Breaking up is hard to time

Natural History, Sept, 2004 by T.J. Kelleher

Ever since plate tectonics began to gain acceptance in the 1960s, investigators have been trying to deduce the distribution of landmasses and life-forms in earlier eras. One persistent puzzle is the breakup of the southern supercontinent known as Gondwana, source of the modern continents of Africa, Antarctica, Australia, and South America, as well as the Indian subcontinent and the large island of Madagascar. When, and in what sequence, did the whole become parts?

One hypothesis has been that Africa split off first, between 140 million and 120 million years ago, and that the two key remaining land bridges--one linking Antarctica with South America, the other linking Antarctica with Australia--broke off between 90 million and 80 million years ago. Based on recent fossil discoveries, however, Paul C. Sereno, a paleontologist at the University of Chicago, and his colleagues reject that hypothesis.

Working in Niger, the investigators uncovered the 95-million year-old fossilized remains of an abelisaurid dinosaur--remains that bear a strong resemblance to fossils found recently in South America. Nearby, Sereno and his colleagues also found fragments of the abelisaurid's forerunners, dating to about 110 million years ago. The finds indicate the persistence of a trans Atlantic land bridge between Africa and the rest of Gondwana, via South America, as late as 95 million years ago. The story of evolution in the ancient Southern Hemisphere may now need some major editing. ("New dinosaurs link southern landmasses in the Mid Cretaceous, "Proceedings of the Royal Society of London B 271:1325-30, July 7, 2004)