dinosaur

Offspring of enormous carnivorous dinosaurs, such as Tyrannosaurus rex, may have out-competed smaller species

UNM Biology

Giant, carnivorous dinosaurs like Tyrannosaurus rex really did dominate Earth. The predacious habits of these animals were so vast, in fact, that they muscled out the competition from smaller predatory species.

In a study of 43 dinosaur communities spanning 136 million years of prehistory, Katlin Schroeder at the University of New Mexico and her colleagues found that carnivorous dinosaur species estimated to have had an adult body weight of between 100 and 1000 kilograms were rare to non-existent in many dinosaur communities.

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This contrasts with the way carnivorous species of different sizes carve out niches in todays ecosystems. In places like the Serengeti in East Africa, carnivorous species form a continuous size gradient of small to large – from the tiny bat-eared fox to mid-sized African wild dogs to burly lions – without the break seen among predatory dinosaurs.

The explanation, Schroder and her colleagues propose, is that the juveniles of giant species – or “megatheropods” – like Allosaurus and Gorgosaurus acted almost like different species, choosing different prey to the adults and so outcompeting mid-sized carnivorous species.

“The implication is that we’re not missing medium-sized dinosaurs from the fossil record because they didn’t fossilise well or haven’t been collected, but that competition from juvenile megatheropods pushed them out of the ecosystem,” says Schroeder.

“I absolutely do agree with their premises, as I find the same basic pattern myself,” says Thomas Holtz at the University of Maryland. Holtz and others have pointed out that juvenile tyrannosaurs, for example, were much slenderer than adults and probably had different preferences for prey, only turning into bone-crushers after a teenage growth spurt.

Why megatheropod dominance was the rule, rather than the exception, might come down to prey availability, says Schroeder. Herbivorous dinosaurs started to get big early in their evolutionary history, setting up an adaptive arms race that resulted in larger carnivores. Since all dinosaurs hatched from eggs and started small, this meant that any medium-sized carnivores would have to compete with the youngsters of the giants.

“Bottom line is that throughout that entire time, it’s difficult to be a medium-sized carnivore,” says Shroeder.

Journal reference: Science, DOI: 10.1126/science.abd9220

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