Oldest Fossil of Sea Reptile Dating Back 94 Million Years Found in Utah

The discovery of the oldest fossil of a marine reptile, dating back 94 million years, is shedding new light on the evolution of animals that would later dominate the oceans.

The fossil of the mosasaur, a predatory sea creature equipped with powerful jaws and teeth designed for cutting prey, was first found in the Kaiparowits—a rock formation in the gray shale badlands of Utah—11 years ago.

Only when examining the fossil more closely, however, did researchers realize its true significance, and its record-breaking age.

They named it Sarabosaurus dahli, or "Dahl's reptile of the mirage," in honor of Steve Dahl, the longest-serving volunteer working on the project, and to reflect the find's origins. "Mirage" alludes both to the vast, ancient seaway that once covered most of the middle of North America, and in which the animal swam, and the mirages that accompany the area's extreme summer heat.

Mosasaurs were marine reptiles that lived in the Late Cretaceous period, between 84 million and 95 million years ago. They could grow up to 56 feet in length, but many didn't exceed the size of a porpoise. They had a long, thin body, not unlike that of a snake, and a long snout.

The oldest of these creatures typically didn't grow larger than 3 to 5 feet long. Later, when they evolved, they became gigantic, apex predators.

The Utah fossil, consisting of skull fragments and vertebrae, was found by Scott Richardson, a trained volunteer working with Barry Albright of the University of North Florida. They were both working to uncover some scattered fragments, the Bureau of Land Management (BLM) said in a statement.

Taking the fossilized remains to the lab, they realized that this was the oldest mosasaur ever found.

"I knew right away we had something special, especially considering it was so old," said Michael Polcyn, a vertebrate paleontologist and mosasaur expert at the Southern Methodist University in Dallas.

BLM Paria River District Paleontologist Dr. Alan Titus, who co-authored the study on the specimen published in Cretaceous Research, said that younger mosasaurs are "relatively abundant."

"But mosasaurs are extremely rare in rocks older than about 90 million years," he said. "Finding one that preserves so much informative data, especially one of this age, is truly a significant discovery."

Part of the significance is what it reveals about the evolution of the creatures' blood supply to the brain, which mirrors that of whales.

"Sarabosaurus sheds light on long-standing questions regarding the relationship of some early-branching mosasaurid species but also provides new insights into the evolution and antiquity of a novel cranial blood supply seen in a particular group of mosasaurs," said Polcyn.

"The way that blood gets to the brain is quite conservative in lizards, with the internal carotid arteries carrying the load. One group of mosasaurs that includes Sarabosaurus did something very different, shifting the primary blood supply from a branch of the internal carotid arteries to arteries entering the brain below the brain stem, a shift similarly observed in the evolution of cranial circulation in whales."

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