The Mongolian Late CretaceousAsiatherium, and the early phylogeny and paleobiogeography of Metatheria

Abstract

The skull and skeleton of the Mongolian Late Cretaceous (Barungoyotian) marsupial Asiatherium are described, illustrated, and compared to various therian taxa. The study supports the distinctness of Asiatherium from deltatheroidans and eutherians, and the metatherian status (sensu stricto) of asiatheriids, based primarily on the dental formula, but also on cranial, dental, and postcranial characters. Its postcanine dental formula (three premolars and four molars), found only in dentally primitive living marsupials, is shown to be a derived condition within the Theria (tribosphenic mammals, sensu stricto). In addition, the closely twinned hypoconulid and entoconid, correlated with a (relative) hypertrophy of the metacone, an alisphenoid component to the bulla (possibly an independently derived trait), oval (not elliptical) fenestra vestibuli, and an elliptical fenestra cochleae, along with other unmistakenly marsupial-like (and therian as well as pre-therian) postcranial taxonomic properties, all attest to the noneutherian status of Asiatherium. The hypothesis of independent origin of the Metatheria and Eutheria from taxa with molars of an aegialodontan stage of development is not supported from functional-adaptive considerations. The derived therian nature of the primitive metatherian dental formula is supported, and it is argued that deltatheroidans are modified for a hypercarnivorous diet, as attested by their molar dentition and skull form. Their narrowed but fully therian molar talonid, like that of various marsupial and placental carnivores, appears to be derived from a relatively wider one, as the well-developed protocone suggests. Deltatheroidan 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 primarily Asiatic clades of Metatheria that suggest a complex and hitherto poorly understood paleobiogeography for therians in the Cretaceous. The biogeographic significance of Asian marsupials, compared to the American, European, and African forms, is that their uniqueness does not preclude a hitherto unrecognized ancient Cretaceous presence of Metatheria in the tropics of southern Asia.