When the going got tough in the Paleozoic Era, trilobites rolled up. Armed with sturdy exoskeletons, these ancient arthropods curled up like armadillos to avoid predators or dangerous environmental conditions on the seafloor.
Many trilobites have been found with their exoskeletons fossilized in a curled position, as if holding a perpetual stomach crunch. But few of these fossils preserve the internal anatomy that trilobites used to form a defensive ball.
“While enrolled trilobite fossils are really common, we don’t have any of the ventral soft tissue preserved,” said Sarah Losso, a Ph.D. candidate at Harvard University who specializes in trilobite evolution.
Ms. Losso and her colleagues may have finally unfurled the mystery of tumbling trilobites by using a cache of impeccably preserved fossils. Their findings, published on Wednesday in the journal Proceedings of the Royal Society B, describe the interlocking anatomy of a rolled-up trilobite for the first time.
The trilobite fossils examined in the new study came from central New York’s Walcott-Rust Quarry, where a mudslide 450 million years ago smothered an entire community of the scuttling sea creatures. Discovered by the paleontologist Charles Doolittle Walcott in 1870, the site yielded the first traces of trilobite appendages and soft-tissue features like gills.
Walcott’s trilobite fossils, and thin sections he sliced out of them, are stored at the Museum of Comparative Zoology at Harvard University. Ms. Losso was analyzing the trilobites’ appendages when she came across a curled Ceraurus trilobite with a set of plates called sternites lining its stomach that rarely survives fossilization. “When I found that specimen, that’s when I got excited,” Ms. Losso said. “We don’t have these plates in enrolled, three-dimensional specimens.”
The researchers used micro-CT scans to analyze the inner anatomy of the fossil, which they describe as enrolled, and examined the thin sections Walcott made in the 1870s. Because Ceraurus trilobites had spiny shells, they folded more than they rolled. “It’s a lot more like a taco than a perfect ball,” Ms. Losso said.
These thin sections provided the researchers with the most complete view yet of how trilobites rolled up, revealing the central roles played by both the arthropod’s stomach plates and appendages.
While sternite plates were not as hard as the trilobite’s calcite-enriched shell, they were still rigid enough to prevent easy rolling. To overcome this, the trilobite most likely flexed its whole body as it rolled up, allowing the sternite plates to slide past one another as the animal performed a motion like a situp. The trilobite’s wedgelike appendages then locked together, allowing the arthropod to tightly curl. “Their little wedge-shaped legs fit together like slices of pizza,” Ms. Losso said.
The team also compared these structures to the anatomies of living arthropods like terrestrial isopods, or pill bugs, and millipedes. They discovered that these modern rollers, although distantly related to trilobites, possessed similar interlocking mechanisms. The researchers also looked at living horseshoe crabs. While they don’t roll, horseshoe crabs use wedge-shaped appendages to crush and move food toward their mouths.
The similarity of these structures is a great example of convergent evolution, said Jorge Esteve, a paleontologist who studies trilobite ecology at the Complutense University of Madrid but was not involved in the paper.
“While these morphological features were totally unknown in trilobites, we have other arthropods that are also able to enclose the body using similar structures,” Dr. Esteve said. “Evolution sometimes reuses the same answer to address similar problems.”