Answer to fossil record puzzle may lie with teenage T rexes, study finds

Absence of smaller dinosaurs may be result of adolescent megatheropods crowding them out

Teenage T rexes and other carnivorous dinosaurs the size of lions or bears may have crowded out smaller species, explaining why there are so few of them preserved in the fossil record, research suggests.

Despite dominating the land for more than 150 million years, dinosaurs were not particularly diverse, and most known species were giants weighing 1,000kg or more – including massive, meat-eating megatheropods such as Tyrannosaurus rex.

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

Now, a study published in Science has provided an explanation: megatheropods may have adopted a “grow fast, die young” approach that meant Earth was crammed with teenage meat eaters occupying ecological niches that would otherwise have hosted smaller carnivores.

To test this theory, Katlin Schroeder at the University of New Mexico and her colleagues analysed a dataset of dinosaur records representing 43 geographically located communities across seven continents, spanning 136m years. This confirmed that those communities containing megatheropods were largely devoid of medium-sized carnivores in the 100-1,000kg range, whereas those without megatheropods did contain these species.

Using existing information about the growth rates of these dinosaurs and the age at which they died, they also calculated that juveniles must have accounted for a substantial proportion of the total population of megatheropods – enough to have outcompeted similar-sized adults of different species.

“Not only were there many more juveniles than adults, they would have been right in this mass range that we’re missing from other species,” Schroeder said. These young megatheropods may well have occupied a different ecological niche to the adults – just as Komodo dragons do today, with their young hatching from eggs, scurrying up trees and eating insects and lizards, until they grow too big and then drop to the ground and start hunting larger creatures, from rodents to water buffalo.

“One thing that stands out about megatheropods is that as they grew they changed a lot,” said Schroeder. “An adult Tyrannosaurus rex was this huge, robust, bone-crunching animal, but as juveniles they were fairly light, fleet-footed, and they didn’t have deep, heavy skulls. They may have been the same genetic species, but they were entirely different in appearance and function.”

Steve Brusatte, a professor of palaeontology and evolution at the University of Edinburgh, said: “This study puts numbers on something we’ve long suspected but haven’t really proven: that the biggest meat-eating dinosaurs filled different niches in the food chain as they grew from miniature hatchlings into adults bigger than buses.

“This seems to be a consistent pattern in dinosaurs, especially those communities in the Cretaceous, towards the end of their reign. There were few meat-eating dinosaur species of moderate adult body size, and that’s because the juveniles and teenagers and subadults of the big bruising dinosaurs were controlling those niches. It’s an ecological structure that is very unlike what we are used to with mammals today.”

Contributor

Linda Geddes

The GuardianTramp

Related Content

Article image
Remains of new flying reptile species spotted in UK museum drawer
Student finds mislabelled fragment of pterosaur, which flew over eastern England up to 66m years ago

Steven Morris

10, Nov, 2020 @3:52 PM

Article image
Archaeopteryx 'flew in bursts like a pheasant', scientists say
The winged Late Jurassic creature would take to the air in frenetic, flapping bounds, fossil x-rays show

Ian Sample Science editor

13, Mar, 2018 @6:03 PM

Article image
Oldest known case of dandruff found in 125m-year-old dinosaur
Scientists have discovered fossilised dandruff on the skin of a feathered microraptor

Ian Sample Science editor

25, May, 2018 @2:05 PM

Article image
Dinosaurs had feathers ruffled by parasites, study finds
Ancient pieces of amber found to contain dinosaur feathers riddled with louse-like insects

Nicola Davis

10, Dec, 2019 @4:00 PM

Article image
Duck egg blue and oviraptor green: study reconstructs colour of dinosaur eggs
A new study of oviraptor eggshell fragments shows remarkable similarities between the reproductive biology of dinosaurs and birds

Hanneke Meijer

11, Oct, 2017 @5:00 AM

Article image
Monster mash: does the Frankenstein dinosaur solve the mystery of the Jurassic family tree?
Chilesaurus diegosuarezi, named after the seven-year-old who discovered it, changes everything we thought we knew about dino evolution …

16, Aug, 2017 @2:57 PM

Article image
Gideon Mantell: forgotten man who discovered the dinosaurs
A new play recalls the battle in the scientific establishment that denied a cobbler’s son credit for a major discovery

Rob Walker

03, Feb, 2019 @9:00 AM

Article image
Stegosaurus back plates differed between sexes, new study reveals
Finding is most convincing evidence so far that male and female dinosaurs looked different and may have had mating selection rituals comparable to birds

Hannah Devlin, science correspondent

22, Apr, 2015 @6:00 PM

Article image
Oldest fossil of bird's voicebox gives new hint at soundscape of the Cretaceous
66m-year-old syrinx of Vegavis iaai suggests that creature could honk and quack and confirms some modern bird groups lived alongside the dinosaurs

Nicola Davis

12, Oct, 2016 @5:00 PM

Article image
Meet Junornis: the tiny Cretaceous bird which reveals the earliest form of bounding flight
Newly-discovered Junornis huoi was the oldest bird capable of bounding flight – and represents an exciting update to what we know about complex flight

Dr Dave Hone

25, Oct, 2017 @12:08 PM