Reptile remains fill in fossil record.

The fossil remains of a sphenodontian, an ancient, lizardlike reptile, are helping span a 120-million-year data gap between its ancestors and today's tuatara, which are the sole survivors of a once prominent group.

Sphenodontians evolved around the same time that dinosaurs did, about 235 million years ago. Sphenodontians flourished until 120 million years ago, when they disappeared from the fossil record known until now, says Fernando E. Novas, a paleontologist at the Argentine Museum of Natural History in Buenos Aires. Many scientists thought that sphenodontians declined in response to competition from lizards, their supposed ecological successors. However, the newly discovered 90-million-year-old fossils--unearthed in northwestern Patagonia and described in the Oct. 9 Nature--suggest that sphenodontians thrived for much longer than previously had been suspected.

Adult members of the newly described species dominated their ecosystem and measured up to 1 meter in length, making them the largest known land-dwelling sphenodontians of any era. Today, only scattered populations of two species of tuatara--each measuring about 65 centimeters long--live on remote islands of New Zealand (SN: 11/8/97, p. 300).--S.P.