"We want to explore braincases if possible, because they are exceptionally rich sources of anatomical information," Coates said. "They're much better than scales, teeth or fin spines, which, on their own, tend to deliver a confusing signal of evolutionary relationships."
The analysis of the sample 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 acanthodian species turned out to be primitive sharks, while others were relatives of the common ancestor of sharks and bony fishes.
This result explains some of the longstanding confusion about the placement of acanthodians in vertebrate history. But additional analyses went a step further. Using more than 100 morphological characters, the researchers quantified the mutual resemblance among the earliest jawed fishes. 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, PhD, 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 aw
|Contact: Rob Mitchum|
University of Chicago Medical Center