The answer to the riddle of the fossil record may lie in adolescent T rex, study finds | Dinosaurs

Adolescent T rex and other carnivorous dinosaurs the size of lions or bears may have displaced smaller species, which explains why there are so few of them preserved in the fossil record, research suggests.

Despite dominating the earth for more than 150 million years, dinosaurs were not particularly diverse, and most of the known species were giants weighing 1000 kg or more, including massive carnivorous megaheropods such as Tyrannosaurus Rex.

Particularly absent from the fossil record are the smallest dinosaurs weighing less than 60 kg. This is very different from other vertebrate communities, which normally contain a wide spectrum of body sizes.

Now a study published in Science has provided an explanation: Megaheropods may have taken a “grow fast, die young” approach that meant the Earth was crammed with carnivorous teenagers occupying ecological niches that would otherwise have been home to 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 communities geographically located on seven continents, spanning 136 million years. This confirmed that communities containing megatheropods largely lacked medium-sized carnivores in the 100-1,000 kg range, while those without megatheropods did contain these species.

Using existing information on the growth rates of these dinosaurs and the age at which they died, they also calculated that the juveniles must have represented a substantial proportion of the total population of megatheropods, enough to have outnumbered adults of similar size. of different species.

“Not only were there many more juveniles than adults, they would have been in this massive range that we lack in other species,” Schroeder said. These young megatheropods may well have occupied a different ecological niche than adults, just as Komodo dragons do today, with their young hatching from eggs, climbing trees, and eating insects and lizards, until they get too big. and then fall to the ground and start hunting bigger creatures from rodents to water buffalo.

“One thing that stands out about megaheropods is that as they got older they changed a lot,” Schroeder said. “An adult Tyrannosaurus Rex It was this huge, sturdy, bone-crunching animal, but when they were young they were quite light, light-footed and did not have deep, heavy skulls. It is possible that they were of the same genetic species, but they were completely different in appearance and function. “

Steve Brusatte, professor of paleontology and evolution at the University of Edinburgh, said: “This study puts numbers on something we long suspected but haven’t really proven: that the largest carnivorous dinosaurs filled different niches in the food chain as they grew from miniature hatchlings to larger-than-bus-sized adults.

“This seems to be a consistent pattern in dinosaurs, especially in Cretaceous communities, towards the end of their reign. There were few species of carnivorous dinosaurs of moderate adult body size, and that’s because the young, teenagers, and sub-adults of the large bruising dinosaurs controlled these niches. It is a very different ecological structure than what we are used to with today’s mammals. “

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