Cranial morphology of Thyrohyrax domorictus (Mammalia, Hyracoidea) from the early Oligocene of Egypt

Abstract

The Jebel Qatrani Formation in the Fayum Depression, Egypt, has yielded a diverse hyracoid fauna that includes both small- and large-bodied forms. Thyrohyrax domorictus is one of the most common hyracoids found in the upper sequence of the Formation, from sites dating to between 29 and 31 Ma. The dental morphology of T. domorictus is more similar to that of living hyraxes than other Paleogene species, but additional morphological evidence is needed to test the hypothesis of procaviid affinities. Two fairly complete crania of T. domorictus are now known and provide a number of additional craniofacial characters for phylogenetic analysis. The specimens are comparable in size to other small-bodied Fayum hyracoids and extant procaviids. T. domorictus has a relatively long rostrum that is perforated by a nasomaxillary fossa, which is also present in Miocene Afrohyrax and Prohyrax and older Thyrohyrax pygmaeus. The anterior border of the orbit is positioned above the molars, and the orbital aperture is closed posteriorly by a complete postorbital bar. Compared to other Fayum hyracoids, T. domorictus exhibits relatively derived, and more procaviid-like, cranial morphology. The cranium of T. domorictus is most similar to that of the extant genus Dendrohyrax, and they share similar morphology of the orbit and cranial roof. Phylogenetic analysis including morphological and molecular data from a large sample of living and extinct afrotherians places Thyrohyrax domorictus as the closest known Paleogene relative of Procaviidae and Pliohyracidae, supporting the hypothesis of procaviid affinities that was originally proposed solely on the basis of dental morphology.

ACKNOWLEDGMENTS

For access to collections we would like to thank P. Chatrath (Duke Lemur Center, Durham, North Carolina), M. Pickford (Muséum National d’Histoire Naturelle, Paris), A. Currant, P. Brewer, and L. Tomsett (Natural History Museum, London), J. Galkin (American Museum of Natural History, New York), and E. Heizmann (Staatliches Museum für Naturkunde, Stuttgart). We thank J. Sertich (Denver Museum of Nature and Science) for providing CT scans of the specimens described here, and the Stony Brook University Medical Center for access to their CT scanning facilities. Recent field work in Egypt has been made possible through an ongoing collaboration with the Egyptian Geological Museum and the Egyptian Mineral Resources Authority and has been funded by U.S. National Science Foundation grants to E.L.S. (BCS-0114856), to E.L.S. and E.R.S. (BCS-0416164), and to E.R.S. (BCS-0819186), as well as Leakey Foundation grants to E.R.S. E.C.B. was funded by a NERC studentship (NER/S/A/2006/14173). E. Gheerbrant and R. Tabuce provided valuable comments on the manuscript. This is Duke Lemur Center publication number 1211.

Handling editor: Blaire Van Valkenburgh