Remains may be an evolutionary relic.

Fossils recently found in southwestern China may be of a lineage that originated long before the Cambrian explosion of biodiversity, when most major groups of animals first appeared in the fossil record.

The frondlike animals were up to 7.5 centimeters long and anchored themselves to the seafloor with a hollow stalk, says Simon Conway Morris, a paleontologist at the University of Cambridge in England. The rocks bearing the fossils were deposited on an ocean bottom about 520 million years ago during the Cambrian period. The fine-grained sediments that covered the organisms preserved details of the animals that haven't been seen in other fossils. Such features include a network of channels that ran between neighboring branches of the frond.

"These [animals] are strikingly similar to older organisms," says Conway Morris. Creatures with multiple frondlike structures lived at least 20 million years earlier, during the Ediacaran period, the interval that immediately preceded the Cambrian.

Conway Morris and his colleagues suggest in the May 5 Science that the Cambrian fronds are members of a lineage that survived the Ediacaran period, although they mention other potential scenarios.--S.P.