A fossil discovered by a Bureau of Land Management team in southern Utah is providing insights into the evolution of a family of reptiles that existed during the latter part of the dinosaur age.
Mosasaurs evolved from a land-based lizard group similar to the modern Komodo dragon, but roughly 94 million years ago, they were beginning to explore the deeper ocean realm, according to Alan Titus, a paleontologist with the bureau’s Paria River district. Over time, they evolved streamlined bodies, fins and shark-like tails that propelled them through the water.
“They very rapidly diversified and basically become the dominant marine reptile for the remainder of the Cretaceous (145 million to 65 million years ago),” Titus said. “And they reached enormous sizes that rival modern whales.”
The fossil was initially found in 2012 by trained volunteer Scott Richardson, who was looking for traces of creatures that once swam in the seaway that covered most of the middle of North America during the Late Cretaceous Period (between 84 and 95 million years ago), according to a news release about the findings. He discovered several small skull fragments and vertebrae of what proved to be an early mosasaur scattered across a broad shale slope in Glen Canyon National Recreation Area.
(Bureau of Land Management) Steve Dahl and Alan Titus at the site of the Sarabosaurus dahli discovery within the gray shale badlands of Glen Canyon National Recreation Area on March 21, 2012.
After sharing his finding with Titus and Barry Albright, an earth science lecturer at the University of North Florida, the team was able to recover nearly 50% of the specimen over the course of several years, including vertebrae and enough skull fragments to determine its exact identity. The researchers estimated that the fossil was of a mosasaur that was about 3 or 4 years old, about a fourth of the way through its lifespan.
“The specimen had been weathering on the surface for quite a few years and was found in pieces so it did take quite a while to just go through the material and identify as much detail in the anatomy as possible to reconstruct ... certain aspects of the animal,” said Dr. Michael Polcyn, vertebrate paleontologist and mosasaur expert at Southern Methodist University.
The team was able to conclude that the specimen is potentially the oldest mosasaur ever found in North America.
This specific species of mosasaur was named after Steve Dahl, the longest-serving paleo volunteer at Grand Staircase-Escalante National Monument. The name is Sarabosaurus dahli, or “Dahl’s reptile of the mirage,” which “alludes to both the ancient seaway in which this animal swam that has long since vanished and the mirages that accompany the region’s extreme summer heat,” according to the news release.
“We chose to honor Steve Dahl for his long support of our program and the immense amount of work that he did on this project,” Titus said. “He was out in the field crews, helped us sieve all those thousands and thousands of pounds of shale fragments looking for bones, and was later involved in sorting pieces out and gluing them back together in the lab which is very tedious, painstaking work.”
Through micro-CT scanning and computer reconstruction, researchers were able to reveal that the Sarabosaurus had evolved a unique blood supply system similar to whales, which allowed them to enhance their underwater survival capabilities — likely a big factor in growing as large as they did.
The location of the discovery and nearby areas in southern Utah are some of the few places in North America where sedimentary layers from millions of years ago are exposed enough to consistently find new fossils, Titus said.
“It’s just a unique corner of the world that we’ve only just started combing through,” Titus said.
Since 2005, there have been 14 new species of dinosaurs named in the nearby Grand Staircase-Escalante National Monument, an area that encompasses a variety of environments from the Crustaceous period — from deep water to shallow water to swamps to dry land.
“All the pages in Earth’s history book are there in this region in one place,” Titus said.