November 28, 2006
Scientists Identify Long-Extinct Fish With the Strongest Jaws
CHICAGO _ A fearsome mega-predator of the ancient seas had the most powerful jaws of any fish that ever lived, according to a new study that makes even the biggest great white shark seem like a slack-jawed weakling.
The new report by scientists at the Field Museum and the University of Chicago offers a unique window into the bizarre aquatic world of 400 million years ago, when the conquest of dry land by animals had barely begun and the first jawed creatures were carving out their evolutionary niches.
The fish in the new study, called Dunkleosteus terreli, was a toothless killer with jaws like straight blades and a huge, armored head. Experts believe it grew up to 30 feet long and weighed as much as 4 tons _ bigger than most modern sharks, and about the size of a killer whale.
But the animal's jaws would have set it apart from any latter-day seagoing carnivores.
Using fossil remains to build a computer model of the fish's bones and muscles, the Chicago researchers found that it could tear apart its food with a force of 1,100 pounds per square inch.
The fish needed that strength to pierce the bony armor that many sea creatures of the era possessed. Only alligators and Tyrannosaurus rex have had more powerful jaws, experts believe.
"This thing was capable of biting anything it came across in half," said study co-author Mark Westneat, associate curator of zoology, fish evolution and biomechanics at the Field Museum.
The work sets a new standard in its precise analysis of how a long-extinct species moved and ate its food, said Adam Summers, a professor of ecology and evolutionary biology at the University of California at Irvine.
Biologists in the past have used biomechanics to study how dinosaurs such as T. rex ran or consumed their prey. Summers said the new report, published in Wednesday's issue of the scientific journal Biology Letters, uses an innovative way of drawing on data taken from modern fish to determine how an extinct ocean predator used its jaws.
"We now have an idea of just how hard a fossil critter can bite," Summers said. "It's the basis for a broader comparison of species. I'm very excited to see what's going to come next."
To help calculate the ancient fish's biting power, the scientists spent a day fitting packing foam inside the jaws of a fossil replica at the Field Museum. That gave them a good estimate of how burly the animal's jaw muscles were.
Graduate student Philip Anderson of the University of Chicago also took extensive measurements of six Dunkleosteus skulls at the Cleveland Museum of Natural History. In addition to showing how the fish's joints fit together, the fossils bear scars from where jaw muscles connected with the bone.
Westneat already had a computer model showing how such attributes affect the power and motion of jaws in modern fish. He plugged the fossil's measurements into that model to simulate how the extinct fish's jaws worked.
The comparison with living species shows that Dunkleosteus was unlike any living fish, both in its appearance and eating habits.
"We don't really have an animal that fills this kind of niche _ a massively armored predator that bites that hard," said Summers of UC Irvine.
In addition to having extremely strong jaws, researchers think the monstrous fish could move its jaws very quickly, creating a suction effect that drew smaller prey into its mouth. The tremendous force of its bite was focused on a fang at the front of its mouth.
Biologists say the predator may have used its strong jaws to hunt armored animals such as the ammonoids, an extinct species that wore a hard shell and was related to the octopus and squid. The fish's imposing jaws and the tough shells of its prey may have evolved in tandem, in what biologists describe as an evolutionary "arms race."
"These guys were probably the first marine predators to be able to bite food into pieces for swallowing," Westneat said. "Other predators were swallowing their prey whole."
Sharks later developed shearing teeth to tear their victims apart, but that did not happen for another 100 million years after Dunkleosteus ruled the seas.
In fact, researchers believe Dunkleosteus was one of the first vertebrates that could be called an "apex predator," meaning that it had no natural enemies and could prey upon virtually any other animal in its environment.
Yet by about 360 million years ago the sea monster had died out, part of the mysterious mass extinction that marked the end of what biologists call the Devonian period. Many armored fish of that epoch died off, though no one really knows why.
"The Devonian extinction is not nearly as well understood as some other extinction events," Anderson said. "Dunkleosteus had thick armor on its skulls and no armor behind, so it would have been very front-heavy. I often wonder if that was the best design."
Even if it was flawed, the mighty fish still sparks the imagination of some biologists.
"It would have been an awe-inspiring predator," Summers said. "We have nothing like that anymore."
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