Where We Split From Sharks

Over 400 million years ago, fish went through an evolutionary divorce that would someday be very relevant to humans. The split produced the two major groups of fish we see in our world today: those with skeletons of bone, which make up the majority of aquatic life, and those with cartilaginous skeletons, which today include the sharks, rays, and skates. All of the animal life on land — including human beings — belongs to the bony fish family, thanks to those first adventurous fish who crawled their way out of the water. So scientists are interested in what the predecessor of bony and cartilaginous fish looked like before they went their separate evolutionary ways, in order to catch a glimpse of the common ancestor of humans, manta rays, and great white sharks.

One clue to this mystery is a group of fossil fish from the Paleozoic era that has long given paleontologists headaches: the acanthodians. These species, known primarily from fossilized scales and elaborate fin spines, have been classified variously as both sharks and bony fish since their discovery. But a new study published in Nature puts that dual identity to good use, proposing that acanthodians provide a window into the common ancestors of all the jawed vertebrates on Earth.

“Unexpectedly, Acanthodes turns out to be the best view we have of conditions in the last common ancestor of bony fishes and sharks,” said co-author Michael Coates, professor of organismal biology and anatomy at the University of Chicago and senior author of the study published in Nature. “Our work is telling us that the earliest bony fishes looked pretty much like sharks, and not vice versa. What we might think of as shark space is, in fact, general modern jawed vertebrate space.”

The study began with a new analysis of a particularly well-preserved acanthodian, Acanthodes bronni. Co-author Samuel Davis created highly detailed latex molds of specimens revealing the inside and outside of the fossil’s skull, which could be used to assess cranial and jaw anatomy as well as the organizations of sensory, circulatory and respiratory systems in the species. The analysis of the molds (which now reside in a tackle box in Coates’ office) combined with recent CT scans of skulls from early sharks and bony fishes led the researchers to a surprising reassessment of what Acanthodes bronni tells us about the history of jawed vertebrates.

“For the first time, we could look inside the head of Acanthodes, and describe it within this whole new context,” Coates said. “The more we looked at it, the more similarities we found with sharks.”

However, analysis of the evolutionary relationships of Acanthodes bronni — even with these new data added — still connected this species to early bony fishes. Meanwhile, some other acanthodian species turned out to be primitive sharks, while others were relatives of the common ancestor of sharks and bony fishes.

But additional analyses, conducted by co-author John Finarelli went a step further to clear up this muddled story. Using more than 100 morphological characters, the researchers quantified the mutual resemblance among the earliest jawed fishes. The heads of acanthodians as a whole, including the earliest members of humans’ own deep evolutionary past, appear to cluster with ancient sharks.

“The common ancestors of all jawed vertebrates today organized their heads in a way that resembled sharks,” said Finarelli, Lecturer in Vertebrate Biology at University College Dublin. “Given what we now know about the interrelatedness of early fishes, these results tell us that while sharks retained these features, bony fishes moved away from such conditions.”

This new revision of the lineage of early jawed vertebrates will allow paleontologists to dig into further mysteries, including how the body plan of these ancient species transformed over the transition from jawless to jawed fishes. The analysis also demonstrated that all of these early members of the modern gnathostomes are clearly separated from what now appear to be the most primitive vertebrates with jaws: a collection of armored fishes called placoderms.

“It helps to answer the basic question of what’s primitive about a shark.” Coates said. “And, at last, we’re getting a better handle on primitive conditions for jawed vertebrates as a whole.”

Davis, S., Finarelli, J., & Coates, M. (2012). Acanthodes and shark-like conditions in the last common ancestor of modern gnathostomes Nature, 486 (7402), 247-250 DOI: 10.1038/nature11080