Oldest Shark Braincase Shakes Up Vertebrate Evolution
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].)
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.
(See photos of modern-day sharks.)
Such differences within the same group rock earlier assumptions that a certain set of species with similar characteristics fit in clearly defined categories.
Grouping fish in this way "gives us a coarse picture of evolution," said Brazeau, whose work will appear tomorrow in the journal Nature.
"We need to find intermediaries between groups, find more fossils, and go and see if we've missed some."
(Related: "Odd Fish Find Contradicts Intelligent-Design Argument" [July 9, 2008].)
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Michael Coates, a professor of organismal biology at the University of Chicago who was not part of the new study, agreed.
"The common perception is that sharks are somehow primitive relative to bony fish"—foundations that are "sketchy at best," he said.
"It's an important piece in the puzzle for trying to understand one of the biggest events not only in our own evolutionary history, but also the vast majority of living animals with backbones," Coates said.
John Maisey, curator in the division of paleontology at the American Museum of Natural History in New York, was also not involved in the research.
"The classical view that we have two types of jawed vertebrates—bony and shark-like—might be true today, but when you look back at the early fossil record, it becomes more complicated," said Maisey, who also studies early shark-like fish.
"This is the first real movement in this part of the evolutionary tree in the last hundred years."
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