An eosimiid primate of South Asian affinities in the Paleogene of Western Amazonia and the origin of New World monkeys

Abstract

Recent fossil discoveries in Western Amazonia revealed that two distinct anthropoid primate clades of African origin colonized South America near the Eocene/Oligocene transition (ca. 34 Ma). Here, we describe a diminutive fossil primate from Brazilian Amazonia and suggest that, surprisingly, a third clade of anthropoids was involved in the Paleogene colonization of South America by primates. This new taxon, Ashaninkacebus simpsoni gen. et sp. nov., has strong dental affinities with Asian African stem anthropoids: the Eosimiiformes. Morphology-based phylogenetic analyses of early Old World anthropoids and extinct and extant New World monkeys (platyrrhines) support relationships of both Ashaninkacebus and Amamria (late middle Eocene, North Africa) to the South Asian Eosimiidae. Afro-Arabia, then a mega island, played the role of a biogeographic stopover between South Asia and South America for anthropoid primates and hystricognathous rodents. The earliest primates from South America bear little adaptive resemblance to later Oligocene-early Miocene platyrrhine monkeys, and the scarcity of available paleontological data precludes elucidating firmly their affinities with or within Platyrrhini. Nonetheless, these data shed light on some of their life history traits, revealing a particularly small body size and a diet consisting primarily of insects and possibly fruit, which would have increased their chances of survival on a natural floating island during this extraordinary over-water trip to South America from Africa. Divergence-time estimates between Old and New World taxa indicate that the transatlantic dispersal(s) could source in the intense flooding events associated with the late middle Eocene climatic optimum (ca. 40.5 Ma) in Western Africa.

Keywords: Brazilian Amazonia; Platyrrhini; paleobiogeography; phylogeny; teeth.

Fig. 1.

Right first upper molar of A. simpsoni gen. et sp. nov. (UFAC-CS 066, holotype) from PRJ-33’ locality, Rio Juruá (Alto Yurúa), State of Acre, Brazil. Buccolingual width: 2.92 mm; mesiodistal length: 2.03 mm. (A) Stereopair in occlusal view; (B) lingual view; (C) distal view; (D) buccal view; and (E) mesial view. Images are renderings of a 3D digital model of the fossil specimen, obtained by X-ray microcomputed (μCT) surface reconstruction (segmented enamel surface).

Fig. 2.

Phylogenetic position of A. simpsoni gen. et sp. nov. in a high-level phylogeny of basal anthropoids (Old World, i.e., Paleogene South Asian and North African known taxa), plus known extinct and extant platyrrhines, deriving from a cladistic assessment of the craniodental and postcranial evidence. Single most parsimonious tree of 2,429.49 steps (consistency index [CI] = 0.36; retention index [RI] = 0.56), which was obtained after analyses performed in considering some ordered and scaled multistate characters, and in applying a molecular scaffold of living taxa relationships (Materials and Methods). The cladogram was then subjected to a Bayesian tip-dating analysis (BTD) for divergence-time estimation between taxa. Abbreviations: Af, Afrotarsiidae; Anth, Anthropoidea; Ap, Amphipithecidae; At, Atelidae; Ca, Catarrhini; Cb, Cebidae; Eo, Eosimiidae; Eos, Eosimiiformes; Ho, Homunculidae; Og, Oligopithecidae; Om, Omomyiformes; Pa, Parapithecidae; Pi, Pitheciidae; Pr, Propliopithecidae; Sim, Simiiformes; Sp, strepsirrhines; Ts, Tarsiidae. Upper molars of some basal anthropoids for comparisons: 1, KEB-1-001 right M2 of Amamria tunisiensis (ref. , their figure 3A); 2, NMMP-85 right M1 and NMMP-81 M2 of Afrasia djijidae (ref. , their figure 2 A and B); 3, UFAC-CS 066 right M1 of A. simpsoni gen. et sp. nov. (this paper); 4, M1 of the NMMP-15 right maxillary of Bahinia pondaungensis (ref. ; here 3D rendering of a cast scan); 5, IVPP V22730 right M1 of Bahinia banyueae (ref. , their figure 1F); 6, IVPP V11993 left M1 (reversed) of Eosimias centennicus (ref. , their figure 9); 7, IVPP V11997 right M2 of Phenacopithecus krishtalkai (ref. , their figure 22), 8, CPI-7937 right M1 and CPI-7938 left M2 (reversed) of Ucayalipithecus perdita (ref. , their figure 1A); 9, CPI-6486 right M1 of Perupithecus ucayaliensis (refs. and , their figure 1D); 10, DT1-31 left M1 (reversed) of Talahpithecus parvus (ref. , their figure 2N; here 3D rendering of a cast scan).