Bones from The Dawn of Dinosaurs

Paleontologists know plenty about the demise of the dinosaurs, not just how it happened--most likely in the aftermath of a cataclysmic comet or an asteroid impact 65 million years ago--but also the variety of species that were around at the time. But there is very little evidence about the other end of the age of dinosaurs. No one knows precisely when the "terrible lizards" arose or what the earliest dinosaurs were like.

Now the shroud is starting to come off that mystery, thanks to a discovery reported last week in the journal Science. Digging in Madagascar, an international team headed by John Flynn of Chicago's Field Museum has unearthed the fossil jaws of two dinosaurs that appear to be around 230 million years old. "These are either the earliest or among the earliest dinosaurs known," comments University of Pennsylvania paleontologist Neil Shubin. The scientists also found fossils of eight other primitive animals, some of which are a key to the evolution of mammals, which arose at about the same time as the dinosaurs.

The dinosaur bones have not yet been fully analyzed, but they appear to belong to early prosauropods, small herbivores that are most likely the ancestors of the giant Apatosaurus (once known as Brontosaurus). Says Flynn of the little beasts: "I like to think of them being somewhat like kangaroos. They were similar in size, and while they didn't hop, they probably walked about on four legs and stood up on two legs to feed." Most of the other fossils come from rhynchosaurs (parrot-beaked reptiles). The rest are cynodonts, cold-blooded, reptile-like animals--the ancestors of modern mammals.

The dating of this find is indirect. The rocks that held the fossils didn't have the right minerals for radiometric dating. So the researchers relied on known ages of kindred vertebrates (for example, the rhynchosaurs appear to be anatomically primitive cousins of ones known to be about 228 million years old). The scientists pegged the Madagascar fauna at about 2 million years older.

Together, the remains open an unprecedented window into a time when nature was setting the stage not only for dinosaurs but also for the age of mammals that followed--and the eventual rise of the human species. Says Shubin: "If you look at the major groups of animals in the world today--mammals, crocodiles, turtles, frogs--most appeared during the Triassic, 220 million to 200 million years ago." With new discoveries making the origin of these groups ever more remote, he adds, "any find dating to this period is clearly very crucial."