New Study Confirms Sharks’ Ancient Ancestors

Research posts

Sharks are ancient predators, and for many species, their body plan hasn’t changed much over the course of millions of years. But even predators as ancient as sharks have predecessors, and a new study published in the journal American Museum Novitates seems to confirm the identity of a long-suspected shark ancestor—a group of bony fishes known as acanthodians.

 

Detailed view of the Doliodus problematicus fossil, with a text overlay of letters and lines pointing to specific areas of the specimen.

A fossil of Doliodus problematicus, now thought to be a shark ancestor.

© AMNH/J. Maisey


“Major vertebrate evolutionary transitions, such as ‘fin to limb’ and ‘dinosaur to bird’ are substantiated by numerous fossil discoveries,” said John Maisey, the lead author of the study and the Herbert R. and Evelyn Axelrod Research Curator in the American Museum of Natural History’s Division of Paleontology.

“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.”

 

John Maisey sits at a desk and looks through a microscope at the Doliodus problematicus fossil, metal storage cabinets and other lab equipment in view

John Maisey studying the Doliodus problematicus fossil in the New Brunswick Museum.

© AMNH/J. Maisey


This longstanding question resurfaced in 2003, when the fossil of a sharklike fish (Doliodus problematicus) was unearthed in New Brunswick, Canada. Dating back to the lower Devonian period, about 400 million years ago, the fossil demonstrated a telltale sign of acanthodian ancestry—paired spines on its pectoral fins.

Further study in recent years by Maisey and his colleagues, though, showed that other features of the fossil fish—such as its head and teeth—were remarkably sharklike.

 

Line drawing of the ventral side of a sharklike fish with highlighted areas that indicate where the fin fossils (illustrated to the right) originate.

The paired fins with spines of Doliodus problematicus next to a body outline. The arrangement of these spines is typical of many acanthodians.

© AMNH/J. Maisey


Now, computed tomography (CT) scans of the fossil have uncovered another typically acanthodian trait—spikes that probably lined the animal’s underbelly in life. Taken together, these discoveries peg D. problematicus as a transitional species, sitting at the evolutionary intersection of ancient acanthodians and modern sharks.

“The arrangement of these spines shows unequivocally that this fish was basically an acanthodian with a shark’s head, pectoral skeleton, and teeth,” Maisey said.