January 19, 2009

Fossil Provides New Clues Into Jaw Evolution

A Swedish scientist has reported the finding of a fossilized specimen that may provide new insight into how jawed vertebrates evolved.

Martin Brazeau, of the Department of Physiology and Developmental Biology at Uppsala University in Sweden, said the fossil was discovered Herefordshire, UK, in the 1940s and is an estimated 415 million years old.

The specimen is the first-known braincase of an Early Devonian acanthodian "“ which is possibly the earliest group of gnathostomes (jawed vertebrates) "“ Brazeau said.

Brazeau said acanthodians feature an array of shark- and bony fish-like characteristics that has long given them prominence in discussions of early gnathostome evolution.

"Because of their superficially shark-like and bony fish-like appearance, acanthodians have played an important role in trying to elucidate the origins of modern jawed vertebrates," he told BBC News.

It has been difficult for scientists to link early gnathostomes with modern ones, partly because their un-mineralized endoskeletons rarely fossilized.

Brazeau presented a fossil of Ptomacanthus anglicus, which he says depicts what braincases of early sharks and armored fish looked like.

"When we look at early bony fishes, the back end of the braincase is very short and the front end is long - which is what Acanthodes were like," said Brazeau.

"This figures in nicely with the emerging idea that acanthodians don't form a group of fishes that are all closely related to each other. Some of these fossils are primitive sharks while others are primitive bony fishes," he told BBC News.

Acanthodian braincases have previously been represented by a single genus, Acanthodes, which occurs more than 100 million years later in the fossil record, Brazeau said.

"The braincase of Ptomacanthus differs radically from the osteichthyan-like braincase of Acanthodes in exhibiting several plesiomorphic features shared with placoderms and some early chondrichthyans."

"We've already got scores of known acanthodians, but braincases are known in only one of these, belonging to the Acanthodes genus," he added.

"Fitting them into the picture of early jawed vertebrate evolution has been extremely difficult because of the lack of data."


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