Molting habits may have led to extinction of once-hardy trilobite

November 25, 2002 – Molting, that periodic ritual in which arthropods shed and replace their outer skeletons, can be a dangerous time for the creatures. Just ask the trilobite.

Research published by an MSU paleontologist suggests that an inconsistent molting style, coupled with inefficient physiology, contributed to the demise of these prehistoric relatives of today’s crabs and lobsters nearly 250 million years ago.

“They would shed their old exoskeleton any way they could,” says Danita Brandt, a faculty member in MSU’s Department of Geological Sciences whose findings were published in the Australian paleontology journal Alcheringa. “They had to improvise.”

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.

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

Brandt’s proposed connection between arthropod molting and evolutionary fate is based on two pieces of evidence: the inconsistency of molting patterns that characterize trilobites, in contrast to the consistent patterns seen in modern arthropods; and her observation that certain trilobites that had fewer body segments tended to live longer ? evolutionarily speaking ? then those that had many segments.

“Trilobites with fewer segments probably had a lower risk of molting-related accidents, and may have shed their old exoskeleton more quickly,” she says. “These are traits of modern arthropods that act to minimize the period during which the animals are vulnerable to predators.”

Brandt also noted that trilobite molting differed from molting in modern arthropods in another potentially important way: many modern arthropods resorb minerals from the old exoskeleton or consume their molted exoskeleton, thus conserving resources.

“There is no evidence that trilobites used these conservation strategies,” she says. “Apparently trilobites were faced with the considerable task of rebuilding a heavily calcified skeleton ‘from scratch’ with each molt.”

At one time, trilobites were one of the more evolutionary successful animals to roam the early world’s oceans. The crab-like creatures, some of which were as small as a fingernail while others were nearly a foot long, thrived, especially during the Cambrian Period. It was at the end of the Paleozoic Era that the trilobite disappeared.

For years the trilobite’s extinction had been blamed on a sudden increase in the numbers of trilobite predators. Fossil records show that the number of trilobites began to drop as other aquatic animals, such as fish and squid, began to increase.

“But it’s highly unlikely that predators ever eliminated an entire group,” Brandt says. “Another argument against predation alone is that other arthropods continue to thrive even today despite the proliferation of predator groups.”

Other theories linked to trilobite extinction include climate change, sea-level fluctuation, and even the effects of meteorite impact. However, the correlation between these possible causes and the pattern of trilobite extinctions is not consistent, Brandt says.

“I think there is a biological ‘wild card’ that complicates the correlation of trilobite extinction with environmental factors, and for the trilobites I think that wild card was the unique challenge they faced during molting,” she says.