A new analysis found out that the controversial class of animals was more shark-like than expected.Categorically, this now-extinct fish was among the first to split from sharks, whose bones are made of cartilage, to evolve into a line of tough-boned species that includes everything from bony fish to human beings. The brain case of a 290-million-year-old fossil fish (Acanthodes bronni) that has long puzzled paleontologists threw light on this evolution.
"Unexpectedly, Acanthodes turns out to be the best view we have of conditions in the last common ancestor of bony fishes and sharks," said Michael Coates, PhD, professor of organismal biology and anatomy at the University of Chicago and senior author of the study.
"The common ancestors of all jawed vertebrates today organized their heads in a way that resembled sharks," study researcher John Finarelli, a vertebrate biologist at University College, Dublin, said in a statement. "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."
Acanthodes bronni left few traces besides its fossil scales and fin spines. But a few fossilized, fragmented skulls survived millions of years in the ground and now it resides in museum collections. The researchers made silicone rubber casts of these fossils to reconstruct the anatomy of the fish's heads.
This closer-than-ever look revealed ridges and grooves never before examined. The researchers took 138 characteristics of the skulls and compared them with both the skulls of the chondrichthyes, the group made up of sharks and rays, and the osteichthyes, or bony fish such as today's sardines and mackerel. They found that on the whole, acanthodian heads fell in with the sharks.
"The more we looked at it, the more similarities we found with sharks." Michael Coates, said in a statement.
The study also revised the relationships between early vertebrates with jaws (whose members range from fish and sharks to birds, reptiles and humans), and the most primitive members of that group, armored fish called placoderms. The researchers found distinctive anatomical differences between placoderms and other early vertebrates with jaws.
All of the updated relationships will allow researchers to look more closely at how fish made the transition from jaw-less to jawed, Coates said.
"It helps to answer the basic question of what's primitive about a shark," he said. "And, at last, we're getting a better handle on primitive conditions for jawed vertebrates as a whole."
The study, "Acanthodes and shark-like conditions in the last common ancestor of modern gnathostomes," will be published on June 14 by Nature. The research was also funded by the Natural Environment Research Council.