Fossils of 3-foot-tall T. rex relatives are evolutionary stepping stone

Fossils found in New Mexico belong to a previously undiscovered species of small tyrannosauroids that lived 92 million years ago, cousins of the much larger Tyrannosaurus rex. They shed light on how these dinosaurs evolved from tiny to the top of the food chain, a new study says.

Tyrannosaurus rex, like the kind wreaking havoc in “Jurassic Park,” reigned as king of the dinosaurs between 66 million and 80 million years ago. But it wasn’t always the top predator.

The origins of tyrannosaurs are hard to determine because high sea levels disturbed the fossil record in western North America, where giant dinosaurs thrived during the Late Cretaceous period. Few fossils exist for the evolutionary period during the mid-Cretaceous.

But two fossils uncovered in New Mexico’s Zuni Basin represent the most complete tyrannosauroids to date. The fossils are detailed in a study published Monday in the journal Nature Ecology & Evolution.

They represent a point in the evolution from the smallest tyrannosaurs to the largest ones that peaked just before dinosaurs went extinct.

Researchers named the creatures Suskityrannus hazelae, combining “suski,” the Zuni word for coyote, with the Latin “tyrannus,” which can mean king. The Zuni Tribal Council allowed the researchers to include “suski” in the name. Hazelae was included for Hazel Wolfe, who made fossil expeditions to the basin possible.

The small tyrannosauroids would have stood 3 feet tall and measured about 9 feet in length. The entire dinosaur was only slightly longer than the skull of a massive Tyrannosaurus rex.

Suskityrannus probably weighed between 45 and 90 pounds, compared with the 9 tons of a Tyrannosaurus rex.

Although the researchers don’t know what Suskityrannus ate, the carnivorous dinosaur probably hunted small animals. An analysis of the fossil’s bone growth suggests that it was 3 years old when it died, but a full-grown Suskityrannus probably wasn’t much larger than this one.

Compared with Tyrannosaurus rex, Suskityrannus had a more slender skull and a specially adapted foot suited for running.

“Suskityrannus gives us a glimpse into the evolution of tyrannosaurs just before they take over the planet,” said Sterling Nesbitt, study author and assistant professor in the Virginia Tech College of Science’s Department of Geosciences, in a statement. “It also belongs to a dinosaurian fauna that just proceeds the iconic dinosaurian faunas in the latest Cretaceous that include some of the most famous dinosaurs, such as the Triceratops, predators like Tyrannosaurus rex, and duckbill dinosaurs like Edmotosaurus.”

Nesbitt’s history with the fossil stretches to 1998. At the time, he was a 16-year-old high school junior with an interest in paleontology who was able to participate in a dig led by Doug Wolfe, head of the Zuni Paleontological Project. James Kirkland of the Utah Geological Survey also assisted with the fossil’s collection at the time.

“[It was] exciting, but at the same time a bit overwhelming,” Nesbitt said. “I was standing over the remains of a dinosaur that was partially weathered out of the ground and I wanted to make sure that I picked up all of the pieces.”

“Following Sterling out to see his dinosaur, I was amazed at how complete a skeleton was lying exposed at the site,” Kirkland said. “Clearly the most complete individual skeleton we had found in the entire basin and we had not even started to collect it.”

Another partial skeleton was found in 1997 by Robert Denton, who is now a senior geologist with Terracon Consultants.

But for 20 years, the fossils’ significance as an evolutionary stepping stone remained a secret. The researchers believed that they had found fossils belonging to a Velociraptor or something like it. The fossil indicated that it had a strong bite force.

At the time, tyrannosaur relatives weren’t known. “Essentially, we didn’t know we had a cousin of Tyrannosaurus rex for many years,” Nesbitt said.

The fossils were just some of many found in the Zuni basin, along with those of fish, mammals, lizards, turtles and crocodiles.

Between 1998 and 2006, the fossils were stored at the Arizona Museum of Natural History. Then Nesbitt took them with him, as both a student and a researcher, to different institutions. And then he began to study the fossils with his colleagues.

Finding the fossil, and working with the dig team members, sparked Nesbitt’s career.

“My discovery of a partial skeleton of Suskityrannus put me onto a scientific journey that has framed my career,” Nesbitt said. “I am now an assistant professor that gets to teach about Earth history.”

Wolfe said, “When I first met Sterling, he was a young student with extraordinary focus and passion. He was always going to be a paleontologist.”

Suskityrannus reveals more about the mid-Cretaceous period, when Earth was warmer and the sea level was higher and how things changed for dinosaurs afterward, Wolfe said.

“It has been a long journey, but I think we have a good understanding of how Suskityrannus fits into the bigger picture of tyrannosauroid and, more broadly, dinosaur evolution,” Nesbitt said. “I always feel that studying these animals and publishing/naming them gives them a voice that was not there until the discovery. If we did not find the two specimens of Suskityrannus we would have never known this animal existed.”