A New Transitional Anthropoid from North Africa?

Readers might recall from our discussion of afrotarsiids that some paleontologists now incorporate this group within the radiation of mostly Asian stem anthropoids known as eosimiiforms. Well, in July, several members of the same team that discovered the early afrotarsiid fossils described a new putative early anthropoid from the middle Eocene (~40 Ma) of Tunisia (Marivaux et al. 2014). They further suggest that this fossil may represent a transitional form linking eosimiiforms and the simiiform radiation of crown and advanced stem anthropoids.

Amamria hologype, KEB-1-001. Highlighting of some key dental traits performed by the blogger. From buccal to lingual, these are the metastyle, the preprotocrista complex, the hypocone, and the pericone. Modified from Marivaux et al. 2014.

Figure 1, Amamria hologype, KEB-1-001. Highlighting of some key dental traits performed by the blogger. From buccal to lingual, these are the metastyle, the preprotocrista complex, the hypocone, and the pericone. Modified from Marivaux et al. 2014.

The species, Amamria tunisiensis, is described from a single upper molar, believed to be M2. This scant representation means the claims made by Marivaux and colleagues are unlikely to make the textbooks until Amamria is better understood from more material, a point they allow. However, they do argue their case suggestively from details of the dental anatomy.

Much of the support for anthropoid affinities in the Amamria molar rest on the presence of a strong, continuous shelf of enamel on the lingual (inside) surface of the tooth known as a lingual cingulum, the particular shapes of two crests known as the hypoparacrista and hypometacrista, and a distinctively U-shaped crest running from the protocone toward the buccal (cheek) edge called the preprotocrista complex. These are features shared with the eosimiids, as is the distinctive shape of the buccal crown margin in Amamria, with the buccal cingulum terminating distally (toward the end of the toothrow) in an enlarged bump known as a metastyle.

However, Amamria also appears derived away from eosimiiforms and toward simiiforms in a few key respects. The tooth is much broader in the midline than in eosimiiforms (teeth A-D in figure 2), among whom it tends to develop some “waisting.” Amamria also has a small hypocone and another accessory cusp called a pericone developed on its lingual cingulum, which develop in some early simiiforms as well (teeth F-I). The hypocone eventually becomes a particularly important cusp and is shared by almost all crown anthropoids.

Comparison of Amamria and several putative anthropoid molars. A) Afrotarsius libycus, B) Eosimias centennicus, C) Phenacopithecus krishtalkai, D) Bahinia pondaungensis, E) Amamria tunisiensis, F) Proteopithecus sylviae, G) Talahpithecus parvus, H) Catopithecus browni, I) Oligopithecus rogeri

Figure 2, Comparison of Amamria with several putative anthropoid molars. A) Afrotarsius libycus, B) Eosimias centennicus, C) Phenacopithecus krishtalkai, D) Bahinia pondaungensis, E) Amamria tunisiensis, F) Proteopithecus sylviae, G) Talahpithecus parvus, H) Catopithecus browni, I) Oligopithecus rogeri. From Marivaux et al. 2014

Based on a single tooth, little can be said about the paleobiology of Amamria. Its estimated body mass is around 100 g, and it probably had a similar frugivore/insectivore dietary pattern as that reconstructed for early afrotarsiids.

It is likewise difficult to state too strongly any systematic or biogeographic hypothesis based on this fossil. Supposing it is a transitional anthropoid, however, it would have significant implications. Amamria is older that Afrotarsius libyicus (37.2-28.4 Ma), but appears to be more derived, implying at least two dispersals of eosimiiforms from Asia into Africa by the middle Eocene. If Amamria is uniquely related to the African simiiforms known from the late Eocene and Oligocene of the Fayum and other North African localities, this would suggest that simiiforms nest within eosimiiforms as an in situ African radiation. According to this model, stem anthropoids evolved first in Asia, then a few species migrated to Africa where one was ancestral to all later fossil North African and living anthropoids. This would make some sense and explain the origins of the incontrovertible anthropoids of the Fayum, which seem to appear without clear “prosimian” progenitors. In the most banal phrase in all of paleontology, however, more fossils will be needed to confidently determine the affinities of Amamria and test these hypotheses.

References

Marivaux et al. 2014. A morphological intermediate between eosimiiform and simiiform primates from the Late Middle Eocene of Tunisia: Macroevolutionary and paleobiogeograpic implications for early anthropoids. Am J Phys Anthropol 154: 387-401.

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