Discovery reveals ancient shark ancestors

March 14 (UPI) -- Scientists are now closer to understanding the evolutionary origins of sharks. X-ray images of a sharklike fish discovered in 2003 suggest the species is indeed a transitional species.

The discovery suggests the earliest ancestors of moderns sharks were acanthodians, a group of sharklike fish that lived during the Denovian period, between 420 and 360 million years ago.

Understanding how and when sharks emerged from the larger fish family has proven difficult for paleontologists.

"Major vertebrate evolutionary transitions, such as 'fin to limb' and 'dinosaur to bird' are substantiated by numerous fossil discoveries," lead study author John Maisey, a paleontologist at the American Museum of Natural History, explained in a news release. "By contrast, the much earlier rise of sharklike fishes within jawed vertebrates is poorly documented. Although this 'fish to fish' transition involved less profound anatomical reorganization than the evolutions of tetrapods or birds, it is no less important for informing the evolutionary origins of modern vertebrate diversity."

In 2003, Canadian scientists discovered the well-preserved remains of a sharklike species from the lower Devonian; it was aged between 397 and 400 million years old. Researchers named the species Doliodus problematicus.

Initial analysis revealed the presence of a double spine in front of the specimen's dorsal fin, a feature unique to acanthodians. Followup reassessments, however, suggested the species' head, skeleton and teeth resembled those of sharks, not acanthodians.

The new CT images, captured by scientists at the French National Museum of Natural History in Paris, revealed even more hidden spines. The vertebrae likely lined the specimen's underside, further evidence of its close relation with acanthodians.

Scientists detailed the latest images in the journal American Museum Novitates.

"The arrangement of these spines shows unequivocally that this fish was basically an acanthodian with a shark's head, pectoral skeleton, and teeth," Maisey concluded.