During their heyday in the Cambrian Period, some 500 million years ago, trilobites were as common and as diverse as their crustacean cousins are today. These shelled arthropods crawled on seafloors the world over. Some were fingernail size; others as long as a foot. Yet despite that early success, by 250 million years ago they had vanished from the world’s oceans. What happened to them? Some researchers speculate that predators-- including primitive jawed fish--wiped them out. There is a precipitous decline in trilobite diversity as the predator groups got more abundant, says Danita Brandt, a paleontologist at Michigan State University. But Brandt doubts that predation alone can explain the trilobites’ demise. Modern crustaceans, she points out, have no trouble surviving in predator- filled waters.
Brandt thinks another factor may have decided the trilobites’ fate: the way they molted. Like all arthropods, trilobites were encased in a hard protective exoskeleton that they had to shed periodically in order to grow. Each molt is a crisis, says Brandt. During molting the animal is very vulnerable to predators. They shed their hard exoskeleton and then run around--actually, they hide--with this soft new skeleton, which can take hours to harden. And things can go wrong in the process of shedding: an appendage can get stuck in the exoskeleton, and the animal can get injured.
Presumably to minimize that danger, all modern crustaceans have standardized the molting process. They molt the same way every time, Brandt says. In shrimp, a suture opens up between the front half of the exoskeleton and the back, and the animal pops out; crabs also have a suture that opens up every time. Even horseshoe crabs, ancient relatives of crustaceans that have remained virtually unchanged for at least the past 400 million years, always molt in exactly the same way.
Not so for the hapless trilobites. When Brandt studied thousands of fossilized molted exoskeletons from the genus Flexicalymene, she found that most of the time a suture would open up across the head, and the animal would neatly fall out. But sometimes, it seems, that clean suture failed to open; instead a crack split the segments on the trilobite thorax, and the trilobite then wriggled out as best it could, sometimes getting entangled in the process. Molting in at least a dozen other trilobites has been studied by other researchers; none show a consistent pattern.
I won’t go so far as to say that this inefficient molting habit is the reason the trilobites are extinct, Brandt says. If it had been a problem from the beginning, we wouldn’t have seen this huge diversity. What I am suggesting is that an inefficient molt habit plus an increase in predators may have been a one-two punch they couldn’t recover from.