Oldest Shark Braincase Shakes Up Vertebrate Evolution

Christine Dell'Amore
National Geographic News
January 14, 2009

The earliest known braincase of a shark-like fish has shown some assumptions about the early evolution of vertebrates are "completely wrong," experts say.

The 415-million-year-old specimen of a Ptomacanthus is only the second known example of a braincase from an Acanthodian, a long-extinct group of fossil fish that existed near the time that bony fish and cartilaginous fish—animals with skeletons made up of a type of connective tissue—split off into separate branches.

The other braincase, from a species called Acanthodes, dates to a hundred million years after the Acanthodian group came into existence, casting most of this period of the group's evolution into shadow.

"We've known about [Acanthodians] for 150 years or more, but the braincase has always been missing," said study lead author Martin Brazeau, a Ph.D. student at Uppsala University in Sweden. "To fit it in now is kind of exceptional."

Before this discovery, most scientists believed that the braincases of Acanthodians resembled those of bony fish, and were thus related to this type of animal.

But data from the new fossil support an emerging idea that the ancient group of fish included a diverse tableau of shapes and characteristics that defy clear-cut categories.

(Related: "Ancient Fish Fossil May Rewrite Story of Animal Evolution" [October 18, 2006].)

Coarse Picture

The primitive fish's softer skeletons did not fossilize well, leaving scientists few braincases or other parts to study.

But Brazeau suspected that in the right circumstances, some bones could withstand time.

So he took a closer look at the well-preserved Ptomacanthus specimen that had been in the literature for 30 years. "Sure enough," he said, "the specimen had its braincase preserved."

The ancient creature has distinct features of a shark—swirling rows of teeth and a short snout, for instance—and looked little like the early bony fish suggested by the Acanthodes braincase.

Continued on Next Page >>




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