When Tyrannosaurs Were Tiny

A new study describes an early T. rex relative that stood about three feet tall and weighed no more than 90 pounds

An artist's rendering of how Suskityrannus hazelae may have looked. Andrey Atuchin

In 1998, when Virginia Tech assistant professor Sterling Nesbitt was 16 years old, he travelled to the Zuni Basin of New Mexico to take part in a dig led by paleontologist Doug Wolfe. As luck would have it, Nesbitt hit upon the fossil of a small dinosaur—though experts weren’t able to identify the species that had left the remains behind. But now, in light of additional discoveries that have been made over the years, Nesbitt and his colleagues have been able to identify the 92-million-year-old fossil as an early and rather tiny Tyrannosaurus rex relative.

Writing in the journal Nature Ecology & Evolution, the researchers note that the “multi-tonne, bone-crunching” T. rex that has become a thundering icon of popular culture thrived in the Late Cretaceous, between 66 million and 80 million years ago. Discoveries of distant T. rex relatives have recently revealed that the tyrannosaur family long consisted of small carnivores—like Dilong paradoxus, which was first found in China in 2004. But in the 1990s, when Nesbitt unearthed his fossil, the T. rex’s less imposing origins were not widely known or recognized.

Even today, the evolutionary history of the T. rex is not well understood, largely because extreme sea level rises during the Late Cretaceous destroyed fossils that had formed during the previous era, as Discover’s Eric Betz explains. The newly described fossil is filling in some of those gaps.

Researchers’ description of the dinosaur is based on two juvenile specimens: the one uncovered by Nesbitt in 1998, and a partial skull found in 1997. Dubbed Suskityrannus hazelae—in part after “Suski,” the Zuni Native American tribe word for “coyote”—the dinosaur stood around three-feet tall at the hip and spanned around nine feet in length. Suskityrannus probably weighed between 45 and 95 pounds. It was not as small as the oldest tyrannosaurs, but it was considerably smaller than the T. rex, which could reach a weight of nine tons and boasted a skull that was about the same length as Suskityrannus’ entire body.

The new dinosaur seems to represent an intermediate phase in tyrannosaur evolution, according to the study authors. Suskityrannus has some features seen in its later relatives—like an “arctometatarsalian foot,” which means that “the three long bones that make up the sole of the foot are pinched together, with the middle bone being particularly skinny,” Brown University PhD candidate Armita Manafzadeh, who was not involved in the new research, tells Gizmodo’s George Dvorsky. The arctometatarsalian foot has been linked to improved running ability, and Suskityrannus is the earliest known tyrannosaur to possess it.

Speaking to Dvorsky, Nesbitt notes that Suskityrannus is not a direct ancestor of the T. rex; it represents more of a “side-branch.” Still, he says, the dinosaur “gives us a glimpse into the evolution of tyrannosaurs just before they take over the planet.”

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