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Fossil illuminates jaw evolution

19 January 09 11:04 GMT
By Tanya Syed
BBC News

A fossil fish is shedding light on the evolution of jawed vertebrates.

It is one of the earliest known jawed fish in the fossil record, a scientist from Uppsala University, Sweden, reports in the journal Nature.

The specimen is the first example of a well-preserved braincase of a group of extinct fish called acanthodians from the Paleozoic era.

The fossil fish was unearthed in Herefordshire, UK, in the 1940s and is an estimated 415 million years old.

The study was led by Martin Brazeau from Uppsala University in Sweden.

"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.

Different proportions

The fossil's dimensions differ from typical Acanthodes fossils in two important ways, Dr Brazeau explains.

The front of the preserved head is short while the back end is long.

"This is really what braincases of early sharks and armoured fish looked like," he says.

"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."

"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."

The study also suggests that some acanthodians are ancestors to all modern jawed vertebrates.

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

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

"Previously, we've had to operate on an assumption that the braincase of the Acanthodes fossil was stereotypical for all the other acanthodians."

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