Researchers Study 'Shark Nursery' Fossils from Upper Midwest, Find Ancient Migration Route

Researchers have found the earliest evidence of shark migration in an ancient river delta system in modern-day Illinois.

The long-snouted shark- Bandringa- lived in this region about 310 million years back. The University of Michigan paleontologists reanalyzed the Bandringa fossils and found evidence of an early migration route. Bandringa is assumed to be the oldest relative of modern sharks.

About 300 million years back, llinois looked very different. It was covered with swampy lowlands and marine bays. According to the latest study, adult Bandringa sharks lived in freshwater and migrated during breeding seasons to saltwater swamps.

The "shark nursery" in these swamps had shark eggs and juveniles preserved in the sediments.

"This pushes migratory behavior in sharks way back," Lauren Sallan, an assistant professor in the U-M Department of Ecology and Evolutionary Biology, said in a news release. "These sharks bred in the open ocean and spent the rest of their lives in fresh water. No shark alive today is known to do that."

The ancient species was discovered in 1969 in the Mazon Creek deposits in northern Illinois. Since then, researchers have believed that the genus had two species- one that lived in freshwater and the other in saltwater.

Sallan and colleagues reanalyzed fossils from 24 specimens and found that the genus had just one species. According to the researchers, the males remained in the swamps while the females migrated to shallow, marine waters to lay eggs.

The physical difference in the shark specimens was due to difference in the process by which fossils are preserved in fresh and salt waters, researchers said. The freshwater specimens had more bones while the marine ones had more soft tissues.

The shark specimens found in Mazon Creek were all juveniles and were found along with their egg cases. Adult sharks have been found exclusively in freshwater swamps in Ohio and Pennsylvania.

"This is the first fossil evidence for a shark nursery that's based on both egg cases and the babies themselves," Sallan said in a news release. "It's also the earliest evidence for segregation, meaning that juveniles and adults were living in different locations, which implies migration into and out of these nursery waters."

Also, scientists have found many other physical features present in the ancient shark such as needle-like spines, downward facing jaw and electroreceptors on the body and snout.

The study is published in the Journal of Vertebrate Paleontology.