New Hominoid Genus Identified

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UIC RESEARCHER HELPS IDENTIFY NEW HOMINOID GENUS

The earliest fossil of a ground-based ape has been identified by a University of Illinois at Chicago researcher and colleagues at Northeastern Ohio Universities College of Medicine, Yale University and Northern Arizona University. They have classified the fossil ape as a new hominoid genus from the middle Miocene named Equatorius. Their findings appear in the Aug. 27 issue of Science.

Recognition of the new genus was prompted by the discovery of the partial skeleton of a fossil ape by a team led by Yale's Andrew Hill and Steve Ward from Northeastern Ohio Universities. The skeleton is from an approximately 15-million-year-old site at Kipsaramon, in the Tugen Hills of western Kenya.

The limb anatomy of Equatorius indicates that members of this genus were at least partly adapted for life on the ground. Facial and jaw features were also key in the characterization of the new genus.

"Recognizing the distinctiveness of Equatorius provides a more accurate framework to assess the wealth of new fossil material from the middle Miocene that is being uncovered," said UIC associate professor of oral biology Jay Kelley, the research team's expert on oral anatomy and one of the Science article's authors.

Prior to this discovery, the species now included in Equatorius had been placed in another hominoid genus from the African middle Miocene, Kenyapithecus. There had been conflicting opinions about the relationship of Kenyapithecus to great apes and humans. Some evidence from the paleontological record showed a close relationship, even suggesting that Kenyapithecus was an early member of this group of species with a common evolutionary ancestry, or clade. Other evidence seemed to show that there was a much more distant relationship between Kenyapithecus and living great apes and humans.

"It is now clear that one of the reasons for this diversity of opinion was that the Kenyapithecus sample actually contained two genera," said Kelley. "Specimens of one of the genera are fairly primitive and those of the other genus are more derived. The sample was giving off mixed signals."

Samples of the more derived genus have a morphological pattern that more closely resembles that of species that appear later in the human/great ape clade. These samples are retained in Kenyapithecus. The more primitive samples contain none of the specialized features that establish a link with the human clade. These have been placed in the new genus, Equatorius.

Kenyapithecus as it was constituted previously contained both the derived and primitive samples. The phylogenetic signal of the very small Kenyapithecus sample from Ft. Ternan in western Kenya was being masked and distorted by being placed with the other, somewhat larger samples of the species now assigned to Equatorius.

The partial skeleton from Kipsaramon includes much of the forelimbs and shoulders, as well as portions of the vertebral column. It also includes most of the lower jaw and dentition, making it the first specimen from the middle Miocene (about 16 to 11 million years ago) to preserve associated cranial and postcranial remains. This association enabled Dr. Kelley and his associates to compare the Kipsaramon skeleton with remains from all of the other African middle Miocene sites, especially the small Kenyapithecus sample from Ft. Ternan, which consists almost entirely of teeth and jaw fragments. Kenyapithecus shares some features with living great apes that the more primitive Equatorius does not. These include relatively slender, high crowned canine teeth and robust incisors, and a restructuring of the bony anatomy of the zygomatic area of the face which, among other things, alters the length and position of some of the chewing muscles. However, whether Kenyapithecus is in fact an early member of the great ape and human clade or simply a closer sister-taxon, remains unclear, according to Dr. Kelley and his colleagues.

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