Fossil discovery changes primate's extinction date

Paleontologists studying a site rich in primate fossils near Cairo found a near-complete molar of an oligopithecid, a species of primates thought to be related to Old World Monkeys.

The depth at which the fossil was found indicates that the species was living millennia later than when it was thought to have been extinct.

Though oligopithecids were once plentiful in the area, they started to disappear near the end of the Eocene epoch about 35 million years ago.

"There was a major climate change at the boundary between the Eocene and the Oligocene," says Erik Seiffert, a paleontologist at Stony Brook University, New York, and the lead researcher on the expedition. "The general view is that these primates had gone extinct at that time due to those changes."

The paleontologists discovered the molar fossil in the Fayum Depression, a fairly large site, just west of the Nile River and south of Cairo. Scientists have combed the basin for primate fossils since 1965, but the last discovery of an oligopithecid fossil was at a layer about 4 million years younger. They describe their findings in the Journal of Human Evolution.

The new fossil, however, is from an animal much smaller than remains of previous oligopithecid remains indicate – which probably weighed about 800g when they were alive.

"This new species was maybe only 100-150g so it actually qualifies as one of the smallest primates ever discovered," says Seiffert. This would make it of similar size to the tiny pygmy marmosets found in South America.

While there are species of monkeys that have become smaller through evolution in South America, this is the first documented case in Africa, explains Seiffert.

The reduction in size may have been an evolutionary adaptation that allowed these primates to utilize food sources that did not put them in competition with the abundance of much larger animals that roamed the area at the time. This allowed them to survive while their ancestors became extinct.

Christopher Beard, curator of vertebrate paleontology at Carnegie Museum of Natural History in Pittsburgh, PA, suggests another likely reason. In Nature in 2010, he described Talahpithecus parvus, a species of tiny oligopithecids found in Libya. The newfound molar may be of an animal of the same species, but the lack of corresponding fossilized body parts means this cannot yet be confirmed.

"If the Fayum oligopithecid is closely related to (or even the same as) Talahpithecus, then there is no reason to believe that it got smaller through time. It simply started out small and stayed small," he suggests.

Rare primates

It is an excellent way for local scientists to collaborate with foreign specialists

The Fayum Depression is a particularly tricky site in which to find small fossils. "In sites that have very fine grained sand it is possible to use screens to go through sand samples. But this site is a very old river channel deposit so the grains of the sand are fairly large, making it quite hard to use a sieve," Seiffert says.

Seiffert says that the scarcity of fossils of this species suggests the animal was already very rare during that period. While oligopithecids generally probably perished at the end of the Eocene, this particular dwarfed species managed to survive among much larger animals afterwards.

Beard, however, suggests they may not have been as rare as the fossil record from the Fayum Depression indicates. There are many environmental factors that determine whether an animal's carcass is fossilized. "I suspect that the sites in the Fayum Depression that yield this tiny primate are biased against the preservation of small primates like this oligopithecid. It is extremely important to work at other sites of the same age in order to control for this important factor," he says.

"We still have a great deal to learn about the early evolution of primates in North Africa and adjacent parts of the Middle East, such as the Arabian Peninsula. This should be an important goal of future research in the region, and it is an excellent way for local scientists to collaborate with foreign specialists," adds Beard.

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