Jul 21, 2019 | Updated: 08:54 AM EDT

Smaller Tyrannosaur Fossils Aid Researchers in T-Rex Lineage

May 07, 2019 11:57 AM EDT

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Sterling Nesbitt was a teenager volunteering on a paleontological expedition back in 1998 when he found much of a two-legged dinosaurs' skeleton in western New Mexico's Zuni Basin. At the time, researchers knew little about what early tyrannosaurs should look like. So, the team confused Nesbitt's fossils for a dromeosaur, a group that includes the relatives of the velociraptor. The fossils sat in storage until 2006, when he began bringing the fossils with him as he moved between research institutions. It wasn't until scientists started finding some truly tiny, ancient tyrannosaur relatives in Asia, that the researchers finally realized what they had.

This new tyrannosaur, named Suskityrannus hazelae, takes its title from the Zuni tribe's word for a coyote. It stood about three feet tall at the hip and stretched nine feet from nose to tail. It was fleet-footed and had a longer, more slender snout shape than the large tyrannosaurs that evolved millions of years later. And another similar, small tyrannosaur from the Southwest was announced earlier this year. That one, named Moros intrepidus, or the harbinger of doom, lived 95 million years ago and is thought to have weighed less than 200 pounds as a full-grown adult. And, when standing erect, its head would've risen less than five feet tall.

"The discovery of Moros and Suskityrannus in the same year is wonderful and just shows that there are lots more out there to discover," says Nesbitt, now a paleontologist at Virginia Tech. He added: "Suskityrannus is a bit younger so it helps us constrain when large tyrannosauroids got big and became top-tier predators. Additionally, we have lots more of the anatomy of Suskityrannus compared to Moros, so it helps fill in our knowledge of the anatomical systems during this transition."

However, because both Suskityrannus specimens were juveniles, the researchers can't say how big the predators would have ultimately grown, but they were still very small compared to their more recent relatives. They were likely about the same size as Moros. And, taken together, the two new species tell paleontologists that tyrannosaurs had yet to start evolving their enormous body sizes.

They both lived at a time when plant-eating prey, like the ancient relatives of triceratops, were also much smaller. Larger animals had likely died off in a mini-mass extinction. And some scientists suspect that after the die-off, tyrannosaurs may have migrated across a land bridge from Asia following their prey. But there's little fossil evidence to support that idea.

"Suskityrannus brings us one step closer to filling in the Late Cretaceous gap in tyrannosaur evolution," says Lindsay Zanno, a paleontologist at North Carolina State University whose team discovered Moros and was not involved in this latest study. "The window of uncertainty has now shrunk to a little over 10 million years."

And scientists say not to read too much into the fact that the two juvenile tyrannosaurs were found about 150 feet apart at the same general site in the same sediment layer. There's no evidence the animals interacted in life. "We really can't say anything other than they died about the same time," Nesbitt says.

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