Who Says Sharks Haven't Evolved?

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

ScienceShot: Who Says Sharks Haven't Evolved?

Sid is a freelance science journalist.

Many scientists have presumed that sharks and their kin have evolved little since they first appeared millions of years ago. But newly described, well-preserved fossils of a creature near the base of the shark’s family tree contradict that notion. The remains of Ozarcus mapesae—embedded within chunks of rock (one example, photo) laid down as sediments 325 million years ago in what is now north-central Arkansas—are the first of the shark lineage from that era to be preserved largely intact with body parts in lifelike arrangements. (The flexibility of the shark’s cartilaginous skeletons usually means such carcasses become flattened if they survive decomposition.) Although Ozarcus clearly lies within the lineage of sharks, rays, and their close relatives, high-resolution CT scans of the fossils (one example, digital image) reveal that the archlike structures supporting the creature’s gills (yellow in digital image) were arranged like those in bony fishes (osteichthyans), the researchers report online today in Nature. Furthermore, gill arches in the Ozarcus fossils were separated by small bits of cartilage found in some species of bony fish and their relatives but previously unknown in any living or extinct chondrichthyans. Ozarcus’s unexpected blend of traits indicates that sharks and their kin have evolved substantially since they first appeared. The last common ancestor of sharks and bony fishes probably didn’t have gill arches arranged like those in modern sharks—which, in turn, suggests that the oldest known species of bony fishes can likely provide more information about the earliest jawed vertebrates (a group that today includes humans) than early chondrichthyans can, the researchers contend.

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Posted in Paleontology, Plants & Animals