Any parent of growing teenagers knows their appetites can reach gargantuan proportions. Now, imagine you had a young T. rex checking the fridge. The outsize appetites of growing dinosaurs reshaped food chains in their environment and squeezed out other carnivores, according to a new study, published today in Science, of hundreds of dinosaurs of all sizes.
The “elegant study puts real numbers on something we’ve suspected for a while,” says Steve Brusatte, a paleontologist at the University of Edinburgh who was not involved with the study. There weren’t many midsize meat eaters “because the juveniles and teenagers and subadults of the big beastly dinosaurs were hoarding those niches.”
Most groups of animals have many small species, somewhat fewer medium-size species, and even fewer large species. In contrast, the extinct dinosaurs—especially carnivores—had plenty of species no bigger than modern-day chickens and also many giant species, but few medium-size ones.
Paleontologists wondered whether juvenile dinosaurs crowded out medium-size adults by exploiting the habitats and food sources those species might have taken. To test the idea, Katlin Schroeder, a Ph.D. student at the University of New Mexico (UNM), Albuquerque, combed a global collection of fossil data called the Paleobiology Database to determine the size distribution of more than 550 dinosaur species in 43 ancient ecosystems across 136 million years and seven continents.
In most communities, herbivorous dinosaurs came in a range of sizes. But “carnivores are completely different,” she says. Carnivores between 100 and 1000 kilograms were consistently quite rare. “It’s as if you went to the savanna and saw nothing in size between a bat-eared fox and lion,” Schroeder says. Patterns in all the dinosaur communities studied “are very similar, which is not what you’d expect from communities separated by 100 million years and half a globe,” she notes. “They did this on a grand scale,” says Gregory Erickson, a paleontologist at Florida State University. “It’s very impressive, and very comprehensive. … We saw the gap [in species’ sizes] for years, but never quantified it.”
To explore the reason, Schroeder and paleontologists Felisa Smith of UNM and Kathleen Lyons of the University of Nebraska, Lincoln, modeled the role that juvenile carnivores such as Tyrannosaurus rex might have played in ecosystems, based on their growth curves and the relative numbers of juveniles and adults found in “mass death” fossil beds. “We said, OK, if the juveniles are really using up this space, how many of them would you expect?” Smith says. Their calculations showed that “the teenagers fill the gap,” she says. “If you fill them in, then you get a community that looks like what you’d expect.”
The effect may be stronger in meat eaters because each carnivorous dinosaur species occupied a wide range of niches. They hatched from relatively small eggs; even the largest ones only weighed about 15 kilograms as hatchlings. Then they grew very quickly, changing diets and hunting methods to accommodate their new sizes and competing with a range of other species along the way, Erickson says.
The study’s emphasis on how animals’ niches can change as they grow offers fresh insights, says Mike Benton, a paleontologist at the University of Bristol. “It will make people look at predator-prey interactions in a different way.”