Early kin of vertebrates found in China.

Looking something like a tiny squashed eel, a newfound fossil from southwest China will not win any beauty contests. Yet this 530-million-year-old specimen from Earth's Cambrian period holds a special allure for paleontologists. It may be the oldest known chordate, the phylum to which humans and all other vertebrates belong.

"We've managed to push the story back considerably," says Simon Conway Morris, a paleontologist at the University of Cambridge in England, who collaborated with D.-G. Shu and X.-L. Zhang at Northwest University in Xian, China. They describe the 2.2-centimeter-long animal, named Cathaymyrus diadexus, in the Nov. 14 NATURE.

Before the discovery of Cathaymyrus, the oldest fossil generally recognized as a chordate was the 520-million-year-old Pikaia from Canada's Burgess Shale fossil beds.

Cathaymyrus comes from the Chengjiang fossil site in Yunnan Province and has a number of key chordate features, according to the researchers. Along its abdomen, the creature has V-shaped segments that closely resemble the stacked muscle blocks in primitive living chordates such as amphioxus. The Chengjiang fossil also has a crease like impression running partway down the back of its body. The scientists interpret this as the imprint left by the animal's notochord--a rodlike structure in primitive chordates that is related to the backbone of vertebrates.

The new fossil is not the only potential chordate from Chengjiang. Last year, paleontologists identified a creature called Yunnanozoon as the earliest known chordate (SN: 2/3/96, p. 74). But Shu and others contested that interpretation, arguing in the April 4 NATURE that Yunnanozoon was a hemichordate, a separate phylum that includes the modern acorn worm.

Although the two phyla appear quite different today, chordates are often linked with the hemichordates and another dissimilar phylum, the echinoderms, which include starfish. All three share a distinctive pattern of embryo development, suggesting a common ancestry. Scientists debate, however, how chordates evolved from such improbable stock.

The Chengjiang animals lived only a few million years after the Cambrian explosion, a biological Big Bang during which most modern animal phyla suddenly appeared in the fossil record. By pushing the record of chordates so close to this pivotal event, the new discovery "accentuates the speed of the diversification," says Conway Morris. He notes, however, that a controversial gene study, published in the Oct. 25 SCIENCE, suggests that modern animal phyla arose about a billion years ago, much earlier than the Cambrian explosion.