We finally know the reason for the absence of medium-sized carnivorous dinosaurs.

This research provides an unprecedented overview of the dynamics that governed their distribution.

— Orla / Shutterstock.com

American scientists have shown that the offspring of massive carnivorous dinosaurs like T. rex, characterized by spectacular growth, have profoundly reshaped their ecosystem, eclipsing smaller rival species.

A TENACIOUS MYSTERY FINALLY CLEARED

Published in the journal Science, this new study helps explain why, unlike what we see in terrestrial animals today, large dinosaur species outnumbered small ones during the 150-million-year-old period. Seen these creatures reign on Earth.

“Dinosaur communities were like shopping malls on a Saturday afternoon, packed with teenagers,” said Kat Schroeder, lead author of the study. “They were an essential part of the individuals of a species and probably had a genuine impact on the resources available within them. “

Even considering the limitations of current fossil records, it is believed that dinosaurs were not exceptionally diverse: only some 1,500 species are known, compared to tens of thousands of modern species of mammals and birds. Moreover, from 252 to 66 million years ago, many more species of enormous dinosaurs weighing a ton or more than species weighing less than 60 kilograms throughout the Mesozoic era.

Infographic illustrating the imbalance between prehistoric dinosaurs and modern carnivores – © UNM Biology Department

Therefore, some scientists have put forward the idea that even the most massive dinosaurs were tiny in size at birth and consequently tapped into different ecosystem resources as they grew, occupying niches in which smaller species. Otherwise, it could have prospered.

“VERY FEW ADULT CARNIVOROUS DINOSAURS WEIGHING BETWEEN 100 AND 1000 KILOS EXIST IN THESE COMMUNITIES”

To test this theory, Schroeder and his colleagues looked at data from fossil sites worldwide, including more than 550 species of dinosaurs. They organized dinosaurs according to their diet (herbivorous or carnivorous) and size. This analysis enabled them to highlight a striking absence of medium-sized adult carnivores within communities, including megatheropods, such as T. rex, which likely had more of a marathon runner’s profile than a sprinter.

“Very few adult carnivorous dinosaurs weighing between 100 and 1000 kilos existed in these communities,” Schroeder points out. “It turns out that the young megatheropods filled that space correctly. “

This conclusion was supported by how dinosaurs’ diversity evolved, with Jurassic communities (200-145 million years ago) exhibiting a less marked shortage of intermediate-sized creatures than those of the Cretaceous (145- 65 million years old).

Tyrannosaurus rex fossil – © UNM Biology Department

“More similar to adults, juvenile Jurassic megatheropods could attack a wide range of long-necked herbivorous sauropods, such as the brachiosaurus,” the study authors detail. “While the Cretaceous was completely dominated by tyrannosaurs and abelisaurids, whose morphology changed enormously during their growth. “

TREAT JUVENILES AS A SPECIES IN ITSELF

To mathematically test this theory, the team multiplied the mass of juvenile megatheropods at given ages by the number of creatures likely to survive annually, based on fossil records collected so far. By treating juveniles as a species in their own right, this statistical method has made it possible to fill the observed shortage of carnivorous dinosaurs of average size.

According to Schroeder, in addition to helping solve a stubborn mystery, this work demonstrates the value of further studying dinosaurs from an ecosystem point of view to better understand the dynamics that governed their distribution.

This new study echoes an analysis conducted in early 2020, which confirmed that two medium-sized T. rex skeletons were juvenile specimens, not hypothetical representatives of a dwarf species.

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