This week we’ll return to our look at the fossil record of tarsiers and their putative relatives with a much more controversial group of primates, the afrotarsiids. This group of three known species was first assigned to the tarsiidae, but is now often argued to represent a group of early stem anthropoids. For my overview of the certain members of the Tarsiidae, see Part I.
If tarsiids represent a restricted radiation, Afrotarsiidae is tiny, including just three species in two genera. The type species, Afrotarsius chatathri, was described as a possible tarsiid from dentognathic material recovered at the famous Oligocene site of the Fayum in Egypt (Simons and Bown 1985). It was in fact the first fossil to be assigned to the Tarsiidae, as Tarsius thailandicus would not be described until 1987. Afrotarsius libycus is known from a single molar collected at Dur At-Talah from the middle Eocene of Libya (Jaeger et al. 2010). Finally, Afrasia djijidae was described from dental material from the middle Eocene of Myanmar, extending the range of the Afrotarsiids into Asia (Chaimanee et al. 2012). All of the afrotarsiids appear to have been small bodied insectivores of around 100 g, in this respect ecologically similar to modern tarsiers. In Africa, where there are no known omomyids, they may have filled the tarsier-like niche, although they appear to have been quite rare if their depauperate fossil record is any indication.
The afrotarsiids as a group are defined almost entirely by details of their dental anatomy, as two of the three species are known only from dental remains. A fused tibiofibula was reported for Afrotarsius chatathri which would have closely linked it with tarsiids (Rasmussen et al. 1998), but this element is controversial and often regarded as not belonging to a primate (White and Gebo 2004). Their suprafamilial relationships are complicated by the general dental similarity of basal anthropoids and tarsiers. Nearly as soon as Afrotarsius was assigned to the Tarsiidae, it was suggested that it may represent a close relative of anthropoids (Ginsburg and Mein 1987). The most recent argument for this link was made by Chaimanee et al. (2012). They performed a maximum parsimony analysis incorporating all of the known afrotarsiids and a sample of early anthropoids, tarsiers, omomyids, and adapiforms, and found afrotarsiids to be sister to the eosimiids, a group of primates generally thought to be stem anthropoids. These authors point especially to features of the upper molars first recovered in Afrotarsius libycus and Afrasia djijidae which more closely resemble eosimiids than the lower molars reported from Afrotarsius chatathri. The lower molars of afrotarsiids may have convergently evolved the high, sharp crests of tarsiers as an adaptation for faunivory.
Early on, Simons (1995) and Rasmussen et al. (1998) pushed back against the association of Afrotarsius chatathri with anthropoids, arguing for near identity between the Afrotarsius holotype and living tarsiers and for the tarsiid affinities of the putative afrotarsiid tibiofibula. More recently, Seiffert (2012) suggested that afrotarsiids show close similarities to Tarsius sirindhornae. As always, more complete fossil material, as well as new phylogenetic analyses that more completely sample Tarsiidae, may be necessary to resolve this issue.
The strong similarity and near contemporaneity of Afrasia from SE Asia and Afrotarsius in North Africa suggest that the group dispersed from its place of origin across the ancient Tethys Sea relatively rapidly in the middle Eocene. Chaminee et al. (2012) believe this place to be Asia, due to the nesting of Afrotarsiidae within the Eosimiiformes, which is otherwise entirely Asian. If the position of Seifert (2012) is correct and afrotarsiids are tarsiers, then this biogeographic interpretation still seems likely as tarsiids are an otherwise Asian radiation. This makes the taxonomic position of afrotarsiids perhaps less vital for understanding the biogeography of anthropoid origins than was once thought. This is especially true as Afrotarsius is unlikely to have been ancestral to later “higher” anthropoids in Africa due to the distant relationship between eosimiiformes and higher anthropoids and their co-occurrence both at Dur At-Talah and in Myanmar (if the amphipithcids are interpreted as higher anthropoids, another controversial assignment). At best afrotarsiids indicate a possible route for the ancestors of higher anthropoids to have dispersed to Africa after having evolved from a more stem anthropoid-like form earlier in the Eocene or Paleocene of Asia.
Different interpretations of the phylogenetic position of afrotarsiids do suggest significantly different biogeographic scenarios for tarsier evolution, however. If afrotarsiids are stem anthropoids, then the “classic” explanation of tarsiers as Asian rainforest specialists, whose range contracted with those rainforests to SE Asia from the Eocene onward, would appear to be correct. If afrotarsiids nest within Tarsiidae, however, perhaps in a sister relationship with Tarsius sirindhornae, this would suggest that either the radiation was capable of throwing off members with a much broader dispersal capability or has an evolutionary history involving a much larger area. It is also possible that tarsiids and afrotarsiids have a separate evolutionary history dating from at least before the middle Eocene but are sister taxa to the exclusion of anthropoids. Their last common ancestor is still likely to have been Asian, but its adaptive profile would be obscure.
Chaimanee, Y., O. Chavasseau, K. C. Beard, A. A. Kyaw, A. N. Soe, C. Sein, et al. 2012. Late Middle Eocene primate from Myanmar and the initial anthropoid colonization of Africa. PNAS 109:10293–7.
Ginsburg, L. and Mein, P. 1987. Tarsius thailandica nov. sp., preimier Tarsiidae (Primates, Mammalia) fossile d’Asie. C R Acad Sci II, 304:1213-1215
Jaeger, J.-J., K. C. Beard, Y. Chaimanee, M. Salem, M. Benammi, O. Hlal, et al. 2010. Late middle Eocene epoch of Libya yields earliest known radiation of African anthropoids. Nature 467:1095–8.
Rasmussen, D., G. C. Conroy and E. L. Simons. 1998. Tarsier-like locomotor specializations in the Oligocene primate Afrotarsius. PNAS 95:14848–50.
Seiffert, E. R. 2012. Early primate evolution in Afro-Arabia. Evolutionary anthropology 21:239–253.
Simons, E. L. 1995. Egyptian Oligocene primates: A review. Yearbook of Physical Anthropology 38:199-238.
Simons, E. L. and T. M. Bown. 1985. Afrotarsius chatrathi, first tarsiiform primate (? Tarsiidae ) from Africa. Nature 313:475–477.
White, J. L. and D. L. Gebo. 2004. Unique Proximal Tibial Morphology in Strepsirrhine Primates. American Journal of Primatology 64:293–308.