The smallest embryo of an extinct reptile ever reported -- the surprisingly well-preserved, two-inch-long skeleton of a lizardlike creature that lived among dinosaurs 230 million years ago -- has been discovered on a museum shelf in Switzerland.

Study of the skeleton, which has the large head and huge eyes typical of most embryonic and young animals, is expected to shed light on methods of reproduction among extinct reptiles. One question, for example, is whether they laid eggs or gave birth to live young.

The discovery is reported in today's issue of Science by P. Martin Sander of the University of Zurich. Sander said the fossil had been on the museum's storage shelves, unrecognized, since it was excavated in the Alps probably 60 years ago.

The skeleton is of a nothosaur, a well-known group of Triassic Period reptiles with long necks and long tails that inhabited warm, shallow coastal waters but which could also crawl on land. The embryo belongs to the species Neusticosaurus, adults of which were about nine inches long. The university's museum has 97 specimens of Neusticosaurus, including a full range of adults and young.

Sander said nothorsaurs were less completely adapted to life in the sea than ichthyosaurs and plesiosaurs, large, air-breathing reptiles whose legs had evolved into paddles. Nothosaurs, although they had feet suited to walking on land, had short legs and the heavier ribs needed by an aquatic air-breather to keep its lungs from collapsing in deep water.

Although excavated from what is today a mountainous region, the embryonic skeleton died well out to sea. The rock in which it was found was formed of sediment laid down long before the sea floor rose to become mountains. Sander said geological analysis indicates the fossil was originally deposited several miles out from the ancient coast.

Sander said Neusticosaurus' ability to crawl on land suggests it may have been an egg layer. Most aquatic reptiles today that walk on land do so chiefly to lay eggs. On the other hand, if the specimen had been in an egg laid on land, it seems unlikely that it could have wound up on the sea floor so far from shore.

Sander said he suspects the embryo was not in an egg but was the product of a spontaneous abortion that happened while the mother was at sea. The skeleton is well preserved, even down to tiny foot bones, but there is no sign of an eggshell in the rock surrounding it.