When the going got tough in the Paleozoic Era, the trilobites rolled over. Armed with strong exoskeletons, these ancient arthropods curled up like armadillos to avoid predators or dangerous conditions in the underwater environment.
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 build a defensive ball.
“While enrolled trilobite fossils are really common, we don’t have any ventral soft tissue preserved,” said Sarah Losso, a Ph.D. candidate at Harvard University specializing in trilobite evolution.
Ms. Losso and his colleagues may have finally solved the mystery of the fall of trilobites by using a cache of unmistakable fossils. Their findings, published Wednesday in the journal Proceedings of the Royal Society Bdescribe the interlocking anatomy of a composite trilobite for the first time.
The trilobite fossil examined in the new study came from central New York’s Walcott-Rust Quarry, where a mudslide 450 million years ago buried an entire community of scuttling sea creatures. Discovered by paleontologist Charles Doolittle Walcott in 1870, the site yielded the first traces of trilobite appendages and soft-tissue features such as gills.
Walcott’s trilobite fossils, and thin sections he cut from them, are stored in the Museum of Comparative Zoology at Harvard University. Ms. studies. Losso lost the appendages of trilobites when he found a curly Ceraurus trilobite with a row of plates called sternites wrapped around its abdomen that rarely survive fossilization. “When I found that specimen, I was so happy,” said Ms. Loss. “We don’t have these plates in enrolled, three-dimensional specimens.”
The researchers used micro-CT scans to study the internal anatomy of the fossil, which they describe as recorded, and examined thin sections made by Walcott in the 1870s. Because Ceraurus trilobites have spiny shells, they fold more than they fold. “It’s more like a taco than a perfect ball,” Ms. Loss.
These thin sections have given researchers the most complete view yet of how trilobites were put together, revealing the key roles played by both abdominal plates and arthropod appendages.
While the sternite plates are not as hard as the calcite-enriched shell of the trilobite, they are still strong enough to prevent easy rolling. To overcome this, the trilobite probably flexed its entire body as it rolled, allowing the sternite plates to slide over each other as the animal performed a movement such as a situp. The trilobite’s wedge-like appendages lock together, allowing the arthropod to flex tightly. “Their little wedge-shaped legs fit together like pizza slices,” Ms. Loss.
The team also compared these structures to the anatomies of living arthropods such as 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. Although they do not roll, horseshoe crabs use wedge-shaped appendages to crush and move food toward their mouths.
The similarity of these structures is a good 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 are completely unknown in trilobites, we have other arthropods that can also attach the body using similar structures,” said Dr. Esteve. “Evolution sometimes uses the same answer to solve similar problems.”
