Teenage T. rexes Outcompeted Smaller Rivals, New Study Suggests
The theory could account for why there are so few fossils of medium-sized dinosaurs.
Prehistoric heavyweights such as the Tyrannosaurus rex may have outcompeted their smaller rivals whilst in their teens, leaving medium-sized dinosaurs missing from the fossil record, researchers from The University of New Mexico and at the University of Nebraska-Lincoln have found.
Palaeontologists have long been puzzled why the number of different dinosaurs types known around the globe is so low, particularly among small and medium-sized species. Now, a new study published in the journal Science suggests this may be because they were outcompeted by adolescent megatherapods that were not yet fully grown.
Despite growing to the size of double decker buses, colossal dinosaurs such as the T. rex started life relatively small – roughly the size of a Chihuahua – on account of being born from eggs. This means that they would likely have been in competition with smaller dinosaurs as they grew, the researchers say.
“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 Kat Schroeder, a graduate student in the UNM Department of Biology who led the study.
“Dinosaur communities were like shopping malls on a Saturday afternoon – jam-packed with teenagers. They made up a significant portion of the individuals in a species and would have had a very real impact on the resources available in communities.”
To investigate the question of decreased dinosaur diversity, the team gathered data from multiple well-known fossil locations scattered across the globe, comprising fossils from more than 550 dinosaur species. They then organised the dinosaurs by mass, diet, size, and location.
They then built up a picture of what the dinosaur communities would have looked like by combining data from growth rates taken from lines found in cross-sections of bones and the number of infant dinosaurs surviving each year based on the fossil record. This enabled them to calculate what proportion of a megatheropod species would have been juveniles.
“There is a gap – very few carnivorous dinosaurs between 100-1000kg exist in communities that have megatheropods,” said Schroeder. “And the juveniles of those megatheropods fit right into that space.”
They also found that the gap was much smaller in Jurassic communities, which ran from 200 to 145 million years ago, than Cretaceous communities, which ran from 145-65 million years ago – the period when the T. rex was king.
“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,” said Schroeder. “The Cretaceous, on the other hand, is completely dominated by Tyrannosaurs and Abelisaurs, which change a lot as they grow.”