Madrid, 17 (European Press)
Medium-sized predators practically disappeared towards the end of dinosaur history as Tyrannosaurus rex and its close relatives took over.
This is the conclusion published by scientists from the University of Maryland in the Canadian Journal of Geosciences.
In these regions, which eventually became Central Asia and western North America, juvenile tyrannosaurs took it upon themselves to fill the ecological niche previously occupied by other carnivores.
Research by Thomas Holtz, Senior Professor in the Department of Geology at the University of Maryland, verified previous anecdotal reports of a sharp decline in the diversity of medium-sized predatory species in tyrannosaur-dominated communities.
In contrast, the diversity of prey species did not decrease. This suggests that the medium-sized predators did not disappear due to the retreat of their prey, and that something else – perhaps small dinosaurs – was stepping in to fulfill its ecological role.
“Early in the history of dinosaurs, in most societies there were many different species of carnivores of varying size ranges, from the size of a small fox to the occasional giant,” Holtz recalls.
“Then something happened between 95 and 80 million years ago, where we see a change – confirms -. Really big carnivores, bigger than an elephant, like tyrannosaurs and their like, became apex predators, and medium-sized ones, let’s say carnivores the size of a tiger or buffaloes, disappear or be very rare.”
Usually, such a drastic change in predators will coincide with some change in their prey. Either prey species will increase dramatically in the absence of predators, or prey species will also decrease, possibly indicating why predators are disappearing. But the fact that Holtz found no change in the diversity of prey species suggests that something continued to perform the ecological role of the lost intermediate predators.
Previous work by Holtz and others provides evidence that small dinosaurs were faster and more agile than their parents and likely hunted prey more similar to those eaten by faster and more agile medium-sized dinosaurs.
It is likely that as tyrannosaurs evolved and achieved dominance, their young outnumbered other medium-sized carnivorous dinosaurs. But it’s also possible that something else wiped out other carnivores and that dinosaurs simply stepped in to fill the void.
The shift to the dominance of tyrannosaurs and the disappearance of medium-sized predators occurred during a long gap in the fossil record, so scientists can’t say exactly what happened.
“Ultimately, the resolution of this will depend on the basic level 1 aspect of palaeontology, which is the aspect of boots on the ground and peaks in the sediments – says Holtz -. We need more sampling sites than this. The interval is between about 95 and 80 million years ago. Year “.
To conduct the study, Holtz examined the current 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 categorized them into size classes, with medium-sized dinosaurs weighing between 50 and 1,000 kilograms and large dinosaurs weighing more than 1,000 kilograms.
Their analysis revealed that tyrannosaurs in 31 communities 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 societies existed from the Jurassic period 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 that Holtz studied, tyrannosaurs were the largest, presumably dominant predators, weighing more than 1,000 kilograms. In these societies, which are all located in Asia and North America, predators weighing 50 to 1,000 kg were scarce or absent during the second half of the Upper Cretaceous period (between 80 and 66 million years ago).
Holtz then analyzed the societies themselves for changes in the number of prey species. He found no statistical difference in the diversity of prey species between tyrannosaur-dominated and non-tyrannosaurus-dominated societies.
So what does this mean? Holtz asked. In those societies in which medium-sized predators have disappeared but the types of prey are quite diverse, can we say that no one preys on this prey? “A medium size? No. This is almost certainly not the case. It is very likely that young tyrannosaurs took over the ecological role of the lost medium-sized carnivores.”
Future studies will delve into the composition of prey species communities to see if there were changes in prey size during the transition into tyrannosaurs’ dominance.
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 effects that affect different types of predator and prey communities.
He says, “These interactions are important for understanding what life was like in the time of the dinosaurs. But in a broader sense, having a greater understanding of changing ecosystems, and in this case, observing the components of predator and prey in an ecosystem, gives us a better and more diverse view of how life’s interactions work in the world, even Today “.