Richard A. Kerr
Science 1998 282: 19-21.
Could paleontologists have missed a third of the preserved history of animals? That's the implication of a startling claim on page 80. Researchers have grown accustomed to competing claims about when multicellular animals first appeared. In February, new fossil embryos from China pushed the date back tens of millions of years to just before 600 million years ago (Science, 6 February, p. 803), and some molecular biologists sorting through animals' genes have inferred an even earlier origin. Now the new find may extend the fossil record of animals more than 400 million years to 1.1 billion years ago, supporting the oldest molecular estimates of the origins of animals.
In this issue of Science, an international team of scientists argues that wiggly grooves on the surface of ancient sandstone from central India are the tracks of burrowing, half-centimeter-thick, wormlike animals. "If it's true, it's staggering," says paleontologist Charles Marshall of the University of California, Los Angeles (UCLA). "It would be the first evidence of macroscopic animals."
For now, experts in such trace fossils--most of whom haven't yet seen these specimens--are divided on the claim, torn between the convincing appearance of the tracks and their appearance in rock radiometrically dated to hundreds of millions of years before any other animal traces. "I'm a believer," says Tony Ekdale of the University of Utah, Salt Lake City, who has seen one specimen. "I find them convincing." Others are not so sure that these squiggles are traces of life. "I wouldn't be surprised if they turn out to be inorganic," says Sören Jensen of Cambridge University.
To the authors of the study--paleontologist Adolf Seilacher of Yale University and the University of Tübingen in Germany, and sedimentologists Pradip Bose of Jadavpur University in Calcutta and Friedrich Pflüger of Yale--the ancient tracings paint a detailed picture of one creature's life 1.1 billion years ago. The wormlike animal, about the thickness of a drinking straw, plowed through the sediment a few millimeters below the floor of a shallow sea, the group suggests. They argue that the creature propelled itself with rhythmic muscle contractions, or peristalsis, leaving open burrows with raised edges like those of modern worms that move by peristalsis. The animal was probably grazing on the decaying base of a thin mat of microbial life on the sea floor, says Pflüger, because the burrows follow the base of a thin veneer of darker sandstone that may be the remains of the mat. (At press time, Seilacher was in the field in Libya.)
Burrowing by peristalsis suggests to Seilacher and his colleagues that the animal was rather complex. Peristalsis implies a fluid-filled cavity that can be contracted by muscles, and they argue that it also implies the existence of a coelom, a lined cavity between the gut and body wall. Coeloms are common to mollusks, annelid worms, and arthropods but are absent in the simpler flatworms and roundworms. If so, the fossil evidence would support one date offered by some molecular biologists: a 1.2-billion-year age for a major evolutionary split among the coelomate animals, between a group including the annelids and one including the echinoderms.
Pflüger admits that distinguishing true trace fossils from all manner of sedimentary cracks, wrinkles, and ripples is a tricky business, but says that he is "85% confident" that the features were left by an animal. He points out that the burrows are too irregular to be the type of cracks commonly found in such sediments and too sharply delineated to be wrinkles in the sediment surface. The grooves vary in width, but each has a constant width throughout its length, unlike a crack. "If they were 700 million years old," says Pflüger, "there would be no reaction [challenging] the paper." But given the antiquity of the finding, "there will be people contesting it."
Indeed there are. "This is not the smoking gun," says paleontologist and early life expert Bruce Runnegar of UCLA. "It is almost impossible to tell trace fossils from tubular body fossils [of large algae] when they are poorly preserved, as these are. I'd say the jury is out."
Paleontologist Mary Droser of UC Riverside is more persuaded, agreeing with Pflüger that "if we found this in the Paleozoic [younger than 544 million years], we would say it is a trace fossil." But she notes that "there have been a lot of examples [of sedimentary features] that people thought were trace fossils and they were not." And because no large worm burrowings turn up again in the rock record until about 600 million years ago, "I wonder why we go 400 million years without another one," she says. Paleontologist Andrew Knoll of Harvard University agrees that "if you see centimeter-scale, coelomate organisms and then don't see them for 400 million years, you have a lot to explain."
It's possible that relatively complex animals did appear very early but died out, says Marshall, only to evolve again later. Or perhaps there are older animal fossils that haven't been found yet, and the gap is only apparent. "I'm not sure enough people have looked at the right rocks for the right thing," he says. "Five or 10 years from now, are the gaps in the record going to be filled in? That will be the proof of the pudding."