Who Says Sharks Haven't Evolved?

(Top, photo) AMNH/F. Ippolito; (Bottom, digital image) AMNH/A. Pradel

ScienceShot: A 300-Million-Year-Old Shark Migration

Just when you thought it was safe to go back in the (prehistoric) water, researchers have identified the fossilized remains of a 310-million-year-old shark nursery in Illinois. One of the earliest close relatives of modern-day sharks, Bandringa (pictured in an artist’s representation above, with its sensitive spoon-billed snout) was a bottom dweller, analogous to today’s sawfish, that lived in an ancient river delta and probably fed by suction feeding, vacuuming prey directly into its downward-facing jaws. Discovered in 1969, Bandringa was previously believed to be two separate species—B. rayi and B. herdinae—with one living in shallow oceans and the other in freshwater swamps. By reevaluating 24 fossil specimens (inset)—including latex peels taken to enhance the extinct shark’s scaly skin—researchers have revealed online today in the Journal of Vertebrate Paleontology that Bandringa was really one species that lived in both environments at different times of life. Apparent variations between the two “species” actually represent different preservation processes acting in freshwater and marine conditions—better fossilizing soft tissue and bones/cartilage, respectively. The marine specimens were all juveniles, about 4 to 6 inches long, and were found alongside egg cases—together forming the first known example of a shark nursery. In contrast, adult Bandringa fossils, which could reach up to 10 feet in length, have only been unearthed from freshwater surroundings. The scientists believe this is evidence of the earliest known example of shark migration to or from a spawning ground. The behavior is still seen in some sharks today, but they all move in the opposite direction, from marine water to fresh water.

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