Revealing ancient secrets: 3D preservation

They have been in the collection of Harvard University’s Museum of Comparative Zoology (MCZ) since the 1870s, when they were first discovered. Unique fossils nestled among the largest collection of trilobites 145 years later, Sara Rosso, a doctoral candidate in Harvard’s Department of Biology and Evolutionary Biology (OEB), begins scouring the collection of trilobites as part of her research. Until then, it had been sitting in a drawer. paper.

“I started my PhD studies looking at all these thin sections of trilobites and trying to image them and figure out what we could actually see,” Rosso said. “And we came across something that would never be seen in a trilobite fossil.”

In a new study published in Proceedings of the Royal Society B, lead author Losso describes an unusual three-dimensional trilobite fossil that was prepared as a thin section showing 3D soft tissue during registration. This study revealed the soft underside of registered trilobites and the evolutionary mechanisms that allow arthropods to register their bodies to protect themselves from predators and adverse environmental conditions.

Trilobites are early arthropods of the Paleozoic era. They were extremely diverse and numerous until they became extinct in the end-Permian mass extinction. Trilobites are named for their three-lobed bodies covered in a durable exoskeleton rich in calcite that is easily preserved. Trilobites have become an iconic part of the Paleozoic fossil record. Their segmented bodies have numerous limb pairs, including walking legs and gills for breathing. Unlike the durable exoskeleton, the underside, including the legs, is much softer, making it unlikely to fossilize unless perfect conditions are met. Trilobites look similar to horseshoe crabs, but they are not closely related. However, horseshoe crabs are useful for comparison because they have similar lifestyles.

The challenges associated with soft tissue fossilization make the trilobites Rosso studied even more special. The fossils date from the Mohawk period of the Ordovician period (462-451 million years ago). They were discovered at the Wolcott Last Quarry in upstate New York near Trenton Falls. The area was originally inhabited by the Iroquois people. The quarry is named in part for scientist Charles D. Walcott, who discovered trilobites registered there as a young man, prior to discovering the famous Burgess Shale while director of the Smithsonian Institution. I did.

The fossils, which Wolcott sold to MCZ and the Smithsonian Institution in the 1870s, were trapped in a slurry of sediment that quickly moved downhill to bury the trilobites and preserve their delicate tissues before being destroyed by decomposition. It was decided that It is unusual in that soft tissues such as the legs and antennae are preserved in 3D. Walcott studied the fossils by cutting them into paper-thin slices of rock and attaching them to glass slides using balsam sap. Although doing his best with what was available at the time, Walcott’s method of preparation makes studying the fossil difficult because his 3D structure is seen as his 2D plane.

“These were the first complete trilobite appendages known. Scientists knew about walking legs until they were discovered in the late 1800s, but they didn’t know what branchial branches looked like. I didn’t know that,” Rosso said. Due to environmental disturbance, trilobites have registered more delicate appendages to protect them. Sediment then surrounded the partially registered trilobite leg, forming an external mold even as the tissue decayed.

Enrollment occurs in a variety of organisms. This is a defense strategy for animals that have a hard exoskeleton and soft tissue underneath. There are also registrations of modern animals such as pill bugs (isopods), pill bug millipedes (millipedes), and even armadillos. By registering their bodies, these animals are able to protect their fragile soft tissues from predators with a hard exoskeleton. In modern terrestrial arthropods, it can also protect against desiccation and water loss.

The registration mechanism of trilobites is well studied, but due to the lack of registered fossils with preserved soft tissue, these observations have only been made by examining the exoskeleton. Of the 20,000 species of trilobites, fewer than 40 have preserved soft tissues. And of those 40, most only have parts of their legs or antennae preserved. Only 12 species are known to have complete appendages, but most are preserved as highly compacted, flattened fossils, as seen in the Burgess Shale of British Columbia.

“These fossils are the first clear demonstration of the three-dimensional organization of trilobite soft tissue, and also represent the first types of trilobites at various stages of registration, allowing them to be “We were able to actually see how it moves its appendages and other structures in sequence,” Rosso said.

Trilobites and other arthropods have rows of dorsal exoskeletal plates on their backs and undersides. The back plate, called tergite, is reinforced and much larger than the lower plate. Granite is a hard slab that runs along the underside, and is rarely found in the fossil record because it is softer and more susceptible to decay. However, the Wolcott Last fossil preserved ventral structures, including the sternum and limbs.

Although a soft surface, the sternum is too long and stiff to register if the animal cannot flex or articulate. To avoid this, the sternums actually slide past each other in a dipping motion, similar to window blinds, curling up into a ball. Trilobite legs have also adapted to allow this movement by evolving into a wedge shape that fits snugly like a pizza slice inside a registered ball.

“Because the part of the leg that is attached to the body is rarely seen, and certainly not in 3D, people often represented the cross-section of the leg as an oval or square,” Rosso said. . “However, with oval or square legs, full registration requires flexibility.”

Rosso compared the Wolcott-Rust fossil to CT scans of modern arthropods, including millipedes, isopods, and horseshoe crabs, also in the MCZ collection. Rosso discovered that modern arthropods use the same sternal movements seen in trilobites to register with their bodies. Trilobites have been discovered throughout the Paleozoic era. Adaptations for registration have allowed them to grow and have evolved structures that aid in registration, such as the correct proportions and number of abdominal segments to maintain body registration, and wedge-shaped legs.

“This fossil has been known for a long time, but no one had put together the idea that we could use these really amazing fossils to study abdominal adaptations for registration,” Rosso said. . “These fossils allowed us to compare trilobites and modern arthropods, and showed that given the body plan of arthropods, there is really only one way to achieve registration. This is a great example of convergent evolution occurring between all these different lineages and over a vast time span, as we see this in the Ordovician and today. It’s a strategy.”

“Sarah’s research will greatly advance our understanding of the key behavioral strategies that have made trilobites so incredibly successful for more than 200 million years, and will also create a historic collection of Wolcott-Rust fossils that have not been studied in the MCZ for more than 100 years. “It has brought new attention to the.” Lead author Javier Ortega Hernández is an assistant professor at OEB and curator of invertebrate paleontology at MCZ. “New data on the three-dimensional morphology of trilobites at the time of their accession allows us to accurately model this complex strategy for the first time, providing a beautiful example of convergent evolution underway between distantly related species. ”