Rare fossils of enigmatic amphibian.

Rare fossils of enigmatic amphibian 

What limbless land vertebrate has a name that sounds like the inhabitants of an Italian island?

Don't despair if the word caecilian fails to come to mind. Even biologists know relatively little about these blind, burrowing amphibians, most of which spend their lives underground and are easily mistaken for worms. Paleonthologists have an even sketchier understanding of the creature's evolution, because the entire fossil record of ancient caecilians has long consisted of only two small vertebrae.

Researchers now report finding a rich cache of ancient caecilian bones that offers an unprecedented look at the early development of these animals. The fossils, found in northeastern Arizona, date to the Jurassic period (about 175 to 180 million years ago) and are some 100 million years older than the other known caecilian remains, report Farish A. Jenkins Jr. of Harvard University and Denis M. Walsh of the University of London.

Containing bones from at least six individuals, the Arizona find includes skulls, vertebrae and, most surprisingly, limb bones. Modern caecilians show no trace of legs even during the earliest phase of the life cycle. But their Jurassic relatives apparently had functional limbs. Because those limbs were unusually small in proportion to total body size, Jenkins surmises that the ancient caecilians also burrowed underground. "Already by Jurassic times, caecilians were beginning to lose their limbs," he says. The Jurassic animals were about 4 centimeters long, he estimates, whereas modern caecilians can reach lengths of more than 1 meter.

Unlike modern forms, which are nearly or completely sightless, the Jurassic caecilians apparently had well-developed eyes, says Jenkins. The ancient burrowers also used tentacles -- as do living species -- to help guide them through their dark world. No other known vertebrate has tentacles, says Jenkins.

Paleontologists have long debated the origins of these cryptic amphibians. Some argue that caecilians evolved from a group of early amphibians called microsaurs, developing separately from frogs and salamanders. Others maintain that all three amphibian groups are more closely related. Unfortunately, says Jenkins, the Arizona fossils "don't necessarily solve the problem over caecilian origins, at least not now."