Most ecosystems (with one astonishing exception) have room for only one large predator, so it is not surprising the rise of the tyrannosaurs displaced other big carnivores. However, a study published in the Canadian Journal of Earth Science makes a much more unexpected case: the tyrannosaurs also wiped out midsized carnivores wherever they established themselves. The modern equivalent would be if leopards, hyenas, jackals, etc all could not co-exist with lions. Compared to the modern-day king of the beasts, T-Rex really was a tyrant.

Some paleontologists noticed major reductions in medium-sized predator diversity in all tyrannosaur-dominated environments. In the Cretaceous context, mid-sized means a carnivore whose adults weighed between 50 and 1,000 kilograms (110–2,200 pounds). On this scale, polar bears, the largest living land predators, are middle-sized, whereas every tyrannosaur species was large (more than 1,000 kilograms (2,200 pounds)). It really was a time of giants.

Dr Thomas Holtz of the University of Maryland decided to investigate the apparent pattern, and he concluded the impression reached by his colleagues was real. "Earlier in the history of dinosaurs, in most communities you'd have a bunch of different types of carnivores of various size ranges from small fox-sized all the way up to the occasional giants," Holtz said in a statement. "Then something happens between 95 and 80 million years ago, where we see a shift. The really big carnivores, larger than an elephant, like tyrannosaurs and their kin, become the apex predators, and the middle-sized predators, say leopard to buffalo-sized carnivores, are either missing or very rare." If Jurassic Park’s velociraptors really had met a T-Rex, it would have been something very new for them, leaving aside the film’s confusion about their size.

Nor was what Holtz observed confined to T-Rexes, with the entire tyrannosauridae genus having similar impacts. Tyrannosaurs never made it to Gondwana, however, where ecosystems maintained a mix of predator sizes.

At the same time, there was no equivalent reduction in the diversity of prey species.

Holtz doesn’t find it credible that herbivores of a certain size scored tens of millions of years of predator-free living, nor that adult tyrannosaurs could catch such small mouthfuls, even if they could be bothered. Instead, he thinks young tyrannosaurs filled the ecological niche once held by other species.

If, as seems likely, young tyrannosaurs were faster and more agile than their fully-grown counterparts, one species could have provided both the mid and large-sized predators. Lions don't do this because they are collective hunters, rather than leaving their young to find food alone.

The question Holtz still wants to answer is whether tyrannosaur young were such lethal hunters they drove competitors to extinction, or if the mid-sized carnivores were wiped out by something else, and teenage tyrannosaurs filled the vacant niche. That, Holts believes will take “Boots on the ground and picks in the sediments,” to answer. “We need more sampling sites from this interval between 95 and 80 million years ago,” he said. 

One measure that may throw light on the question is whether there was a change in prey size, even as the total diversity stayed the same. Holtz plans to investigate this but has no answers yet.