Ancient Shark Migrated From Fresh Water To The Seas: Long-Snouted Bandringa Shark Used River Deltas As Nurseries
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Two researchers have stumbled upon fossil evidence of one of the earliest known shark nurseries. Dr. Lauren Sallan, paleontologist from the University of Michigan and Dr. Michael Coates of the University of Chicago reanalyzed all known specimens of the Bandringa shark, and came to the conclusion that the long-snouted sharks (the earliest close relatives of modern sharks) migrated from freshwater swamps to downstream tropical coastline 300 million years ago. Although other sharks (such as tiger sharks in Hawaii) do migrate, this is the only known example of a shark migrating from freshwater to saltwater.
After analyzing the Bandringa fossil, the researchers also discovered the earliest known example of shark nurseries. Interestingly, juvenile sharks and fossilized egg cases were found preserved in the same sediments.
The long-extinct Bandringa shark had a long snout up to half its body length, resembling the present day sawfish and paddlefish. First discovered in 1969, the adult Bandringa are thought to have been as long as 10 feet while the juveniles were barely 4 to 6 inches long. The shark attracted the attention of scientists soon after its discovery in Mazon Creek deposits in northern Illinois, becoming among the most prized fossils from. According to a press release, the authors believe that current research will most likely replace the opinion among the scientists that there were two species of Bandringa - one that lived in freshwater and the other inhabiting the shallow ocean.
Apart from pushing way back the migratory behavior in sharks, the researchers also found that "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," Sallan said in a press release.
The researchers evaluated fossil evidence from 24 individuals to conclude that it was a single species that lived in fresh, brackish, and salt water during the various phases of its life cycle. The mistaken identity of two species could have possibly emerged due to the way preservation process worked at marine and freshwater locations. While marine sites preserved soft tissue, freshwater preserved bones and cartilage.
All the adult Bandringa fossils so far have been identified from freshwater locations, including several in Ohio and Pennsylvania, while Bandringa fossils from the Mazon Creek marine sites were juveniles found along egg cases. Females laid their eggs in shallow marine waters traveling downstream to a tropical coastline. The females then departed after laying eggs, returning upstream to freshwater sources.
"This is the first fossil evidence for a shark nursery that's based on both egg cases and the babies themselves," Sallan said. "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."
As a result of new study, Sallan and Coates also identified several previously unreported features unique to this extinct species. Their downward direct jaw was highly useful for suction feeding. They had needle-like spines on the head and cheeks and their extended snouts and body contained a complex array of sensory organs to detect prey in murky water. The study will be published in the Journal of Vertebrate Paleontology.
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