Moros, a newly discovered tyrannosaurJorge Gonzalez

In 2012, Lindsay Zanno was searching for dinosaur fossils in the hillsides of eastern Utah when she found a bone protruding from the hillside. Most of it was still wreathed in rock, but Zanno, who heads the paleontology division at the North Carolina Museum of Natural Sciences, could already tell that it was the limb of some two-legged meat eater. “That was a thrilling moment,” she says.

It took several years to fully free the bone, along with a few others, from the rock, and several more to work out that they were once the right leg of a tyrannosaur—a cousin of the famed Tyrannosaurus rex. But at just 170 pounds and six feet long from nose to tail, this new human-size dinosaur was much smaller than its more famous relative. Growth rings in the bones, much like those in a tree trunk, showed that the individual was at least seven years old and nearly mature. “It’s certainly not a very young individual of a very large species,” Zanno says. Instead, it was an adult—just a small one.

Zanno named it Moros, after the embodiment of impending doom in Greek mythology. It’s a rather dramatic name for such a diminutive dinosaur, but it’s apt considering the creature’s age. Moros lived 96 million years ago, preceding Tyrannosaurus by a good 30 million years. It was a miniature harbinger of the bone-crunching tyrants to come—impending doom, indeed. And its age and size offer important clues about one of the most dramatic plot twists in the dinosaur story.

During the late Jurassic period, at a time when Asia and North America were connected to each other, the first tyrannosaurs evolved in the former continent before crossing over into the latter. At first they were just one of many groups of small-bodied hunters, all skulking subordinately in the shadow of far bigger predators, such as the allosaurs, a family of toothy, two-legged dinosaurs with dangerous claws. But at some point during the Cretaceous period, the allosaurs died out. The tyrannosaurs quickly usurped them, evolving into apex predators that ruled unchallenged in the northern continents until an asteroid strike (perhaps in combination with volcanic activity) ended their reign.

That switch from allosaurs to tyrannosaurs “was a defining event in dinosaur evolution, but we still don’t know very much about it,” says Steve Brusatte from the University of Edinburgh. “We’re not really sure exactly when it happened, if it happened quickly or was more of a prolonged battle, or if it happened across the northern continents all at once.”

These mysteries remain because of a lack of tyrannosaur fossils, especially in North America. Paleontologists have found many small-bodied species that are about 150 million years old, but the record then goes dark until the biggest species appear about 80 million years ago. During the 70 million intervening years: nothing, except for a few teeth. “It’s like a historian trying to understand how the Ming dynasty gave way to the Qing dynasty, but with only a few scraps of parchment to go by,” Brusatte says.

Moros, however, lived squarely within that tyrannosaur-free zone. Its discovery means that 96 million years ago, North American tyrannosaurs were still pretty small. That dramatically narrows the timing of their eventual ascension to a much shorter 15-million-year span. “That’s very quick,” Zanno says, for an animal to increase in mass by more than 100 times.

“This doesn’t completely solve the mystery of why the tyrannosaurs took over from allosaurs, but like a partial fingerprint at a crime scene, it provides important context and helps rule out some theories,” Brusatte says.

A similar scenario played out in Asia. Three years ago, Brusatte and his team announced the discovery of a tyrannosaur from Uzbekistan called Timurlengia, named after the central-Asian conqueror. It lived 90 million years ago, and was the size of a horse, suggesting that Asian tyrannosaurs were also small and medium-size for most of their history. “There is still a gap of about 10 million years between Timurlengia and Moros and [the later] huge tyrannosaurs,” Brusatte says. “Filling that gap, and hopefully with more complete skeletons, will be the next big breakthrough.”

In the meantime, Zanno and her colleagues are continuing their work in eastern Utah, focusing on sites from the early and middle Cretaceous. These rocks are much older than those that harbor celebrities such as Tyrannosaurus and Triceratops, and since many of the fossils within them are badly preserved, fewer paleontologists have studied them. That the sun can bake the ground to about 130 degrees Fahrenheit doesn’t help. “It’s a very difficult place to work,” Zanno says.

But a decade of hard labor has already paid off. Aside from Moros, the team also recently announced the discovery of Siats—a 30-foot-long allosaur whose name (pronounced see-ots) comes from a man-eating monster of Ute mythology. They’re also working hard to describe three new species of dinosaur and one new turtle. “We’re working to uncover an entirely new ecosystem,” Zanno says.

By unearthing this poorly understood world, she hopes to learn more about why allosaurs such as Siats died out. At the time, global temperatures were rising, as were sea levels. Flowers appeared around the world, and several Asian animals expanded into North America. Perhaps some combination of these changes conspired to dethrone the allosaurs, creating open opportunities that the tyrannosaurs then seized.

After all, Moros’s leg suggests that these animals were fast, agile hunters, while Timurlengia and other early tyrannosaurs show that the group already had well-developed senses. “As soon as the allosaurs went extinct, these early tyrannosaurs like Moros were primed to make the jump to being top predators,” Zanno says.

If this story is right, it means that the world’s most famous dinosaur was the beneficiary of good luck. “Tyrannosaurs are such iconic predators that people think they somehow outcompeted their rivals and were destined to be the top predators of the day,” Zanno says. “But Moros helps us understand that their success was linked to the extinction of allosaurs. Had that not happened, these tiny animals wouldn’t have been able to assume the top predator roles. There was no destiny in the ascent of tyrannosaurs.”

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