A new study by the University of Chicago found that a prevalent species of 335-million-year-old shark had sophisticated jaws that functioned in much the same way as modern nurse sharks.
Researchers used CT scans of fossils embedded deep in rock to create 3-D models of the movements and mechanics of the prehistoric sharks. The findings revealed that the sharks had evolved the capabilities as suction feeders some 50 million years before previous thought.
According to the study, the findings show “both the evolutionary versatility of sharks, and how sharks responded quickly to new ecological opportunities in the aftermath of one of the five big extinctions in Earth’s history.”
“Among today’s aquatic vertebrates, suction feeding is widespread, and is often cited as a key factor contributing to the spectacular evolutionary success of ray-finned fishes,” said Michael Coates, Ph.D., professor of organismal biology and anatomy at the University of Chicago and senior author of the new study. “But here we show that high-performance aquatic suction feeding first appeared in one of the earliest known sharks.”
The study, published this week in Science Advances, describes the fossil of Tristychius arcuatus, a 2-foot long shark similar to a dogfish. It was first discovered by Swiss biologist Louis Agassiz in 1837, and later described in detail by John Dick, a former classmate of Coates’, in 1978. Tristychius, and other Devonian period sharks like it, are found in ironstone rock nodules along the shores of the Firth of Forth near Edinburgh, Scotland.
Shark fossils are rare because their cartilage skeleton usually rots away before there’s any chance of fossilization. For decades, researchers studying ancient sharks have been limited to isolated teeth and fin spines. Even if they do find a more complete skeleton, it’s usually flattened, or, if it’s encased in one of these stones, it crumbles when they try to remove it.
Coates and his lab have been leading the field in applying modern imaging technology and software to study these challenging fossils. CT scanning allows them to create 3-D images of any fossilized cartilage and the impressions it left while still encased in the stone. Then, using sophisticated modeling software originally developed to study structure and function in modern-day fish, they can recreate what the complete skeleton looked like, how the pieces fit together and moved, and what that meant for how these sharks lived.
“The combination of both physical and computational models has allowed us to explore the biomechanics in a Paleozoic shark in a way that’s never been done before,” Coates said. “These particular sharks were doing something sophisticated and new. Here we have the earliest evidence of this key innovation that’s been so important for multiple groups of fishes and has evolved repeatedly.”
