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.

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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.

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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.”