Monday, August 2

When the Tyrannosaurus rex dominated, the average predators disappeared

Medium-sized predators practically disappeared by the end of dinosaur history where the ‘Tyrannosaurus rex’ and its close relatives reached dominance.

It is the conclusion that scientists of the University of Maryland published in the Canadian Journal of Earth Sciences.

In these areas, which eventually became central Asia and western North America, Juvenile tyrannosaurs were in charge of filling the ecological niche previously occupied by other carnivores.

Research carried out by Thomas Holtz, Senior Lecturer in the Department of Geology at the University of Maryland, verified previous anecdotal reports on a drastic decline in the diversity of species of medium-sized predators in communities dominated by tyrannosaurs.

In contrast, the diversity of prey species did not decrease. This suggests that medium-sized predators did not disappear due to the decline in their prey, and that something else – probably young tyrannosaurs – intervened to fulfill their ecological role.

“Early in the history of dinosaurs, in most communities there would be a lot of different types of carnivores of various ranges in size, from small fox-sized to occasional giants,” Holtz recalls.

“Then something happens between 95 and 80 million years ago, where we see a change – he emphasizes -. The really big carnivores, bigger than an elephant, like tyrannosaurs and their congeners, become top predators, and the medium-sized ones, let’s say carnivores the size of a leopard or a buffalo, disappear or are very rare. ”

Normally, such a drastic change in predators would coincide with some change in their prey. Either the prey species would increase dramatically in the absence of predators, or the prey species would decrease as well, perhaps indicating why the predators disappeared. But the fact that Holtz found no change in the diversity of prey species indicates that something continued to fulfill the ecological role of the missing medium predators.

Previous work by Holtz and others provides evidence that young tyrannosaurs they were faster and more agile than their parents and probably hunted prey more similar to that eaten by medium-sized dinosaurs, faster and more agile.

It is possible that, as tyrannosaurs evolved and achieved dominance, their young ones outnumbered other medium-sized carnivorous dinosaurs. But it’s also possible that something else wiped out the other carnivores and the tyrannosaurs simply stepped in to fill the void.

The shift toward tyrannosaurus dominance and the disappearance of medium-sized predators occurred during a long gap in the fossil record., so scientists cannot say exactly what happened.

“Ultimately, solving this will depend on the most basic aspect of the first level of paleontology, which is that of the boots on the ground and the peaks in the sediments,” Holtz said. “We need more sampling sites for this. interval between about 95 and 80 million years ago. ”

To carry out the study, Holtz examined the existing record of 60 dinosaur communities -groups of animals that lived 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 into size categories, with medium-sized dinosaurs weighing between 50 and 1,000 kilograms and large dinosaurs weighing more than 1,000 kilograms.

A wide range of predators

Their analysis revealed that in 31 communities tyrannosaurs were not the largest predators, and that there were a wide range of predators in the 50 to 1,000 kilogram category. In Asia and North America, these communities existed from the Jurassic to the early part of the Upper Cretaceous (between 201 and 80 million years ago). Outside of Asia and North America, they continued to exist until the end of the late Cretaceous period (80 to 66 million years ago).

In the other 29 dinosaur communities studied by Holtz, tyrannosaurs were the largest predators, and presumably dominant, weighing more than 1,000 kilograms. In these communities, all located in Asia and North America, predators between 50 and 1,000 kilograms were rare or absent during the second half of the Upper Cretaceous (between 80 and 66 million years ago).

Holtz then analyzed the same communities for changes in the number of prey species. He found no statistical difference in the diversity of prey species between tyrannosaur-dominated and non-tyrannosaur-dominated communities.

So what does this mean? Holtz wondered. In those communities where the medium-sized predators have disappeared but the prey species are just as diverse, can we say that no one is preying on these medium-sized prey? “No. That is almost certainly not the case. It is quite likely that the juvenile tyrannosaurs assumed the ecological role of the missing mid-size carnivores.”

Future studies will delve into the composition of species communities of prey to see if there were changes in the size of the prey during the change to the dominance of the tyrannosaurs.

Holtz also plans to study the size distribution of carnivores during the Triassic period, between 251 and 201 million years ago. Understanding changes in size distribution and species diversity can help paleontologists understand the influences that affect different types of predator and prey communities.

These interactions are important to understanding what life was like in the time of the dinosaurs. -he assures-. But in a broader sense, having a greater understanding of changing ecosystems, and in this case, looking at the predator and prey components of an ecosystem, gives us a better and more diverse view of how life’s interactions work in the environment. world, even today. ”

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