![]() To conduct the study, Holtz surveyed the existing record of 60 dinosaur communities - assemblages of animals living in the same area at the same time - from the Jurassic and Cretaceous periods (201 to 66 million years ago). "We need more sampling sites from this interval between about 95 and 80 million years ago." "Ultimately resolving that is going to rely on the most basic first-level aspect of paleontology, which is boots on the ground and picks in the sediments," Holtz said. The shift to tyrannosaur dominance and the disappearance of middle-sized predators happened during a long gap in the fossil record, so scientists can't say exactly what occurred. But it is also possible that something else eliminated the other carnivores and tyrannosaurs simply stepped in to fill the gap. It is possible that as tyrannosaurs evolved and grew to dominance, their juveniles outcompeted other carnivorous dinosaurs in the middle-size range. Previous work by Holtz and others provides evidence that young tyrannosaurs were faster and more agile than their parents and likely hunted prey more similar to that eaten by the faster, more agile medium-sized dinosaurs. But the fact that Holtz found no shift in prey species diversity indicates that something continued to fill the ecological role of the missing middle-sized predators. Either the prey species would increase dramatically in the absence of predators, or the prey species would also drop off, perhaps indicating why the predators disappeared. Typically, such a dramatic change in predators would coincide with some change in their prey. 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." "Then something happens between 95 and 80 million years ago, where we see a shift. "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. The study was published online on June 17, 2021, in the Canadian Journal of Earth Sciences. ![]() This suggests that medium-sized predators did not disappear because of a drop off in their prey, and that something else - likely young tyrannosaurs - stepped in to fill their ecological role. Diversity of prey species, on the other hand, did not decline. The research conducted by Thomas Holtz, a principal lecturer in the University of Maryland's Department of Geology, verified previous anecdotal reports of a dramatic drop-off in diversity of medium-sized predator species in communities dominated by tyrannosaurs. In those areas - lands that eventually became central Asia and Western North America - juvenile tyrannosaurs stepped in to fill the missing ecological niche previously held by other carnivores. ![]() A new study shows that medium-sized predators all but disappeared late in dinosaur history wherever Tyrannosaurus rex and its close relatives rose to dominance.
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