Back in 1969, a group of scientists unearthed what appeared to be fossils of juvenile sharks in Mazon Creek near the Braidwood Nuclear Generating Station in Illinois. More than 300 million years ago, the entire area was part of a prehistoric inland sea. The fossils discovered in the area included spiral egg casings as well as the remains of small baby sharks around 4 to 6 inches long. However, a recent scientific paper published by a scientist from the University of Michigan may shed new light on the subject matter.

Back when the shark fossils were first discovered in Mazon Creek, scientists who worked on the dig initially theorized that the remains were from miniature spoon-billed sharks. Due to the size of the fossils, the scientists assumed that they were full-grown sharks that only grew up to around 6 inches long. They dubbed the mini sharks as Bandringa rayi. However, recent studies indicate that the remains are actually part of a shark nursery for prehistoric spoon-billed sharks with long bills that account for around half of the sharks' bodies. While initially thought to be from different shark species, it seems that all of the fossils in the area belonged to Bandringa sharks.

"When you account for the different preservation modes, there is nothing that distinguishes them at all," says Lauren Sallan, who published the new paper.

According to the new paper published in the Journal of Vertebrate Paleontology, the entire area served as a Bandringa nursery for laying eggs and bringing up young sharks. The Bandringa species is also one of the early common ancestors of many sharks today.

Early scientists may have been confused by the fact that there were no large, fully-grown fossils in the area. However, Sallan believes that this may have been due to the fact that adult members of the species spent most of their time in freshwater rivers. The adults of the species may have visited costal areas to lay their eggs.

The new paper brings to light the possible breeding habits of prehistoric sharks. Many modern sharks also share the same patterns. While there is no record of modern sharks moving to and from salt water seas and freshwater rivers, it is a well known fact that sharks often make use of nurseries in shallow waters to lay eggs and raise their young.