Teenage rexes and other carnivorous dinosaurs the size of lions or bears may have displaced smaller species, which explains why so few of them are preserved in the fossil record.
Despite dominating the country for more than 150 million years, dinosaurs were not very diverse, and most known species were giants weighing 1,000 kg or more – including massive, carnivorous megateropods such as Tyrannosaurus rex.
Particularly absent from the fossil record are smaller dinosaurs weighing less than 60 kg. It is very different from other vertebrate communities, which usually have a wide range of body sizes.
Now a study published in Science gave an explanation: megateropods may have followed a “grow fast, die young” approach, which meant that the earth was chock-full of teenage carnivores occupying ecological niches that would otherwise house smaller carnivores.
To test this theory, Katlin Schroeder of the University of New Mexico and her colleagues analyzed a dataset of dinosaur records representing 43 geographically located communities across seven continents, spanning 136 million years. It confirmed that the communities containing megateropods were largely without medium carnivores in the 100-1,000 kg range, whereas those without megateropods contained these species.
Using existing information on the growth rates of these dinosaurs and the age at which they died, they also calculated that juveniles made up a significant portion of the total population of megateropods – enough to surpass similar adults of different species.
“Not only were there many more juveniles than adults, they would have been right in this mass range that we lack in other species,” Schroeder said. These young megateropods may have had a different ecological niche than adults – just as Komodo dragons do today, with their young hatching eggs, scrubbing trees and eating insects and lizards, until they grow too big and then fall to the ground. and began hunting larger creatures, from rodents to water buffalo.
“One thing that stands out about megateropods is that they have changed a lot as they have grown,” Schroeder said. “An adult Tyrannosaurus rex was this huge, robust, debilitating animal, but as juveniles they were fairly light, navy-footed, and they did not have deep, heavy skulls. They may have been the same genetic species, but they differ completely in appearance and function. ”
Steve Brusatte, a professor of paleontology and evolution at the University of Edinburgh, said: ‘This study has numbers on something we have long suspected but have not yet really proven: that the largest dinosaurs that eat meat filled different niches in the food chain as they grew from miniature hatcheries to adults larger than buses.
‘This seems to be a consistent pattern in dinosaurs, especially in the Cretaceous communities, towards the end of their reign. There were few carnivorous dinosaur species with a moderate adult body size, and that is because the juveniles and teens and large adults of the large bruised dinosaurs controlled the niches. It is an ecological structure that is very different from what we are used to with mammals today. ‘