Abstract

A new hyracoid genus and species, Dimaitherium patnaiki from the early Late Eocene (early Priabonian) Birket Qarun Formation in the Fayum Depression, Egypt, is described. The material is approximately 37 million years old and three million years older than any other hyracoid known from the Fayum area. A partial cranium preserves features that are likely primitive within Paenungulata, such as a postorbital process made up solely of the frontals (without a parietal contribution), a restricted jugal contribution to the anterior orbital margin, and an anteriorly positioned orbit. The internal mandibular fenestra, a peculiar feature seen in many younger Fayum hyracoids, is primitively absent in Dimaitherium, but a coronoid canal is present at the base of the ascending ramus as in later hyracoids. The dentition of Dimaitherium shows several primitive characteristics, such as poorly molarised premolars, but otherwise differs little from many younger hyracoids. Phylogenetic analysis consistently places younger hyracoids, including procaviids, in a clade to the exclusion of Dimaitherium and the much older Seggeurius and Microhyrax. Given the phylogenetic hypotheses proposed here, the presence of 'broken' hypocristids on the lower molars is likely to have evolved more than once during hyracoid evolution. Evidence from the humerus, astragalus and calcaneum indicate that Dimaitherium may have been an agile climber, and was likely capable of rapid movement but was not cursorially adapted. Of the other two small Fayum hyracoids, Dimaitherium was more similar to Thyrohyrax than to Saghatherium. The presence of only this single genus and species in the Birket Qarun localities cannot be explained by depositional biases, and makes an ancient origin for the Fayum hyracoid lineages unlikely. The poorly known large-bodied Early Eocene hyracoids 'Megalohyrax' gevini and 'Titanohyrax' mongereaui and tantulus from Algeria and Tunisia were not placed with Fayum species of Megalohyrax or Titanohyrax in our phylogenetic analyses. It is unlikely that these species belong to those genera and provide no compelling evidence for an Early Eocene or Paleocene divergence of the Fayum hyracoid lineages.