Could paleontologists have overlooked a third of the history of animals preserved in the fossil record? That's the startling implication of what appear to be worm tracks in 1.1-billion-year-old sandstone, reported in tomorrow's issue of Science.
Researchers have grown accustomed to competing claims on the question of 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. The new find, in rocks from central India, may extend the fossil record of animals more than 400 million years. "If it's true, it's staggering," says paleontologist Charles Marshall of the University of California, Los Angeles.
To the study's authors--paleontologist Adolf Seilacher of Yale University and the University of Tübingen, Germany, and sedimentologists Pradip Bose of Jadavpur University in Calcutta and Friedrich Pflüger of Yale--the ancient tracings come from wormlike animals, about the thickness of a drinking straw, that plowed through the sediment a few millimeters below the floor of a shallow sea. The creatures probably grazed 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.
Pflüger admits that distinguishing true trace fossils from sedimentary cracks, wrinkles, and ripples is a tricky business but says he is "85% confident" 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 have different widths, but the width is constant for any particular trace, 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 the University of California, Los Angeles. "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." And paleontologist Andrew Knoll of Harvard University wonders why there would be such a gigantic gap in the fossil record. "If you see centimeter-scale, [complex] organisms and then don't see them for 400 million years, you have a lot to explain," he says.