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We now have a better idea of what T. rex’s ancestors may have been like. Two fossils have been found of a new tyrannosaur species, dubbed Suskityrannus hazelae, that lived around 90 million years ago, before the tyrannosaurs evolved into giants like T. rex.

The fossils show that tyrannosaurs developed many of their characteristic features such as a muscular skull, broad mouth and a shock-absorbing foot when they were still small. “Suskityrannus gives us a glimpse into the evolution of tyrannosaurs just before they take over the planet,” says palaeontologist Sterling Nesbitt at Virginia Tech.

T. rex, one of the biggest predators ever to walk the earth, is probably the most famous of all dinosaurs. What many people don’t realise, says Nesbitt, is that the reign of the giant tyrannosaurs like T. rex was surprisingly short – they only evolved around 15 million years before the dinosaurs were wiped out.

Palaeontologists have found very early, small tyrannosaurs that lived up to 150 million years before the giants appeared. But what happened right before the giants evolved has been a bit of a mystery, because of a gap in the fossil record.

S. hazelae fills in this gap. The first, very incomplete skeleton was found in New Mexico in 1997. A second, more complete fossil was found by Nesbitt in 1998 while he was still at high school. Nesbitt and colleagues have now finally published a paper describing the find.

The team estimates that this second individual was 1 metre high at the hip, 3 metres long and weighed 20 to 40 kilograms when alive. The bones show it was still a juvenile when it died, so the species may have grown a bit bigger but definitely not as large as a giant like T. rex.

Why the tyrannosaurs that roamed what is now North America and Asia remained medium-sized for 150 million years before becoming giants isn’t clear. But some other kinds of dinosaurs like duck-billed dinosaurs and ceratopsians also grew larger at that time, so perhaps the tyrannosaurs were keeping pace with their prey.