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5/1/97
MSU Researcher Presents Findings at National Conference
MOLTING PATTERNS MAY HAVE CONTRIBUTED TO THE DEMISE OF THE ONCE-HARDY TRILOBITE
MADISON, Wis. -- Research by a Michigan State University paleontologist has shed new light on why the trilobite, a prehistoric arthropod that inhabited the Earth for nearly twice as long as the dinosaurs, met the same fate as the dinosaur nearly 250 million years ago.
The key, said Danita Brandt of MSU's Department of Geological Sciences, is molting, the act in which an arthropod sheds and replaces its outer skeleton. Unlike crabs, lobsters and shrimp, the trilobite's evolutionarily successful cousins, the trilobite -- every grade schoolers' favorite fossil -- molted in a very inconsistent manner. "They got out any way they could," she said. "They winged it."
On the other hand, today's modern arthropods molt the very same way every time. The same suture opens every time, letting the animal out. And 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. Phacops, a common trilobite.
"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.
At one time, trilobites were one of the more evolutionarily successful creatures that roamed the early world's oceans. The small, crab-like creatures thrived, especially during the Cambrian Period. It was at the end of the Paleozoic Era that the trilobite disappeared.
"Trilobites got out of the evolutionary gate with a bang and then whimpered out," Brandt said. "Modern arthropods, on the other 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."
Brandt presented her findings at a recent regional meeting of the Geological Society of America in Madison, Wis.
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