Extinct Freshwater Sharks That Spawned in Saltwater Had Been Mistaken for Separate Species

In 1967 the fossilized remains of a strange new shark species were discovered in Will County, Ill., an area known for its marine fossils dating back more than 300 million years. The shark had an “enormously elongated” snout, according to the paper that first described the species, Bandringa rayi. Several years later, a similar-looking shark was found nearby and christened B. herdinae, a related but different species.

In a new study, published on January 7 in the Journal of Vertebrate Paleontology, however, researchers have reanalyzed all available specimens of these sharks and determined that they are in fact a single species. The confusion arose in part due to this shark’s unusual trait: adults lived in freshwater swamps but migrated to nearby marine waters to spawn.

These different environments—freshwater and saltwater—resulted in markedly different preservations of the adult and juvenile sharks, says Lauren Sallan, lead author of the paper and assistant professor at the University of Michigan Department of Ecology and Evolutionary Biology. “In the marine fauna you only find soft tissue,” she says. However, in the freshwater sites “you only find skeletal tissue.”

In the more acidic marine settings the soft tissue is preserved but the skeleton is not. “They’re a bit like peat bog mummies from the U.K.,” Sallan says. “In those mummies all the bones are melted away because the acidity that preserves them, preserves the soft tissue, actually destroys the bone.”

The researchers analyzed eleven different categories of body structures in the fossils, including their long snouts. They found nothing to indicate that B. rayi and B. herdinae were separate species, suggesting that the perceived differences were entirely due to the differences in preservation.

Sallan says that not only is this finding important for the taxonomic study of sharks, but it is also the only known example of a shark that lived in freshwater but spawned in saltwater. They believe the sharks used their long snouts for finding fish to eat, using a network of electroreceptors in much the same way some paddlefish do today.

—Geoffrey Giller