When T. rex dominated, medium-sized predators disappeared – replaced by juvenile tyrannosaurs

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New research from UMD shows that the ecology for tyrannosaurs changed as they grew up. Slender, agile young tyrannosaurs (left) hunted other prey and did so in a different way than the much larger, powerful adult jaws (right). Photo credit: Zubin Erik Dutta

A new study from the University of Maryland suggests that wherever tyrannosaurs rose to dominance, their juveniles took over the ecological role of medium-sized carnivores.

A new study shows that medium-sized predators have almost disappeared throughout dinosaur history Tyrannosaurus rex and his close relatives rose to dominance. In these areas – areas that eventually became Central Asia and western North America – juvenile tyrannosaurs entered to fill the missing ecological niche previously occupied by other carnivores.

Research, conducted by Thomas Holtz, a senior lecturer in the University of Maryland’s Department of Geology, confirmed previous anecdotal reports of a dramatic decline in the diversity of medium-sized predators in communities dominated by tyrannosaurs. The species diversity of the prey, however, did not decrease. This suggests that medium-sized predators did not disappear because of a drop in their prey, and that something else – likely young tyrannosaurs – intervened to fulfill their ecological role. The study was online on June 17, 2021 Canadian Journal of Earth Sciences.

“Back in dinosaur history, most communities had a number of different types of carnivores of varying sizes, from small foxes to the occasional giant,” Holtz said. “Then something happens 95 to 80 million years ago where we see a shift. The really large carnivores that are larger than an elephant, such as tyrannosaurs and their relatives, become the top predators, and the medium-sized predators, such as leopard to buffalo-sized carnivores, are either missing or very rare. “

Size distribution of carnivorous dinosaurs

The images above and below show the difference between the size distribution of carnivorous dinosaurs 71 million years ago and 151 million years ago. Photo credit: Thomas Holz / UMD

Typically, such a dramatic change in predators would coincide with a change in their prey. Either the type of prey would increase dramatically in the absence of predators, or the type of prey would also decrease, possibly suggesting why the predators disappeared. But the fact that Holtz did not find a shift in prey diversity suggests that something continued to fill the ecological role of the lack of medium-sized predators.

Previous work by Holtz and others shows that young tyrannosaurs were faster and more agile than their parents and likely hunted prey similar to that of the faster, more agile medium-sized dinosaurs.

It is possible that with the development and dominance of the tyrannosaurs, their young displaced other carnivorous dinosaurs in the middle range. But it’s also possible that something else eliminated the other carnivores and tyrannosaurs simply stepped in to fill the void. The shift to tyrannosaur dominance and the disappearance of medium-sized predators happened during a long void in the fossil record that left scientists unable to pinpoint exactly what happened.

“Solving this will rely on the most fundamental aspect of paleontology at the first level, boots on the ground and picks in the sediments,” said Holtz. “We need more sampling points from this interval around 95 to 80 million years ago.”

To conduct the study, Holtz examined the existing records of 60 dinosaur communities – collections of animals living in the same area at the same time – from the Jurassic and Cretaceous periods (201 to 66 million years ago). First he counted the number of carnivorous species and classified them in size classes, with medium-sized dinosaurs weighing between 50 and 1,000 kilograms and large dinosaurs weighing more than 1,000 kilograms.

His analysis found that tyrannosaurs were not the largest predators in 31 communities and there was a wide range of predators in the 50 to 1,000 kilograms category. In Asia and North America, these communities existed from the Jurassic to the early part of the Late Cretaceous (201 to 80 million years ago). Outside of Asia and North America, they existed until the end of the Late Cretaceous Period (80 to 66 million years ago).

In the other 29 dinosaur communities that Holtz studied, tyrannosaurs were the largest and arguably most dominant predators, weighing over 1,000 kilograms. In these communities, all found in Asia and North America, predators weighing 50 to 1,000 kilograms were rare or absent in the second half of the Late Cretaceous Period (80 to 66 million years ago).

Next, Holtz analyzed the same communities looking for shifts in the number of prey species. He found no statistical difference in the variety of prey types between the tyrannosaur-dominated and non-tyrannosaur-dominated communities.

“So what does that mean?” asked Holtz. “In these communities where medium-sized predators have disappeared but the types of prey are just as diverse, can we say that no one is hunting these medium-sized prey? No. That is almost certainly not the case. It is very likely that juvenile tyrannosaurs have taken over the ecological role of the missing medium-sized carnivores. “

Future studies will dig deeper into the makeup of prey communities to see if there were changes in prey size during the transition to tyrannosaur domination. Holtz also plans to study the size distribution of carnivores during the Triassic 251 to 201 million years ago. Understanding changes in size distribution and biodiversity can help paleontologists understand the influences that affect different types of predator and prey communities.

“These interactions are important to understand what life was like in the times of the dinosaurs,” said Holtz. “But in a broader sense, by better understanding the changing ecosystems, and in this case looking at the predator and prey components of an ecosystem, we get a better and more diverse view of how the interactions of life in the world work, even” today.

Reference: “Theropod Guild Structure and the Tyrannosaurid Niche Assimilation Hypothesis: Implications for Predatory Dinosaur Macroecology and Ontogeny in later Late Cretaceous Asiamerica” ​​by Thomas R. Holtz Jr., June 17, 2021, Canadian Journal of Earth Sciences.
DOI: 10.1139 / cjes-2020-0174



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