By Sean Henahan, Access Excellence

The extinction of the trilobite has remained an even deeper mystery than the extinction of the dinosaurs. Trilobites existed during the Paleozoic era, from the Cambrian to the Permian period about 540-245 million years ago. A model of successful evolution for some 300 million years, the marine arthropod suddenly became extinct.

The missing piece in the puzzle, according to a new study, is molting. the periodic shedding and replacing of the outer skeleton. Unlike modern counterparts like crabs, lobsters and shrimp, the trilobite molted in a very inconsistent manner.

"They got out any way they could," said Danita Brandt of Michigan State University's Department of Geological Sciences. "They winged it."

Modern arthropods molt the same way every time. The same suture opens every time, letting the animal out. This, Brandt said, could be the reason why they live on today while trilobites are extinct.

"When the same technique is used, there is less of a chance that things will go wrong," she said. "Molting is a very dangerous time for an arthropod. A lot of things can go wrong."

Studying thousands of fossilized trilobite remains, Brandt hypothesized that poor molting habits, along with the increased numbers of trilobite-eaters, did them in.

"Take this less effective molting style and add to that the increased predation pressure, and I think that spells the formula for extinction," she said.

Until recently, the demise of the trilobite had been blamed on a sudden increase in the numbers of trilobite predators. Fossil records show that the numbers of trilobites began to drop as other aquatic animals, such as fish and cephalopods (squid and octopus), began to increase.

"But it's highly unlikely that predators ever eliminated an entire group," Brandt said. "Another argument against predation alone is that other arthropods continue to thrive even today despite the proliferation of predator groups. Why are these modern arthropod groups doing so well, even in the face of adversity, while trilobites have become extinct?" she asked.

"Trilobites got out of the evolutionary gate with a bang and then whimpered out," Brandt said. "Modern arthropods, on theother hand, took a long time to establish an evolutionary foothold and have been very successful."

Brandt said there is some indication that, as extinction loomed, the trilobite made an attempt to correct this evolutionary flaw. After the Cambrian Period, the trilobites became smaller, having fewer body segments.

"If there were more segments, each could be shed individually, providing more of an opportunity for something to go wrong," she said. "Reducing the number of thoracic segments could have been an attempt at improving the design.

"Was this a step toward more efficient molting? Why didn't it take the next step? We may never know," she said.

Dr. Brandt presented her findings at a recent regional meeting of the Geological Society of America in Madison, Wis.