Scientists at the University of New Mexico and the University of Nebraska-Lincoln in the United States have solved the mystery of why medium-sized predatory dinosaurs are practically absent in the Cretaceous, and large theropods such as tyrannosaurs dominate. The researchers’ article was published in the journal Science.
Modern predator communities include animals with a wide range of body sizes. However, the fossil record indicates that there were no mesocarnivorous predators among the dinosaurs, that is, species with an average body size. According to new data, the reason for this is the special developmental biology of large theropods, including tyrannosaurs, when the young were originally the size of tiny chicks. As they grew, tyrannosaurs sequentially occupied niches of predators of other sizes, limiting biodiversity.
Researchers have determined the effect of size mismatch between newborn and adult dinosaurs on 43 dinosaur communities on seven continents over 136 million years. Paleoecologists analyzed the fossils of 550 dinosaur species, distributing them by weight and diet, and estimating the number of small, medium and large dinosaurs in each community.
As a result, it turned out that it was the mega-pods (giant carnivorous dinosaurs) that had a specific effect on the structure of communities. Despite the fact that herbivorous dinosaurs could be of any size, there were no carnivorous species with a body weight of 100 to 1000 kilograms in the communities. Instead, this niche was occupied by young individuals of tyrannosaurs. This gap is most typical for the Cretaceous period (145-65 million years ago), and in the Jurassic (200-145 million years ago) it was even smaller.
In the Jurassic period, the difference between young and adult theropods (for example, Allosaurus) was not as high, unlike the Tyrannosaurs and Abelisaurs of the Cretaceous period. In addition, there was a wide variety of sauropods, giant herbivorous dinosaurs that could be hunted by various predators.