Scientists have discovered evidence of an ancient shark nursery -- 310 million years old -- along the shores of a prehistoric interior sea.
Long before the dinosaurs, long-nosed Bandringa sharks were leaving their freshwater homes to lay eggs in the shallow coastal waters of a long-gone sea that stretched over most of the American Midwest, according to a new paper published in the Journal of Vertebrate Paleontology.
In the gallery above you will see a painting of what these sharks might have looked like, as well as a photo of a fossilized shark.
In 1969, researchers discovered beautifully preserved fossils of baby Bandringa sharks, as well as their spiral egg casings, along Mazon Creek in northeastern Illinois. The juvenile sharks, just 4 to 6 inches long, had pronounced spoon-billed snouts that stretched half as long as their bodies. At the time, scientists thought the babies might be adult specimens of a mini-shark species that they dubbed Bandringa rayi.
Ten years later, in 1979, the same scientists discovered more juvenile Bandringa shark fossils in a different part of Mazon Creek that had once been a brackish swamp. Thinking they were a new species, the scientists called them Bandringa herdinae. That same year, researchers in Pennsylvania found a 10-foot-long adult shark that also had the same long spoon-billed snout and lived in freshwater rivers. A few years later, another large adult was discovered in what was once a freshwater river in Ohio.
In a paper published online Tuesday, University of Michigan paleontologist Lauren Sallan argues that all of the sharks are members of the same species.
In an interview with the Los Angeles Times, Sallan explained that the sharks were preserved differently depending on where they were found. The fossils from marine waters showed soft-tissue preservation, but there was no trace of endoskeletons. The sharks found in freshwater and brackish water had their endoskeletons preserved, but not their soft tissue.
"When you account for the different preservation modes, there is nothing that distinguishes them at all," Sallan said.
Since there were no adult shark fossils found in the marine sites, Sallan and her colleauges think that Bandringa sharks followed a strange breeding pattern - spending most of their adult lives in freshwater rivers and deltas and going to coastal waters to lay their eggs in shallow shark nurseries.
Although no sharks living today are known to travel from fresh water to salty water to lay their eggs, most sharks do use shark nurseries.
"Almost all sharks and their relatives today use nursery waters," Sallan said. "They usually use an environment very near the shore because the shallow area protects the juveniles from other sharks that may be too big to enter them."
Bandringa sharks were bottom feeders who used electro-receptors on their long snouts to help them detect food. The upper part of their bodies were covered in sharp pointed scales, including needle-like spines on their cheeks and on the tops of their head. Those sharp points were likely used for self-defense, Sallan said.
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