THE MONGOLIAN LATE CRETACEOUS ASIATHERIUM, AND THE EARLY PHYLOGENY AND PALEOBIOGEOGRAPHY OF METATHERIA

The skull and skeleton of the Mongolian Late Cretaceous (Barungoyotian ) marsupial Asiatherium are described, illustrated, and compared to va rious therian taxa. The study supports the distinctness of Asiatheriur n from deltatheroidans and eutherians, and the metatherian status (sen su stricto) of asiatheriids, based primarily on the dental formula, bu t also on cranial, dental, and postcranial characters. Its postcanine dental formula (three premolars and four molars), found only in dental ly primitive living marsupials, is shown to be a derived condition wit hin the Theria (tribosphenic mammals, sensu stricto). In addition, the closely twinned hypoconulid and entoconid, correlated with a (relativ e) hypertrophy of the metacone, an alisphenoid component to the bulla (possibly an independently derived trait), oval (not elliptical) fenes tra vestibuli, and an elliptical fenestra cochleae, along with other u nmistakenly marsupial-like (and therian as well as pre-therian) postcr anial taxonomic properties, all attest to the noneutherian status of A siatherium. The hypothesis of independent origin of the Metatheria and Eutheria from taxa with molars of an aegialodontan stage of developme nt is not supported from functional-adaptive considerations. The deriv ed therian nature of the primitive metatherian dental formula is suppo rted, and it is argued that deltatheroidans are modified for a hyperca rnivorous diet, as attested by their molar dentition and skull form. T heir narrowed but fully therian molar talonid, like that of various ma rsupial and placental carnivores, appears to be derived from a relativ ely wider one, as the well-developed protocone suggests. Deltatheroida n dental similarities to sparassodontan borhyaenids are considered to be convergent, as suggested by the differences in the crania of these two groups. Deltatheroidans and asiatheriids are separate and primaril y Asiatic clades of Metatheria that suggest a complex and hitherto poo rly understood paleobiogeography for therians in the Cretaceous. The b iogeographic significance of Asian marsupials, compared to the America n, European, and African forms, is that their uniqueness does not prec lude a hitherto unrecognized ancient Cretaceous presence of;Metatheria in the tropics of southern Asia.