Ameridelphia and Stagodontidae is not considered to be closely related to borhyaenoids (see Table 5.1).

Pediomyidae
Fig. 5.3. (A) Skeleton of the probable basal metatherian Sinodelphys. (B) Hands and feet of Sinodelphys (left) and the basal eutherian Eomaia (right). (From Luo et al., 2003.)

emphasized (A, B, Bp and E; Kielan-Jaworowska, 1975b), the molar talonids are narrow, the hypoconulid and ento-conid are not twinned, and the last molar (M 4) is vestigial. In addition, there is one less incisor above and below (four incisors over three incisors) than in primitive marsupials. These features indicate that Deltatheridium is not a marsupial in the strict sense. Deltatheroidans are usually considered to represent either an independent clade of metatheri-ans (together with Asiadelphia, composing Holarctidelphia; Szalay, 1994), or alternatively to lie just outside of crown therians (Metatheria + Eutheria; e.g., Luo et al., 2002).

Asiadelphia

This higher taxon is based principally on Asiatherium (Fig. 5.5), a mouse-sized animal known from a skull and articulated skeleton from the Late Cretaceous of the Gobi Desert, Mongolia (Szalay and Trofimov, 1996). Like delta-theroidans, Asiatherium resembles marsupials in having three premolars and four molars and a somewhat inflected man-

dibular angle. It is more marsupial-like than deltatheroidans in having paraconids lower than metaconids and twinned hypoconulid-entoconid cusps on the lower molars. However, the stylar shelf of the upper molars is narrower and the stylar cusps much weaker than in primitive marsupials. The upper molars further differ from those of marsupials in having expanded precingula and postcingula, the latter resembling an incipient hypocone shelf. Thus, Asiatherium seems to be related to marsupials, but, like deltatheroidans, it probably belongs to a separate clade of metatherians. The skeleton of Asiatherium is similar to that of generalized terrestrial therians. Epipubic bones are present.

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