New fossils push back primate origins.

New Fossils Push Back Primate Origins

The discovery of four skulls belonging to mouse-sized, saucer-eyed primates that lived in North America 50 million years ago has dramatically pushed back estimates of when early primate groups first evolved.

Anatomical features of the nearly complete fossil skulls indicate that this animal, called Shoshonius cooperi, was a primitive form of tarsier -- a tree-dwelling primate today found only in the forests of Southeast Asia. Thus, the discoverers conclude in the Jan. 3 NATURE, Shoshonius and modern tarsiers evolved from a common ancestor that split off from the forerunners of simians -- monkeys, apes and humans -- sometime before 50 million years ago.

Paleontologists K. Christopher Beard and Leonard Krishtalka of the Carnegie Museum of Natural History in Pittsburgh, working with Richard K. Stucky of the Denver Museum of Natural History, plucked the ancient skulls from quarries in Wyoming's Wind River Basin between 1984 and 1987. Animal bones found in the area and the age of volcanic ash straddling the site place the specimens at 50 million years old, Krishtalka says.

The oldest known simian fossil skull, dubbed Catopithecus by its discoverers, turned up in an Egyptian deposit in 1988 and dates to about 40 million years ago. Until now, many scientists assumed that the evolutionary parting of tarsiers and simians occurred around that time.

"This important find of the first Shoshonius skulls significantly pushes back in time the existence of tarsiers as a separate [primate] group," says anthropologist Elwyn L. Simons of Duke University in Durham, N.C.

Like living tarsiers, and in contrast to simians, Shoshonius had a short snout, enlarged bony ear chambers, and enormous eyes well suited to nocturnal activity. "These features were probably evolutionary novelties [of the tarsier lineage] that arose from a common ancestor," Krishtalka asserts. The geographic origin of that common ancestor remains unclear, he adds.

Scientists divide forest-dwelling primates living 55 million to 36 million years ago into two families: the adapids, including many species resembling modern lemurs; and the omomyids, comprising several animals with anatomical ties to modern tarsiers.

Because most fossils assigned to the omomyid family consist only of teeth and jaw fragments, Krishtalka contends the omomyids represent a "wastebasket group" into which scientists dump all sorts of fossilized bits and pieces merely because the specimens come from small primates living in the same distant time period.

Many scientists regard omomyids as the forerunners of simians. Dissenters, such as Simons, view the adapids as more likely simian ancestors.

Simons led excavation teams that uncovered the 40-million-year-old Catopithecus skull and three skulls from 35-million-year-old primates called Aegyptopithecus. Neither of these creatures displays anatomical links to omomyids such as the Wyoming specimens, he argues.

In any case, the omomyids apparently included several independent primate lineages, observes anthropologist Robert D. Martin of the University of Zurich, Switzerland, in a commentary accompanying the research report. One omomyid branch may incorporate Shoshonius and living tarsiers, although confirmation of the link must come from lower-body bones as well as cranial fossils, he says.

Last summer, Krishtalka and his colleagues uncovered trunk and limb remains for Shoshonius, as well as three additional Shoshonius skulls. Preliminary, unpublished analyses of these finds suggest the tiney tree-dweller employed a tarsier-like posture and limb movements, Krishtalka says. "These new specimens show that the fossil record does not account for at least 10 million years of primate evolution," he contends.