PLIOCENE GIRAFFIDAE (MAMMALIA) FROM THE HADAR FORMATION OF HADAR AND LEDI-GERARU, LOWER AWASH, ETHIOPIA

摘要

The Hadar giraffids belong to the genera Giraffa and Sivatherium. We recognize two species in the former genus; distinguishing between them on dental elements is straightforward, but it is more difficult on other remains. Giraffa jumae is about the size of the modern G. camelopardalis, but has a less pneumatized skull with more conical ossicones, more slender and often longer limb bones, and also differs in some dental features, making it an unlikely ancestor of the modern giraffe. Giraffa stillei has teeth that are always smaller than those of the modern form, less molarized premolars, ossicones smaller but otherwise similar to those of the larger Hadar species, and slender limb bones that are probably relatively long, thus not much shorter than those of the modern form. It might be the ancestor of the later G. gracilis from the Turkana Basin, and of G. camelopardalis. We assign no specimen to Giraffa pygmaea. Sivatherium maurusium, a well-known form of the African Pliocene and Pleistocene, is a rare form at Hadar. In contrast to most bovids, giraffids are more common in the Sidi Hakoma than in the overlying Denen Dora Member, perhaps as a result of grassland expansion at Hadar at that time.