A frog that lived nearly 40 million years ago has been found beautifully preserved in a chunk of amber from the Dominican Republic.

Many ancient insects and two extinct lizards have been found entombed in amber, the hardened resin of certain trees, but the frog -- a type of tree frog -- is the first amphibian and the oldest vertebrate.

The preservation is so good that the frog's eyes and large parts of its skin are intact. Much of the skin has become transparent, making the skeleton easily visible.

The discovery, reported in yesterday's issue of Science magazine by two biologists at the University of California at Berkeley, is expected to help in analyzing how the Caribbean islands came to be inhabited by such distinctive animal species as exist there today.

Biologists consider the Antilles remarkable because each island has a large number of "endemic" species found on no other island. There are two leading theories as to how this situation arose.

One is that a larger land mass broke up into the islands of the Caribbean, each of which acquired some of the original land mass's species. New species, unique to each island, may then have evolved. The other, more traditional theory is that the original land mass was relatively devoid of species and that the new islands acquired their diversity as species arrived from elsewhere, blown by winds or carried on floating debris. Chance landfalls gave each island distinctive species.

The amber frog discovery, along with many other amber-preserved animals from the Dominican Republic -- an unusually rich source of such things -- supports the view that a fairly diverse fauna was already present on the land that would become the Antilles, the biologists said.

Several lines of evidence indicate that the amber formed, trapping the frog, 35 million to 40 million years ago. This is about 20 million years before a large "paleoisland" broke up to form Cuba, Puerto Rico and the northern half of Hispaniola, the island shared by the Dominican Republic and Haiti.

The antiquity of the frog is not remarkable. The earliest known fossils of frog bones were found in sedimentary deposits laid down about 200 million years ago.

The frog was found by a local worker in an amber mine in the mountains near Santiago, a source of many earlier discoveries, mainly insects and spiders. The specimen was bought by an amber dealer, who lent it to Berkeley's George O. Poinar Jr., a specialist in amber animals, and David C. Cannatella, a vertebrate zoologist. The specimen is valued at $25,000.

"Events leading to the entombment of the frog were probably traumatic," Poinar wrote in his Science report. He suspects that the frog, which is just under an inch long, became trapped when an animal, possibly a bird, carried it to its nest in a hole in a tree. The frog's left leg and right arm are broken.

There the frog came into contact with soft resin and was covered before it could be eaten. Adjacent pieces of amber contain parts of a similar frog, several insects and plant matter that appears to have been part of a nest.

Five years ago Poinar startled many biologists when he discovered that a 40-million-year-old fly in amber retained intact cells and intracellular structures.

Poinar attributes amber's preserving qualities to the fact that it surrounds an animal so closely that all air is excluded, minimizing damage from oxidation, and to the fact that amber contains chemicals that would kill decay-causing bacteria. The ancient Egyptians wrapped mummies in resin-soaked cloths.