Affinities of the Primate Altanius from the Early Tertiary of Mongolia

Abstract

Primates are rare in the early Tertiary record of Asia. The few known species are represented by only one or two fragmentary specimens, and their relationships are controversial (Table 1). Probably the oldest of them is Altantus orlovi, based on an incomplete dentary with P4M1i3 (the only described specimen) from the upper part of the Naran Bulak Formation (early Eocene) at Tsagan Khushu, Mongolia (Dashzeveg and McKenna, 1977). The Naran Bulak faunas are believed to be roughly equivalent in age to the North American Clarkforkian and Wasatchian (latest Paleocene-early Eocene) (Gingerich and Rose, 1977; Dashzeveg and McKenna, 1977; Rose, 1980, 1981). Altanius has been considered one of the oldest known tarsiiform primates (Omomyidae) (Dashzeveg and McKenna, 1977; Szalay and Delson, 1979). Apart from this record, the oldest known tarsiiform is Teilhardina from the early Eocene Sparnacian of Europe and early Wasatchian of North America-a form generally believed to approximate more closely the primitive omomyid morphology (Savage et al., 1977; Szalay and Delson, 1979; Gingerich, 1981). Dashzeveg and McKenna (1977) found the greatest affinity between Altanius and the North American early Eocene anaptomorphine omomyids Anemorhysis (including Tetonoides), Tetonius, and Pseudotetonius, citing particularly the presence in all of them of a high, incipiently blade-like P4. They also observed similarities to the middle Paleocene Elphidotarsius (a member of the archaic primate suborder Plesiadapiformes, which also includes the Plesiadapidae, Saxonellidae, Paromomyidae, Microsyopidae, and Picrodontidae) and stated: carpolestid molars are somewhat exodaenodont, a presumably derived condition shared with the (Dashzeveg and McKenna, 1977:130). Altanius is one of the smallest known primates: its molars are only two-thirds to three-fourths the size of those in Anemorhysis or Elphidotarsius, and it probably weighed no more than 30 g, or about one-third as much as Anemorhysis (see Gingerich, 1981). It differs from both anaptomorphines and carpolestids in having unreduced molar paraconids (fully lingual on M3 and probably on Ma) but closely resembles certain members of each group in numerous other details of dental morphology (Figs. 1 and 2). Our reappraisal of the evidence indicates that significant resemblances of Altanius to Elphidotarsius are at least as numerous and detailed as are those to any omomyid, thus suggesting that the relationships of Altanius lie with Plesiadapiformes rather than with Omomyidae. Particularly important is the structure of Mi. In Altanius its trigonid is high and compact buccolingually; the paraconid and metaconid are well separated. This situation exists also in anaptomorphines and Elphidotarsius. The cristid obliqua on M1 extends anterolingually to the tip of the metaconid, forming a large shelf on the posterior wall of the trigonid (Fig. 1A) and demarcating two offset wear facets (1 and 5) (Dashzeveg and McKenna, 1977: plate 2D). This condition develops to a variable degree in primitive anaptomorphines but it is particularly characteristic of carpolestids (Rose, 1975). The talonid of Ml in Altanius is broad buccolingually but markedly compressed anteroposteriorly (Fig. 1A). The entocristid is very short and steep, the metaconid and entoconid are relatively close together, and the angle between them is decidedly acute. In these features, Altanius is virtually identical to Elphidotarsius, but unlike any known omomyid. The teeth of Altanius are exodaenodont (i.e., the crowns protrude buccally over the edge of the dentary), a derived condition also found in anaptomorphines and carpolestids (Fig. 1). More significantly, the lower molars in Altanius have very high lingual cusps (especially evident on Ms and the talonid of M2, but surely characteristic also of the M2 trigonid, which is broken) that project conspicuously higher than the labial cusps (Fig. 2). This peculiar trait is particularly distinctive of carpolestids but occurs only to a very limited degree or not at all in anaptomorphines. Furthermore, the lower molars of Altanius and carpolestids are generally higher-crowned than in anaptomorphines (Fig. 2). November 1984 721