Heirs of ancient enigmas.

Living organisms have populated the Earth for more than 3.5 billion years, but for most of that time they remained exceptionally small. Then sometime during the late Proterozoic era, about 580 million years ago, life got big. In rocks dating from this time, paleontologists have found a wide variety of large impressions left by a group of puzzling soft-bodied organisms called the Ediacara fauna, which later disappeared from the fossil record.

Scientists have long puzzled over the fate of the Ediacara: Did they leave relatives that evolved into animals seen in later periods of Earth's history, or did these early organisms represent a failed evolutionary experiment -- a separate offshoot of life that did not succeed? One researcher now describes evidence that some of the Ediacara fauna survived.

Simon Conway Morris of the University of Cambridge in England reports that one type of Ediacaran organism, known as Charniodiscus, bears a close resemblance to a newly identified animal from the Burgess Shale, a fossil site dating to the middle Cambrian period, roughly 520 million years ago. The Burgess Shale animal is a sea pen-like creature (a relative of the sea anemone) that apparently lived on the ocean bottom, using branched fronds to filter food out of the water. Because of the striking similarity between Charniodiscus and the Burgess Shale animal, Conway Morris suggests the two are related. Charniodiscus may even represent an ancient member of the sea pen group.

He also points out that several bag-like Ediacaran organisms resemble an enigmatic creature from the Burgess Shale called Mackenzia costalis, which looks a little like a zeppelin. Paleontologists do not know how to classify M. costalis, though Conway Morris suggests it is similar to cnidarians -- members of a phylum that includes sea anemones, jellyfish, and coral.

While debating the fate of the Ediacara fauna, paleontologists have also puzzled over the exact nature of these organisms, wondering whether they were animals, plants, fungi, or something different. The close resemblances between several Burgess Shale organisms and some Ediacaran ones leads Conway Morris to suggest that at least some members of the early group were indeed animals that survived into the Cambrian period. But he also allows that other types of Ediacaran fossils may represent separate branches of evolution that disappeared.