Adolescence was volatile for T rexes, other young dinosaurs

Adolescence is a time of great change for most of us. But it was particularly volatile for young T rexes. Before they became fearsome, bone-crushing adults, they had to pass through a number of stages — two-foot hatchling, gangly preteen, bulky young adult. At each phase, they hunted different prey and filled different niches.

As a new study in Science reveals, juvenile T rexes and the youth of other large carnivores called megatheropods transformed their communities as they fumbled through their own physical changes. Their rapid shifts in size and roles shaped their ecosystems, the study suggests, and could help to explain some of the perplexing mysteries of dinosaurdom, from the relative lack of species diversity to the strange preponderance of huge body sizes. Considering dinosaurs ruled the planet for 179 million years, there were fewer distinct species than you might expect. While today’s world is positively fuzzy with mammals — at the moment, nearly 7,000 different types — we only know of about 1,500 non-avian dinosaur species, said Kat Schroeder, a Ph.D. student at the University of New Mexico and a co-author of the new paper.

There are also “some very weird things about their mass distribution,” Schroeder said. Within contemporary animal classes, small-bodied species tend to vastly outnumber big ones. (For instance, there are currently twenty species of elephant shrew, and just three species of elephant.) But for dinosaurs, it’s the opposite: “Most of them are large,” she said. Some paleontologists looking into these dynamics over the years have tentatively blamed the youth. Juvenile T rexes were light and agile before they levelled up into the adults we’re more familiar with. (The physical discrepancies between younger and older T rexes can be so vast that experts have argued over whether certain specimens are different species altogether, rather than different ages.) Other megatheropods, including abelisaurs and tarbosaurs, also grew from turkey-like hatchlings to bus-sized behemoths over the course of their lives.

For this reason, the presence of just one of these species in an ecosystem meant that “a large number of different-sized predators existed” there, hunting progressively larger prey as they themselves grew up, said Dr. Marcus Clauss, head of research at the Clinic for Zoo Animals in Zurich, who has published theoretical work on this concept but was not involved in the new study. Perhaps the ecological real estate that might have been filled by mid-sized dinosaur species was instead taken up by these in-betweeners.

The new study “represents an enormous feat in testing this concept,” Dr. Clauss said. Theoretically, he said, these same dynamics might have made it harder for dinosaurs to repopulate large niches after a mass extinction event: When the big dinosaurs died, the relative lack of small and medium-sized species meant that mammals were better positioned to take over. Broad analyses like this one are “truly transforming the field” of palaeontology, said Lawrence M. Witmer, a professor of anatomy and palaeontology at Ohio University who was not involved in the study.

“The notion that youngsters were different kinds of predators than their monster parents was out there,” Dr. Witmer said. But while many paleontologists had been focusing on one species at a time in addressing this question, this new study instead connects “thousands of dots,” he said, to show “how whole communities of dinosaurs evolved.”