Mosasaur fossil proves the early lizards swam like sharks

Prehistoric mosasaur lizards didn’t slither like eels. Instead, these late Cretaceous sea monsters swam like sharks, scientists say.

ERMNH HFV 197 reveals, for the first time in a mosasaur, the soft tissue outline of a tail fin, to demonstrate that mosasaurs are convergent with ichthyosaurs and whales in the development of a semilunate propulsive surface for enhanced locomotor efficiency [Credit: Johan Lindgren and Stefan Solberg]

Mosasaurs ruled the oceans about 98 million to 66 million years ago. Most paleontologists have argued that these reptiles, which grew up to 33 feet long, had tapered, whip-like tails similar to those of snakes, their closest living relatives. Others thought the downward bend in mosasaur fossils’ tail regions suggested they had shark-like fins. But without a clear picture of these tail fins, they couldn’t say for sure.
Now, an analysis of a mosasaur fossil with unusually well-preserved soft tissue impressions reveals that these reptiles had crescent-shaped tail fins, making them speedy swimmers that would have chased down their prey much like modern-day sharks.   
Scientists discovered the 6-foot-long juvenile specimen encased in a limestone boulder in Amman, Jordan, in 2008. They cut and hauled slabs of the fossil to the Eternal River Museum of Natural History in Amman, where they sat on display until Johan Lindgren examined them three years later.
Lindgren, a paleontologist at Lund University in Sweden, saw a faint outline in the rock surrounding the skeleton’s tail, indicating a soft tissue impression.
“I had a gut feeling that this was something special,” said Lindgren, who led a group that published the findings Tuesday in the journal Nature Communications.
So the researchers chipped  away the remaining sediment, revealing imprints of chevron-shaped scales and dense fiber bundles. Sure enough, the impressions formed a broad, crescent-shaped outline of a fin around the skeleton’s tail bones.
“It was amazing, just mind-blowing,” Lindgren said.
He and his colleagues then compared the skeleton’s dimensions to those of  modern-day marine animals. The mosasaur’s body proportions most closely resembled those of sharks, suggesting that these reptiles had a similar streamlined shape and bilobed tail fin that allowed them to quickly chase their prey over long distances. On the other hand, a serpentine shape would have limited them to short, occasional bursts of speed typical of predators who ambush their food.
The discovery is consistent with earlier fossil evidence of mosasaurs eating squid and large fish, which they could catch only if they had a body shape designed for efficient swimming, said Takuya Konishi, a paleontologist at the University of Alberta in Canada.
“If you want to be a very good swimmer, you need to have a streamlined shape” with a bilobed tail fin, said Konishi, who wasn’t involved in the study.
The tail fin is “a mechanism that really works,” said Anne Schulp, a paleontologist at the Maastricht Natural History Museum in the Netherlands, who wasn’t involved in the study.  “It’s exciting to see it show up time and time again in the evolution of very different species.”
The new finding may not offer a complete picture of the mosasaur’s body shape, since it represents only one specimen among 70 species of mosasaur, Konishi said.  And since it was a juvenile, its fin probably differed slightly from those of adults, Lindgren added.
So far, paleontologists have discovered thousands of mosasaur fossils and will probably unearth many more in the future, Schulp said. The new findings remind scientists to “take a very careful look” at these specimens, he said. Imprints are hard to spot, which may partly explain the lack of soft tissue evidence until now.
“We have to keep our eyes peeled,” he said.