According to paleontologists from the University of Chicago and the University of Michigan, long-extinct Bandringa sharks migrated downstream from freshwater swamps to the ocean to spawn in shallow coastal waters and left behind fossil evidence of one of the earliest known shark nurseries.

The long-snouted Bandringa shark (Elasmobranchii, Chondrichthyes) – a bottom-feeding predator that lived in an ancient river delta system in what is today the Upper Midwest – is likely one of the earliest close relatives of modern sharks.

It resembled present-day sawfish and paddlefish, with a spoon-billed snout up to half its body length. Juveniles were 4 to 6 inches long and grew into adults of up to 10 feet.

Bandringa sharks were discovered in 1969 and soon became one of the most prized fossils from the well-known Mazon Creek deposits in northern Illinois.

Until now, paleontologists believed that the genus Bandringa contained two species – B. rayi and B. herdinae, one that lived in freshwater swamps and rivers and another that lived in the shallow ocean.

But after reevaluating fossils from 24 individuals, Dr Lauren Sallan and Dr Michael Coates concluded that Bandringa was a single species that lived, at various times during its life, in fresh, brackish and salt water.

“The physical differences between the two purported species were due to different preservation processes at marine and freshwater locations. The freshwater sites tended to preserve bones and cartilage, while the marine sites preserved soft tissue,” explained Dr Sallan and Dr Coates, co-authors of the paper published in the Journal of Vertebrate Paleontology.

By combining the complementary data sets from both types of fossil sites and reclassifying Bandringa as a single species, the team gained a far more complete picture of the extinct shark’s anatomy and discovered several previously unreported features. They include downward-directed jaws ideal for suction-feeding off the bottom, needle-like spines on the head and cheeks, and a complex array of sensory organs on both the extended snout and body, suited for detecting prey in murky water.

The Bandringa fossils also reveal the only known example of a freshwater to saltwater shark migration, as well as a 310-million-year-old shark nursery where fossilized egg cases and juvenile sharks were preserved in the same sediments.

“Adult Bandringa sharks lived exclusively in freshwater swamps and rivers,” the scientists said.

“Females apparently traveled downstream to a tropical coastline to lay their eggs in shallow marine waters, a reverse version of the modern-day salmon’s sea-to-stream migration. At the time, the coastline of the super-continent Pangaea ran diagonally between the Mazon Creek freshwater and marine sites.”

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Lauren Cole Sallan & Michael I. Coates. 2014. The long-rostrumed elasmobranch Bandringa Zangerl, 1969, and taphonomy within a Carboniferous shark nursery. Journal of Vertebrate Paleontology, vol. 34, no. 1, pp. 22-33; doi: 10.1080/02724634.2013.782875