Adding to a growing list of spectacular fossils emerging from the African island nation of Madagascar, two reports in the current issue of the journal Nature reveal intriguing new finds. The 70-million-year-old specimens provide key insights into the some of the largest and smallest vertebrates ever to walk the earth.
In the first report, Catherine Forster of the State University of New York at Stony Brook and Kristina Curry Rogers of the Science Museum of Minnesota announced the discovery of a nearly complete skeleton representing a new species of sauropod dinosaur. Named Rapetosaurus krausei, after both a mythical character in Malagasy folklore and expedition leader David Krause (also at Stony Brook), the creature belongs to group of dinosaurs known as titanosaurs. Importantly, Rapetosaurus provides the best picture yet of the herbivorous giants. "What's been particularly frustrating to paleontologists who study these beasts is that we haven't had a clue what a titanosaur skull looks like," Curry Rogers remarks. "Rapetosaurus gives us our first view of a titanosaur from head to tail."
In the second study, Krause describes a tiny fossil molar with sizable implications for scientists' understanding of early mammal biogeography and plate tectonics. Telltale features of the tooth identify it as that of a marsupial mammal. As such, Krause notes, it represents the only marsupial, fossil or living, from Madagascar.
A leading theory of mammal evolution holds that marsupials and placentals (the group to which we belong) arose in the Northern Hemisphere and later spread to the southern supercontinent of Gondwana via South America. If marsupials had already arrived in Madagascar by the end of the Cretaceous period, as the new specimen indicates, they most likely entered Madagascar through Antarctica. This suggests that South America, Antarctica and Madagascar were connected much later than most models of plate tectonics estimate. The tooth discovery also bolsters the argument that the ancestors of the mammals that inhabit Madagascar today, all of which are placental, were not on the island during the late Cretaceous. Rather, Krause concludes, these creatures most likely arrived after the Cretaceous, and that "probably involved crossing a significant marine barrier."