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Fossil find sparks debate on primate origins

A 37-million-year-old jaw raises questions about primate evolution and about a highly publicized primate fossil

6:06pm, October 21, 2009
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Pieces of ancient primates can still pack a surprising punch. Consider a 37-million-year-old lower jaw that still sports many of its teeth and was found in Africa by paleontologist Erik Seiffert of Stony Brook University in New York and his colleagues. This newly unearthed creature had skeletal features that resembled those of higher primates, but it didn’t belong to the lineage that led to higher primates, Seiffert’s team reports in the Oct. 22 Nature.

Seiffert’s group assigns its fossil discovery to a new genus and species, Afradapis longicristatus. The work follows this summer’s highly publicized announcement of another primate find: The 47-million-year-old primate Darwinius is regarded by some as filling in an important gap in human evolution (SN: 6/20/09, p. 8). But Seiffert says Darwinius belongs to the same group as his team’s fossil: adapiforms, a separate and now-extinct primate group.

“It is only with the discovery of Afradapis that we have the first strong evidence indicating that adapiform primates were common at that time and place, successfully living alongside primitive anthropoids in Africa,” Seiffert says.

Higher primates include anthropoids — monkeys, apes, humans and their fossil ancestors. The ancient, lemur-like Afradapis turned up in a part of Egypt’s Fayum desert that has already yielded anthropoid fossils dating from between 37 million and 32 million years ago.

Afradapis weighed between about 2.2 kilograms and 3.3 kilograms (4.8 pounds and 7.2 pounds), the researchers estimate. Most primates from more than 30 million years ago weighed considerably less than that. Darwinius tipped the scales at between 650 grams and 900 grams (3.2 ounces and 6.3 ounces).

Afradapis and other adapiforms independently evolved some traits that later appeared in anthropoids, probably because they competed with anthropoids for access to fruit in dense African forests, Seiffert hypothesizes.

Afradapis displays several jaw and tooth characteristics that began to appear in anthropoids about 30 million years ago, Seiffert says. These traits include the absence of second premolar teeth and the presence of a lower third premolar tooth with a honing surface for a long upper canine, a thick jaw and full fusion of the jaw’s right and left halves.

Large cheek teeth allowed Afradapis to chew tough fibers and eat lots of leaves, he says. But the ancient primate could also eat fruit, pitting it against African anthropoids from the same time that mainly consumed fruit and insects, in Seiffert’s view.

But adapiforms were probably not capable of eating enough fruit to have been dietary rivals of ancient anthropoids, counters paleontologist K. Christopher Beard of the Carnegie Museum of Natural History in Pittsburgh. Beard doubts that Afradapis’ unusually large body and teeth specialized for leaf eating put it in an ecological league of its own, he says.

Beard leads a team that has excavated 40-million-year-old Asian fossils that they regard as the oldest known anthropoids (SN: 4/16/94, p. 245). In a computerized reconstruction of evolutionary relationships among 117 living and extinct primates — based on measurements of 360 skeletal features — Seiffert’s group also concludes that the oldest known anthropoids lived in Asia.

Most importantly, Beard says, Seiffert’s team demonstrates that “Darwinius was a fairly generic adapiform primate only distantly related to living and fossil anthropoids.”

Paleontologist Philip Gingerich of the University of Michigan in Ann Arbor, a member of the team that analyzed the Darwinius fossil, entirely rejects Seiffert’s conclusions and regards Darwinius as a possible anthropoid precursor. Afradapis’ jaw looks much like that of a male monkey, not a lemur, thus putting it in a primate group that led directly to anthropoids, Gingerich contends.

Darwinius shows other anthropoid-like traits, he says, including flat front teeth and interlocking canines.

Seiffert’s proposal that adapiforms evolved traits that mirrored those of anthropoids “seems implausible to me,” Gingerich says.

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