First skulls of the Early Eocene primate Shoshonius cooperi and the anthropoid-tarsier dichotomy

Abstract

The phylogenetic relationships of living tarsiers and extinct omomyid primates are critical for deciphering the origin and relationships of primate higher taxa, particularly anthropoids. Three competing phylogenetic hypotheses are: (1) tarsiers are most closely related to early Cenozoic Omomyidae, particularly genera such as Necrolemur from the late Eocene of Europe; (2) tarsiers share a more recent common ancestry with anthropoids than they do with any known omomyid; (3) tarsiers and/or omomyids are most closely related to strepsirhines. The anatomy of four skulls of the early Eocene omomyid Shoshonius cooperi--the first cranial material recovered for this genus--strongly suggests that Shoshonius shares a more recent common ancestry with Tarsius than do either anthropoids or other Eocene omomyids for which cranial anatomy is known. If the primate suborder Haplorhini (anthropoids, omomyids, tarsiids) is monophyletic, the phylogenetic position of Shoshonius requires that anthropoids and Tarsius diverged by at least the early Eocene, some 15 million years before the first appearance of anthropoids in the fossil record.

Frontal Fusion: Collapse of Another Anthropoid Synapomorphy

“…It is impossible to tell if this represents a crest, a ridge, or a crack between the frontal bones. The local anatomy of the Tetonius frontal closely resembles the four new, relatively well-preserved crania attributed to Shoshonius (Beard et al, 1991;Beard and MacPhee, 1994). One of them presents a thin crisp line between the frontals but all are cracked in one way or another.…”

Elucidating geological and biological processes underlying the diversification of Sulawesi tarsiers

“…There is still no consensus about the affinities of extant tarsiers to the Omomyidae, a family of extinct haplorhine primates with an Eocene distribution throughout most of the northern hemisphere (16). Many phylogenetic reconstructions place omomyids as ancestors to tarsiers, giving rise to only tarsiids (17), to independent tarsiid and anthropoid lineages (16), or to a tarsier-anthropoid clade (5,6). Recent evidence, however, suggests a sister group relationship between extant haplorhines and omomyids (18).…”

Cranial remains of an Eocene tarsier

“…Alternatively, if tarsiids are more closely related to omomyids than they are to anthropoids (6)(7)(8)(9)(10)(11), then their hypertrophied eyes and associated facial morphology are simply an exaggerated state of a trend toward enlarged orbits that is already evident in omomyids such as Shoshonius (6,9). The new material of T. eocaenus described in this article indicates that middle-Eocene tarsiers were at least as derived in this respect as the slightly older Shoshonius and far more derived than basal omomyids such as T. asiatica (28).…”

“…Tarsiers and anthropoids exclusively share some degree of postorbital closure, an anterior accessory chamber of the middle ear, and loss of the stapedial artery (1)(2)(3)(4)(5), whereas omomyids and tarsiers share other features not seen in anthropoids, such as a narrow central stem of the basioccipital, a ''peaked'' choanal region, and basioccipital flanges that overlap the auditory bulla (6)(7)(8)(9)(10)(11). The homology of these features is unclear because few cranial specimens are known for early members of these groups, and none are known for fossil tarsiers (12,13).…”