U. MICHIGAN (US)—A paleontologist searching for dinosaur fossils has found the skull of an ancient primate. The discovery offers new insights into what the last common ancestor of apes and monkeys may have looked like and when the two lineages went their separate ways.
It is well known that Old World monkeys and apes share ancestry, but exactly when the two branches split from the common trunk has been unclear. Debates also have swirled around the question of what sort of facial structure the progenitor of apes and monkeys had.
Both lineages belong to the primate group known as catarrhines. The earliest catarrhines in the fossil record, creatures that were neither monkey nor ape, date back to the late Eocene to early Oligocene epochs, 35 to 30 million years ago.
Later fossils, from around 23 million years ago, suggest the split had already occurred by that time. But few fossil catarrhines from the interval between 30 million to 23 million years ago have been found, making it difficult for scientists to know precisely when monkeys and apes became distinctly separate groups and what catarrhines looked like around the time of the split.
The new fossil catarrhine, Saadanius hijazensis, dates from 29 million to 28 million years ago and lacks the specialized features that distinguish modern apes and Old World monkeys, suggesting that the split had not yet occurred.
The researchers’ analysis of the fossil leads them to believe its physical features are much like those of the last common ancestor of Old World monkeys and apes.
Zalmout, a postdoctoral fellow working with University of Michigan paleontologist Philip Gingerich, found the fossil in 2009, during a trip focused on finding fossil whales and dinosaurs. Working with the Saudi Geological Survey, he was exploring an area where geological maps indicated the rocks might contain fossils from the Cretaceous period (145 to 65 million years ago), a time when dinosaurs dominated the land.
His first clue that the maps were wrong came when he saw a jawbone sticking out of the sediments and realized it was from a hippo-like animal that lived more recently—around 35 to 33 million years ago.
The next day, he explored a nearby area that seemed more likely to yield older fossils, but again the first thing he found was another fossil from a more recent hippo-like creature.
“I didn’t know whether to be disappointed or not, but I thought, well, maybe something interesting will pop up here, so I started looking around,” Zalmout says. “Within minutes, I found teeth sticking out of the ground, and when I realized what they were I was shocked. I had worked with Phil on terrestrial mammals in the Bighorn Basin, and my first look at the size and shape of these teeth told me I had found a primitive primate.”
Zalmout e-mailed a photo to Gingerich, an expert on early primates as well as ancient whales.
“I knew right away what it was, and I was thrilled,” says Gingerich.
The Saadanius skull should help resolve an ongoing debate about the facial anatomy of the ancestral stock of apes and Old World monkeys, says Laura MacLatchy, an associate professor of anthropology.
One view is that the oldest common ancestor’s face was like that of modern gibbons: dainty and button-nosed. Alternatively, the ancestor may have had a baboon-like, long snout, like that of the oldest true apes and monkeys. The Saadanius fossil supports the second hypothesis, MacLatchy says.
Also of interest is the tympanic bone, a part of the skull that surrounds the ear drum. In Aegyptopithecus this bone is ring-shaped, but in Saadanius it’s a tubular outgrowth like that of apes and Old World monkeys.
“That tells us that Saadanius is probably closely related to catarrhines at the base of the ape-monkey split,” MacLatchy explains.
The research was funded by the National Science Foundation.
University of Michigan news: www.ns.umich.edu/