When conditions got tough in the Paleozoic Era, trilobites sprang up. Armed with tough exoskeletons, these ancient arthropods curled up like armadillos to avoid predators and dangerous environmental conditions on the ocean floor.
Many trilobites are found with their fossilized exoskeletons curled up, as if they were constantly squeezing their bellies. However, few of these fossils preserve the internal structures used by trilobites to form their defensive balls.
“Registered trilobite fossils are very common, but the ventral soft tissue is not preserved,” says Dr. Sala Rosso. candidate at Harvard University, specializing in the evolution of trilobites.
Using a treasure trove of perfectly preserved fossils, Rosso and her colleagues may have finally solved the mystery of the fallen trilobite.Their findings were published Wednesday in the journal Proceedings of the Royal Society Bdescribed for the first time the interlocking anatomy of a curled trilobite.
The trilobite fossils examined in the new study were unearthed from the Wolcott Last Quarry in central New York, where a landslide 450 million years ago suffocated entire communities of marine life. Discovered in 1870 by paleontologist Charles Doolittle Wolcott, this site is where the first traces of trilobite appendages, gills, and other soft tissue features were found.
Walcott’s trilobite fossil and the thin sections he cut from it are housed in the Museum of Comparative Zoology at Harvard University. While analyzing the trilobite’s appendages, Rosso found a curled-up Ceraurus trilobite with a set of plates called sternites on the lining of its stomach, making it unlikely to survive fossilization. “I was excited when I found that specimen,” Rosso said. “These plates are not present in the registered three-dimensional specimen.”
The researchers used micro-CT scans to analyze the internal anatomy of the fossils they describe as registered, and examined thin sections that Walcott made in the 1870s. Ceraurus trilobites had spiny shells, so they folded more than they rolled. “It’s more like a taco than a perfect ball,” Rosso said.
These thin sections give researchers the most complete picture yet of how trilobites roll up, revealing the central role played by both the arthropod’s stomach plate and appendages. Became.
Although the sternite plates were not as hard as the calcite-rich shells of trilobites, they were still hard enough to prevent them from rolling easily. To overcome this, the trilobite probably curled up while flexing its entire body, allowing its sternal plates to slide past each other during movements such as sit-ups. The trilobite’s wedge-shaped appendages were then fixed together, allowing the arthropod to curl tightly. “Their little wedge-shaped legs stick together like pizza slices,” Rosso says.
The researchers also compared these structures to the anatomy of living arthropods such as terrestrial isopods, pill bugs, and millipedes. They found that these modern-day brown rollers, although distantly related to trilobites, have similar interlocking mechanisms. The researchers also examined live horseshoe crabs. Horseshoe crabs do not roll, but use their wedge-shaped appendages to crush and move food toward their mouths.
These structural similarities are a good example of convergent evolution, said paleontologist Jorge Esteve, who studies trilobite ecology at Madrid’s Complutense University but was not involved in the paper.
“These morphological features were completely unknown in trilobites, but there are other arthropods that can use similar structures to wrap their bodies,” Dr. Esteve said. “Evolutionary theory sometimes reuses the same answers to address similar problems.”