An article published in the journal “American Museum Novitates” describes a research that provides the strongest evidence yet that sharks descended from a very ancient group of fish called acanthodians. A team of researchers led by John Maisey of American Museum of Natural History’s Division of Paleontology analyzed fossil remains that were exceptionally preserved of an ancient shark-like fish called Doliodus problematicus identifying it as a transitional species between acanthodians and sharks.
Acanthodians – class Acanthodii – lived between the lower Silurian period and the upper Permian, between 430 and 250 million years ago. They had characteristics in common with both bony fish and cartilaginous fish and their relationships were complex to establish. The discovery in 2003 of a specimen of Doliodus problematicus that lived about 400 million years ago in an excellent preservation state marked the beginning of a new series of research on the relationship between acanthodians and sharks.
Early studies of that Doliodus problematicus specimen showed the presence of paired spines in front of its pectoral fins, an characteristics existing especially in acanthodians. Subsequent research conducted in 2009 and 2014 by John Maisey and various colleagues determined that this fish’s head, skeleton and teeth were more similar to those of sharks than acanthodians.
For this new research the Doliodus problematicus specimen was givben a CT scan at the French National Museum of Natural History in Paris. This analysis technique allowed to examine the inside of the fossil remains without destroying them revealing other spines that were hidden. The layout of those spines on the fish’s underside is typical of acanthodians, a confirmation that this species had characteristics of both sharks and acanthodians.
John Maisey commented that the arrangement of the spines show unequivocally that Doliodus problematicus was essentially part of the class of acanthodians with shark’s head, pectoral skeleton and teeth. This combination of characteristics makes it a transitional species that proves that sharks evolved from acanthodians.
There are very few fossils that document the evolution of sharks and that’s why the relationships with other fish were at least so far difficult to establish. For this reason, the Doliodus problematicus specimen found in 2003 could really be the key to establishing the relationship between sharks and acanthodians.
