Trilobites, the long-extinct ancestors of modern crabs and lobsters, may have evolved much earlier than previously believed, according to new research. The finding suggests that the Cambrian radiation, the so-called "big bang" of life more than 500 million years ago, started earlier and took longer than thought. The research also has implications for the highly debated breakup of an ancient supercontinent.
The traditional view of the Cambrian explosion is that life underwent an extraordinary, rapid diversification that resulted in the nearly simultaneous appearance of the ancestors for most major types of animals. To investigate when this explosion of life began, paleontologist Bruce Lieberman of the University of Kansas in Lawrence, used a computer algorithm to determine the degree of relatedness of more than 100 trilobite species by comparing traits such as length and number of body segments from thousands of fossils. The analysis suggests that trilobites were already well-diversified by the time most researchers thought the Cambrian radiation began.
Although the cause of the Cambrian radiation is unknown, many scientists suspect that the breakup of a southern supercontinent called Pannotia could have isolated populations and created new ecological niches that spurred rapid evolution. Exactly how and when Pannotia split apart, however, is highly contested. One prominent theory holds that the split began 523 million years ago and sent the continents speeding from the South Pole to the equator in just 15 million years--a rate roughly 13 times faster than the 5 centimeters per year average of today's continents. But the trilobite relationships suggest the split began much earlier and was more gradual.
So Lieberman teamed up with geologist Joseph Meert of the University of Florida in Gainesville who studies the past locations of continents using the magnetization of rocks. When a rock forms, its magnetic field lines up with Earth's, creating a record of the rock's latitude at that moment. Combining information about the locations of the various trilobite families with the magnetic data, the team concluded that Pannotia began rifting 580 million years ago near the South Pole and the continents took 80 million years to drift to the equator--a more reasonable pace of 15 centimeters per year, the pair reports in the May issue of the Journal of the Geological Society, London.
The new study could help reveal the truth about a time period that has a dearth of appropriate rocks for quality magnetization studies, says paleomagnetist John Geissman of the University of New Mexico in Albuquerque. "This time period is a real bugger," he says. "But Meert and Lieberman's approach gives a hint at which [breakup theory] is more viable."