The discovery of a 70-million- to 65-million-year-old molar in Madagascar is raising questions about whether mammals from Northern Hemisphere invaded the supercontinent Gondwana during the time of the dinosaurs, as commonly thought. The marsupial tooth, reported in the 3 August issue of Nature, is the earliest ever found in the Southern Hemisphere and suggests that these mammals may in fact have originated there.
Most mammal fossils from the Late Cretaceous period come from northern continents. This has led to a dominant theory that marsupials and placental mammals arose in the Northern Hemisphere and over time displaced archaic groups of mammals living on the southern continents, such as South America and Australia, that made up Gondwana. Platypuses, echidnas, and other monotremes were the exception; they existed on southern landmasses during the Early Cretaceous. In 1993, David Krause, a paleontologist at the State University of New York, Stony Brook, decided to further investigate the mammals of the Southern Hemisphere living before the postulated northern invasion. "We didn't know what to expect," Krause says.
Krause and his team worked 6-week stints during the summers in the semiarid Mahajanga Basin of northwestern Madagascar. To look for mammal fossil remains, Krause and his team washed sediment through a screen and then sorted through the leftovers under a microscope. Over the course of six expeditions, the team has found seven ancient mammal teeth. The team identified one piece of the new marsupial molar in 1998 and turned up another part of it a year later. Krause believes the tooth adds significantly to the accumulating evidence that marsupials were already broadly distributed on Gondwana before lemurs and other modern placental mammals arrived on Madagascar about 88 million years ago.
"This tooth is to existing models as a square peg is to a round hole," says Richard Cifelli, a paleontologist at the Sam Noble Oklahoma Museum of Natural History in Norman. Along with post-Cretaceous marsupials identified in recent years from South America, Antarctica, Africa, and Australia, as well as a Late Cretaceous placental mammal from India reported in 1994, the new molar suggests that southern landmasses have an unexpected story to tell. As Cifelli remarks, "Krause and colleagues are exploring a paleontological twilight zone."