New Study Reveals Alvarezsaur Dinosaurs Rapidly Shrank In Size After Ant-based Diet

The size of dinosaurs is an important aspect of dinosaur paleontology. The typical image of a dinosaur in our minds is a massive animal, but a new study of the unusual alvarezsaurs reveals that they shrank in size about 100 million years ago when they evolved into specialized ant-eaters.

Zichuan Qin, a Ph.D. student at the University of Bristol and Vertebrate Paleontology and Paleoanthropology in Beijing, led this study.

Alvarezsaurs were slender theropods, a diverse group of two-legged dinosaurs with hollow bones and three-toed limbs that included Tyrannosaurus rex and Velociraptor. There are 21 confirmed species of alvarezsaurs dating from the late Jurassic period around 160 million years ago to the end of the Cretaceous period around 66 million years ago.

For the study, Zichuan measured the body sizes of dozens of specimens and discovered that for the majority of their lives, they ranged in size from 10-70 kg, or roughly the size of a large turkey to the size of a small ostrich, and then plummeted rapidly to chicken-sized animals at the same time they adopted a remarkable new diet: ant-eating.

"My calculations show how body sizes went up and down for the first 90 million years they existed, ranging from turkey to ostrich-sized, and averaging 30-40 kg," Zichuan explained. "Then, 95 million years ago, their body size suddenly dropped to 5 kg, and their claw shapes changed from grabbing and cutting to punching." 

Paleontologists studying the fossils of these dinosaurs discovered that the group's size decreased dramatically between 110 million and 85 million years ago. Previously, alvarezsaurs weighed between 22 and 154 pounds (10 and 70 kilograms), but they quickly shrank to less than 11 pounds (5 kg), with one species shrinking to just 0.3 pounds (0.15 kg).

"Interestingly, alvarezsaur dinosaurs were indeed not small in size or anteaters at the start," said Professor Jonah Choiniere, a co-author of this paper, who was the first to report the earliest alvarezsaurs in China. "Their ancestors, like Haplocheirus, are relatively large, close to the size of a small ostrich, and their sharp teeth, flexible forelimbs, and big eyes suggest they had a mixed diet."

Professor Michael Benton of the University of Bristol, one of Zichuan's supervisors, suggested that the drastic change in the diet of alvarezsaur could have resulted from increased competition for food.

"This is a very strange result, but it seems to be true," said Professor Xing Xu. "All other dinosaurs were getting bigger and bigger, but one group of flesh-eaters miniaturized, and this was associated with living in trees and flying. They eventually became birds. We've identified a second miniaturization event—but it wasn't for flight, but to accommodate a completely new diet, switching from flesh to termites."

During the Cretaceous period, the ecosystem was rapidly evolving, and flowering plants took over the landscape, Professor Xing Xu claims. The dinosaurs did not eat these flowering plants, but they did cause an explosion of new types of insects, such as ants and termites.

The study was published in the journal Current Biology.