The first skull of Sivameryx africanus (Anthracotheriidae, Bothriodontinae) from the early Miocene of East Africa

ABSTRACT

Here we describe the first known skull and associated postcrania of the small bothriodontine anthracotheriid Sivameryx africanus, which fills an important gap in the current knowledge of morphological diversity within the Anthracotheriidae of Africa. The skull was recovered from the latest early Miocene sediments of the Kalodirr Member in the Turkana Basin of Northern Kenya, and the age is well constrained between two tephra dated to 17.5 ± 0.2 and 16.8 ± 0.2 Ma. A partial mandible of Sivameryx from Kalodirr is also described because it may belong to the same individual. The new anatomical data were incorporated into a genus-level phylogenetic analysis of Bothriodontinae that reveals that Sivameryx is the sister taxon to Hemimeryx and belongs to a clade of advanced bothriodontine anthracotheriids alongside the genera Merycopotamus, Libycosaurus, and Afromeryx. The new material of Sivameryx from Kalodirr greatly expands our knowledge of the cranial anatomy of the genus, because the skull of Sivameryx does not reveal any specializations for aquatic habitats, such as those seen in Libycosaurus. Here we suggest, based on the preserved cranial and postcranial evidence, that Sivameryx may have been a small browser that inhabited denser stands of vegetation at Kalodirr based on evidence from its narrow snout and well-developed labial musculature. However, its masticatory muscles were also relatively well developed, suggesting repetitive loading of the jaw when chewing tough vegetation.

ACKNOWLEDGMENTS

We thank E. Mbua and staff at KNM for access to specimens in their care, M. G. Leakey for encouragement and information about the Kalodirr fauna, and H. Glowacka and E. R. Miller for helpful comments on earlier versions of the manuscript. We thank JVP technical editor J. M. Harris, JVP editor G. W. Rougier, and the reviewers of this manuscript, S. Ducrocq and M. Pickford, for their most helpful comments.