Pre Monkey-Ape Split Primates From Chattian Oligocene Tanzania Found?

From the human perspective, few events in evolution were more momentous than the split among primates that led to apes (large, tailless primates such as today's gorillas, chimpanzees, and humans) and Old World monkeys (which today include baboons and macaques). DNA studies of living primates have estimated that the rift took place between 25 million and 30 million years ago, but the earliest known fossils of both groups date no earlier than 20 million years ago. Now, a team working in Tanzania has found teeth and partial jaws from what it thinks are 25-million-year-old ancestors of both groups. If the interpretations hold up, the finds would reconcile the molecular and fossil evidence and possibly provide insights into what led to the split in the first place.

Researchers have long been frustrated by a paucity of fossils from this key period in evolution, which sits at the borderline between two major geological epochs: the Miocene (about 23 million to 5 million years ago) and the Oligocene (about 34 million to 23 million years ago). The earliest known fossils of early apes and Old World monkeys date from the early Miocene and have been found in just a handful of sites in Kenya, Uganda, and North Africa. Meanwhile, molecular studies of existing primates consistently suggest that these two groups arose during the Oligocene, leading scientists to wonder whether the molecular dates are wrong or if paleontologists have been looking in the wrong places.

For more than a decade, researchers from the United States and Tanzania have been combing Tanzania's Rukwa Rift Basin, searching for fossils of all kinds. During the 2011 and 2012 seasons, a team led by Nancy Stevens, a vertebrate paleontologist at Ohio University in Athens, discovered fossils that it identified as belonging to two previously unknown species of primates: one, an apparent ape ancestor the team has named Rukwapithecus fleaglei; the other, a claimed Old World monkey ancestor dubbed Nsungwepithecus gunnelli. (The species were named after two notable primatologists, John Fleagle of Stony Brook University in New York, and Gregg Gunnell, who is now at Duke University in Durham, North Carolina.)

Both specimens, consisting of teeth and partial jaws, were found in Rukwa Rift sediments dated by several techniques, including the often used argon-argon method, to 25.2 million years ago. The team identified them as ape and Old World monkey ancestors from the features of their molars, which paleontologists routinely use to tell primates apart. For example, Stevens says, Nsungwepithecus "has a much more triangular outline" of its last lower molar than Rukwapithecus, and there are "a number of other major differences in the shape and position of the cusps and crests that run along the chewing surface of the teeth." The two species also show other dental features that group them with later Old World monkeys and apes, but are still different enough to be classified as separate—and more ancient—species.

paper link.