There are small predator dinosaurs and huge predator dinosaurs (1000 kg to 8000 kg), but none with adult sizes between 100 kg and 1000 kg.
Because that ecological gap was filled by adolescent dinosaurs that are the size of house cases when born but grow to the size of a city bus as an adult.
Because they were born from eggs, dinosaurs like T. rex necessarily were born small -- about the size of a house cat. This meant as they grew to the size of a city bus, these "megatheropods," weighing between one and eight tons, would have changed their hunting patterns and prey items. It's long been suspected by paleontologists that giant carnivorous dinosaurs would change behavior as they grew. But how that might have affected the world around them remained largely unknown."We wanted to test the idea that dinosaurs might be taking on the role of multiple species as they grew, limiting the number of actual species that could co-exist in a community," said Schroeder.The number of different types of dinosaurs known from around the globe is low, particularly among small species."Dinosaurs had surprisingly low diversity. Even accounting for fossilization biases, there just really weren't that many dinosaur species," said Felisa Smith, professor of Biology at UNM and Schroeder's graduate advisor.To approach the question of decreased dinosaur diversity, Schroeder and her coauthors collected data from well-known fossil localities from around the globe, including over 550 dinosaur species. Organizing dinosaurs by mass and diet, they examined the number of small, medium and large dinosaurs in each community.They found a strikingly clear pattern:"There is a gap -- very few carnivorous dinosaurs between 100-1000kg [200 pounds to one ton] exist in communities that have megatheropods," Schroeder said. "And the juveniles of those megatheropods fit right into that space."Schroeder also notes that looking at dinosaur diversity through time was key. Jurassic communities (200-145 million years ago) had smaller gaps and Cretaceous communities (145-65 million years ago) had large ones."Jurassic megatheropods don't change as much -- the teenagers are more like the adults, which leaves more room in the community for multiple families of megatheropods as well as some smaller carnivores," Schroeder explained. "The Cretaceous, on the other hand, is completely dominated by Tyrannosaurs and Abelisaurs, which change a lot as they grow."To tell whether the gap was really caused by juvenile megatheropods, Schroeder and her colleagues rebuilt communities with the teens taken into account. By combining growth rates from lines found in cross-sections of bones, and the number of infant dinosaurs surviving each year based on fossil mass-death assemblages, the team calculated what proportion of a megatheropod species would have been juveniles.Schroeder explained that this research is important because it (at least partially) elucidates why dinosaur diversity was lower than expected based on other fossil groups. It also explains why there are many more very large species of dinosaurs than small, which is the opposite of what would be expected. But most importantly, she added, it demonstrates the results of growth from very small infants to very large adults on an ecosystem.
From Science Daily. The paper with its significance statement and abstract are as follows:
Not enough roomModern carnivore communities include species that span a range of body sizes. For example, on the African savannah, there are small species (mongooses), medium species (wild dogs), and large species (lions). This variation reflects available prey sources that best suit each group. Carnivorous dinosaur communities, however, were missing species that fall into the middle, or mesocarnivore, group as adults. Schroeder et al. looked across communities, space, and time and found that this absence appears to have been driven by the distinctive biology of dinosaurs, in which giant adults start out as tiny hatchlings. Growing juvenile dinosaurs thus filled the other niches and limited trophic species diversity.AbstractDespite dominating biodiversity in the Mesozoic, dinosaurs were not speciose. Oviparity constrained even gigantic dinosaurs to less than 15 kg at birth; growth through multiple morphologies led to the consumption of different resources at each stage. Such disparity between neonates and adults could have influenced the structure and diversity of dinosaur communities. Here, we quantified this effect for 43 communities across 136 million years and seven continents. We found that megatheropods (more than 1000 kg) such as tyrannosaurs had specific effects on dinosaur community structure. Although herbivores spanned the body size range, communities with megatheropods lacked carnivores weighing 100 to 1000 kg. We demonstrate that juvenile megatheropods likely filled the mesocarnivore niche, resulting in reduced overall taxonomic diversity. The consistency of this pattern suggests that ontogenetic niche shift was an important factor in generating dinosaur community structure and diversity.
Katlin Schroeder, S. Kathleen Lyons, Felisa A. Smith. "The influence of juvenile dinosaurs on community structure and diversity." 371 (6532) Science 941 (2021). DOI: 10.1126/science.abd9220