94-million-year-old fossil provides clues to ancient reptile evolution

A fossil unearthed by a Bureau of Land Management (BLM) team is unlocking new information about the evolution of the most successful marine reptile family during the later part of the dinosaur age. The new research, published in Cretaceous Research, focuses on a 94-million-year-old mosasaur discovered in the gray shale badlands of the National Park Service Glen Canyon National Recreation Area.

The journey began nearly 11 years prior as Scott Richardson, a trained volunteer working under Dr. Barry Albright of the University of North Florida, looked for traces of creatures that once swam in the vast seaway that covered most of the middle of North America during the Late Cretaceous Period between 84 and 95 million years ago. In March 2012, Richardson found numerous small skull fragments and vertebrate of what proved to be an early mosasaur scattered across a broad shale slope.

After that “Eureka!” moment, a joint BLM-National Park Service crew recovered nearly 50 percent of the specimen over the course of the next two field seasons, including enough skull to determine its exact identity. Back in the lab, BLM Paria River District Paleontologist Dr. Alan Titus and his crew of BLM staff and volunteers traded the unforgiving summer heat of the Kaiparowits for the air-conditioned comfort of the lab to meticulously clean and glue the fossils to prepare them for research.

The team included Steve Dahl, the longest-serving paleo volunteer at the monument. Dahl’s contributions to the project were later honored in the species name, Sarabosaurus dahli, or “Dahl’s reptile of the mirage.” The name 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.

Dr. Titus sent the specimen to vertebrate paleontologist and mosasaur expert Michael Polcyn of Southern Methodist University in Dallas, who confirmed that it was the oldest mosasaur known from the Western Interior Seaway.

“I knew right away we had something special, especially considering it was so old,” said Polcyn.

“Mosasaurs from younger rocks are relatively abundant, but mosasaurs are extremely rare in rocks older than about 90 million years,” said Dr. Titus, a co-author of the study. “Finding one that preserves so much informative data, especially one of this age, is truly a significant discovery.”

The earliest mosasaurs were about 3-5 feet long, but they evolved into gigantic lizards that dominated the oceans during the latter part of the dinosaur age. Their land-dwelling ancestors were similar to the modern Komodo Dragon that, through time, evolved streamlined bodies, fins and shark-like tails that propelled them through the water. Early forms were more lizard-like in appearance and retained relatively primitive tails and limbs.

“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, who led the study.

Using micro-CT scanning and computer reconstruction to analyze the braincases of major mosasaur groups and map their vascular patterns showed how Sarabosaurus evolved a novel blood supply to the brain; a feature linking it with a group of better-known and geologically younger mosasaurs. 

“The way that blood gets to the brain is quite conservative in lizards, with the internal carotid arteries carrying the load,” said Polcyn. “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.” 

Authors of this recently published research include Michael J. Polcyn of SMU and the University of Utrecht, Netherlands; Dr. Nathalie Bardet of the Centre de Recherche en Paléontologie, Paris, France; Dr. Barry Albright, University of North Florida, Jacksonville, Florida; and Dr. Alan Titus, Bureau of Land Management, Paria River District, Kanab, Utah.

The Grand Staircase-Escalante National Monument paleontology program works to preserve and protect paleontological resources for the benefit of current and future generations. The paleontology laboratory, located in Kanab, Utah, is the only fossil laboratory within BLM. -- Bureau of Land Management (BLM)