The secret of most mammals' extraordinary evolutionary success, paleontologists believe, lies in the teeth. Placental and marsupial mammals have a so-called tribosphenic molar that both slices and grinds, improving and expanding their diet. Now three paleontologists propose that this type of molar was not a singular key innovation, but evolved twice. "It shakes a bedrock principle that we've held for a long time," says Andy Wyss of the University of California (UC), Santa Barbara.
The importance of tribosphenic molars seemed clear, initially, from studies of fossils found in Asia, Europe, and North America. They showed a step-by-step progression toward more and more tribosphenic features. Paleontologists concluded that mammals with this type of tooth had most likely arisen from a common ancestor that lived in the Northern Hemisphere during the Early Cretaceous. In contrast to the successful placental and marsupial mammals, the more primitive monotremes--an extremely ancient group that includes the platypus--were thought to have lacked tribosphenic molars and evolved in the southern continents.
Cracks in the theory appeared in 1985, with a report of the jaw of a fossil mammal, called Steropodon, from Early Cretaceous rocks in Australia. The jaw clearly belonged to a monotreme, but it bore relatively advanced teeth that vaguely resembled tribosphenic molars. In the late 1990s, unquestionably tribosphenic molars turned up in Australia and Madagascar--in the wrong place at the wrong time. One of these animals was found in mid-Jurassic rock, meaning it lived in the southern hemisphere tens of millions of years earlier than animals with transitional molar forms in the north.
Trying to resolve the puzzle, three researchers--Richard Cifelli of the Oklahoma Museum of Natural History in Norman; Zhexi Luo of the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania; and Zofia Kielan-Jaworowska of the Polish Academy of Sciences--examined the bones and teeth of 21 living and fossil mammals. The paleontologists divided the fossils, based on skeletal and tooth features, into two evolutionary groups, one southern and the other northern. The tribosphenic molar originated independently in both groups, they propose in the 4 January issue of Nature.
The hypothesis is "going to be very, very stimulating," says Bill Clemens of UC Berkeley. But not everyone is convinced. "I think they're sticking their necks out pretty far," Wyss says, noting that the remains of the southern fossils include only teeth and jaws--no upper teeth, skulls, or other bones. "There's a tremendous amount of missing information here," he adds.