Monkey skull key to evolutionary puzzle.

The oldest known and most complete skull of an extinct South American monkey has been discovered high in the Andes Mountains of Chile. The find provides a missing piece in the puzzle of evolution and clues to what the environment was like in South America at the time.

The 20,000,000-year-old skull represents a new species and is amazingly well-preserved - with both eye sockets and every tooth in the upper jaw intact. It helps to fill a huge gap in the knowledge of the fossil record of New World monkeys.

"All of the primate fossils from South America wouldn't even fill a baseball cap," notes John J. Flynn, chairman of the Department of Geology and curator of fossil mammals Field Museum, Chicago. "They're so rare, you never really expect to find one." The new-found species is named Chilecebus carrascoensis: "Chile" for its country of origin, "cebus" for an ending commonly applied to New World monkey names, and "carrasco" for Gabriel Carrasco. the Chilean fossil hunter who first spotted the specimen.

There are about 85 species of New World monkeys living today, including tamarins, marmosets, and capuchins. Scientists disagree on how the major groups of living New World monkeys are related to each other and how they fit into the anthropoid suborder, which also includes Old World monkeys (found mainly in Africa and Asia), apes, and humans.

A large time gap in the fossil record of South America, from about 55,000,000 to 30,000,000 years ago, represents a crucial period in the evolution of New World monkeys. It was during this era that monkeys first arrived on the continent, as did rodents. How they got there is a mystery, as South America was an island then, completely surrounded by oceans. One theory holds that the animals "rafted" across the Atlantic Ocean from Africa on giant...