for National Geographic News
A gang of ancient sharks took on an enormous "sea monster" 85 million years ago, according to a new fossil analysis.
The bones of the prehistoric reptile, known as a plesiosaur, were found in Japan in 1968. But a lack of comparative samples and other resources meant that scientists didn't release a formal description of the fossil until recently.
"As a child in Japan, I had heard that there were some shark teeth embedded in the plesiosaur," recalled Kenshu Shimada, a paleontologist at DePaul University in Chicago.
"But the [new] description revealed over 80. That is a lot of teeth to have in a fossil," he said.
After reading the report, Shimada wanted to take a closer look at the types of shark teeth. Based on his findings, he estimates that at least seven sharks of different ages attacked the plesiosaur.
The scientist was even more shocked when he identified the species of attacking shark: the extinct, nine-foot-long (three-meter-long) Cretalamna appendiculata.
By contrast, the sharks' plesiosaur prey was a roughly 23-foot-long (7-meter-long) animal armed with a mouth full of razor-sharp teeth and powerfully muscled, paddlelike limbs.
Sharks regularly lose some of their many teeth whenever they bite prey. These teeth fall out and are regrown throughout their lives.
Shimada and colleagues found that the teeth they removed from the plesiosaur were from the same species but of different sizes and shapes.
This suggests there was a multigenerational feeding frenzy, said Jürgen Kriwet, a paleontologist at the State Museum of Natural History in Stuttgart, Germany.
Among modern-day great white sharks, for instance, juveniles will ravenously bite prey in a group but flee as soon as larger adult sharks arrive, said Kriwet, who was not involved in the new research.
A similar event may have taken place when the ancient sharks attacked, he said.
Harder to determine is whether the sharks went after a live, wounded, or dead plesiosaur.
"In the modern day, we usually see sharks of this size scavenging on animals larger than themselves," Kriwet said.
"And if they do attack, they are often attacking large animals that are already wounded."
DePaul University's Shimada said he thinks the plesiosaur was dead or nearly so.
"A healthy plesiosaur," he said, "would have put up a pretty good fight."
Findings reported last week at the annual meeting of the Society of Vertebrate Paleontology in Bristol, England
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