T.rex ancestor sheds light on predator evolution

As every school child knows, Tyrannosaurus rex was an enormous and ferocious apex predator. Right now, however, at least among paelaeontologists, it is a tiny little relative of the big beast that is attracting all the attention.

In stark contrast to T.rex, most tyrannosauroid dinos were small animals – subordinate hunters that enjoyed survival, if not exactly dominion, for 100 million years.

Just how the group developed an evolutionary offshoot that produced several supersized species remains an open question, in part because tyrannosauroids are quite poorly represented in the fossil record.

Hence, the considerable excitement generated by the recent discovery in Emery County, Utah, US, of Moros intrepidus – a previously unknown T.rex ancestor that is small even by tyrannosauroid standards.

Writing in the journal Communications Biology, scientists led by Lindsay Zanno of the North Carolina Museum of Natural Sciences calculate the animal to have been around seven years old at the time of its death. It had a limb length of 1.2 metres and weighed just 78 kilograms.

The dinosaur, say the researchers, “ranks among the smallest Cretaceous tyrannosauroids”, but it is not just its size that is generating interest.

One of the big reasons the evolutionary journey from tiny tyrannosauroid to earth-shaking Jurassic Park star is poorly understood is that, in North America at least, there is a 70-million-year gap between the small animal fossils found and the T.rex remains at the other.

M. intrepidus fills in another 15 million years of the picture. It shows physical similarities with some species found in Asia, supporting the notion that tyrannosauroids may have originally become established in North America through gradual migration.

The find also provides further evidence that tiny tyrannosaurs constituted a highly successful group. M. intrepidus, Zanno and colleagues write, “reveals an evolutionary strategy reliant on speed and small size during their prolonged stint as marginal predators”.

Perhaps most importantly, the researchers note, the dating of the new fossil restricts the window during which one branch of the tyrannosauroids underwent the changes, particularly rapid and huge increases in mass, that resulted in in the iconic super-predator.

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