The stomachs of some of the juvenile sharks even had tiny crustaceans still in them, indicating that these sharks died a sudden death before their meals digested. It remains a mystery for now as to what did them in.
Based on the location of the adult Bandringa sharks and the nursery, the researchers conclude that the adults migrated downstream from freshwater swamps in what are now Pennsylvania and Ohio to a tropical coastline to spawn. That prehistoric coastline is where the nuclear power plant now is.
The defined route marks the earliest known example of shark migration, according to Sallan and Coates. This early route additionally reveals the only known example of a freshwater to saltwater shark migration.
"It's also the earliest evidence for segregation, meaning that juveniles and adults were living in different locations, which (further) implies migration into and out of these nursery waters," Sallan said.
As to why sharks then, and now, rely upon nurseries, she explained that the more isolated regions chosen for nurseries protect juveniles from larger sharks and other potential predators.
The gestation period for sharks can also be quite long -- up to two years -- making the protection all the more important.
Paleontologist John Maisey of the American Museum of Natural History told Discovery News, "The paper offers some interesting speculation about breeding behavior, based upon a literal interpretation of the fossil distributions -- adults in one place, babies and eggs in another."
Maisey, however, said that it is challenging to infer how such ancient species behaved based on tangible evidence.
He explained, "Behavior does not readily fossilize."