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Relationships of the Cretaceous mammal Deltatheridium

1971; Cambridge University Press; Volume: 45; Issue: 3 Linguagem: Inglês

1937-2337

Malcolm C. McKenna, James S. Mellett, Frederick S. Szalay,

Morphological variations and asymmetry

The dental formula of the Mongolian Cretaceous mammal Deltatheridium has previously been interpreted to be Is, C1, Ps8, M3s or I_24 C1, P4t, M'. We suggest that it may be I2', C1, P44, M2'ort.. If so, Deltatheridium is not the ancestor of later mammals possessing M' or more than a vestigial Ms, although the genus can still be regarded as an early member of an Asian palaeoryctoid radiation. MESOZOIC MAMMALS were discovered in the Cretaceous Djadochta Formation of southern Mongolia in 1923 and 1925 by the Central Asiatic Expeditions led by Roy Chapman Andrews. Additional collections have been made in southern Mongolia recently by Polish-Mongolian cooperative expeditions (Kielan-Jaworowska, 1969). The Djadochta Formation is famous for its well preserved dinosaur skeletal remains and for well preserved nests of dinosaur eggs, but its fossil mammals have come to occupy an important position in all studies of early mammalian phylogeny and in all studies of the origin of eutherian mammalian dental pattern, function, and formula. Our purpose here is to suggest that in all previous work the dental formula and phylogenetic significance of one of the most often discussed Djadochta genera, Deltatheridium, may have been interpreted on the basis of false assumptions. Deltatheridium was described originally by Gregory and Simpson (1926). They realized correctly that the genus is related to Didelphodus, an early Tertiary American genus now known to be a member of the insectivoran family Palaeoryctidae. In 1926 what are currently classified as American Tertiary palaeoryctids were known chiefly from Palaeoryctes itself and from Didelphodus, but at that time Didelphodus was generally regarded as a leptictid. American Cretaceous palaeorcytids were known but not recognized as such. Much taxonomic confusion has prevailed in this poorly understood group. Gregory and Simpson (1926) tacitly accepted the idea that Deltatheridium was a primitive genus not only in its dental pattern but also in its molar formula. Less than the usual seven postcanine eutherian teeth were present and they therefore knew that reduction had occurred, but, inasmuch as other primitive eutherians possessed three molars, it was assumed that a premolar, P,1, had been lost. Two lower incisors were depicted in figure 4 of Gregory and Simpson's paper, but the dental formula was conservatively concluded to be I?, C11, P33, M33 in the text. Van Valen (1966), as the result of additional preparation, recently presented evidence that would indicate a formula of I24, C11, P4?, M33, based in part upon his observation of four precanine premaxillary alveoli and in part on what appears to us to be a misidentification resulting from a miscount of the number of postcanine lower teeth. He also concluded that Deltatheroides and Hyotheridium, Djadochta genera described at the same time as Deltatheridium and placed by Gregory and Simpson (1926) in the Deltatheridiidae, are not so closely related to Deltatheridium as had previously been thought. Van Valen referred Deltatheroides to the didelphodontine palaeoryctids, but was unable to assign Hyotheridium with confidence to any palaeoryctoid subgroup. We concur with Van Valen's (1966) conclusion that Deltatheridium is significantly different from other Djadochta genera, even under the rubric of a dental formula specifying three molars. At the same time that the Djadochta genera were discovered, a peculiar Paleocene fauna was found nearby at Gashato, Mongolia. Among its endemic components were the genera Sarcodon Matthew and Granger, 1925a, Opisthopsalis Matthew, Granger, and Simpson, 1929, and Hyracolestes Matthew and Granger, 1925a. Work in progress by Szalay and McKenna (in press) demonstrates that Sarcodon (= Opisthopsalis) and Hyracolestes have a postcanine dental formula of P44, M22. The Central Asiatic Expeditions also collected extensively from Eocene and Oligocene deposits in the 1920's, and from these sediments a number of specimens representing the peculiar endemic Asian family Didymoconidae were recovered. Described genera of this family are Ardynictis Matthew and Granger, 1925b, Didymoconus Matthew and Granger, 1924 (= Tshelkaria Gromova, 1960), Kennatherium Mellett and Szalay, 1968, and Mongoloryctes Van Valen,