Tyrannosaurs Dominated Their Cretaceous Ecosystems

Tyrannosaurus rex may be the most perfect dinosaur name ever coined. What else would you call a carnivorous, bipedal reptile that could grow more than 40 feet in length and weigh more than nine tons? The sheer size and apparent ferocity of T. rex has been apparent from the outset, but, paleontologists have learned, this Cretaceous meat-eater and its relatives really did have a tyrannical clawhold on the ecosystems they lived in. The differences between adult and adolescent tyrannosaurs were so great that the animals almost lived like different species, pushing out mid-sized carnivores in a prehistoric takeover. That’s part of the key findings of a study published earlier this year in the Canadian Journal of Earth Sciences, in which Royal Tyrrell Museum of Paleontology researcher François Therrien and colleagues found that young tyrannosaurs behaved—and bit their prey—in a different way than adults.

The idea that T. rex and related dinosaurs such as Gorgosaurus had an outsize influence on the habitats they stalked hasn’t come from a single discovery, but from years of fortuitous fossil finds, analyses of dinosaur anatomy and placing these clues into an ecological context. A host of new papers have arrived at the same conclusion—the way big tyrannosaurs grew up allowed them to vastly influence their ecosystems in ways no other predators ever had before. Young, small, svelte tyrannosaurs had different predatory capabilities than the adults and chased after smaller fare. It was only in their teenage years, during a dramatic growth spurt, that these dinosaurs gained their taste for big game. This dinosaurian happenstance allowed tyrannosaurs to nudge other carnivores out of the way—creating ecosystems unlike anything seen today, dominated by a single large predator.

Even though tyrannosaurs have a very long history, the first of their storied family evolving about 170 million years ago, these carnivores stayed small for tens of millions of years. With few exceptions, it was only about 80 million years ago that a tyrannosaur subset—the tyrannosaurids—began to achieve giant sizes, their success underwritten by terrible, crushing bites that allowed them to both hunt and dismember carrion with ease. And it’s these dinosaurs, especially the tyrannosaurs that roamed western North America during the last 14 million years of the Cretaceous, that fundamentally altered the landscape around them.

If you were to visit ancient Alberta around 75 million years ago, chances are you’d run into some big, lanky tyrannosaurs such as Albertosaurus and Gorgosaurus. These dinosaurs were the top predators of their ecosystems, much like their more famous relative T. rex. Even though juvenile specimens of the great T. rex are rare, Therrien and coauthors note, experts have uncovered multiple juveniles of Albertosaurus and Gorgosaurus. That’s allowed paleontologists to piece together a fuller picture of how these tyrants grew up. In the case of the new study, Therrien and coauthors found that for their first decade of life Albertosaurus and Gorgosaurus had relatively weak bites and thin, blade-like teeth. Around age 11, however, when their lower jaws got to be about 23 inches long, the teeth that constantly-replaced themselves in the dinosaurs’ jaws began to change. Instead of being thin and blade-like, the teeth took on a more circular shape resistant to breaking and able to deliver more punishing bites to struggling prey. But that wasn’t all. During their teenage growth spurt, between 11 and 20, the skulls of these tyrannosaur became deeper and better able to distribute the stresses involved with powerful bites.

Even though an 11-year-old Gorgosaurus was far from all-grown-up, it was still an impressive animal. At this age, Therrien says, these tyrannosaurs would have been over 18 feet long and over 1,500 pounds. That’s big enough to hunt young duckbilled dinosaurs, as well as some of the mid-sized prey in the same habitat like the “ostrich-mimic” dinosaurs called ornithomimids. From there, the tyrannosaur menu only kept expanding. Whereas a 10-year-old Gorgosaurus was a slasher than went after small prey and had only about 13 percent of an adult’s maximum bite force, another 10 years would make the same animal a deep-skulled carnivore able to bust up bones. In fact, Therrien notes, tyrannosaurs evolved to put more emphasis into devastating bites than other large dinosaur carnivores that retained large claws and arms for subduing prey. “My results show that the bite force of an adult T. rex was about 15 times that of an alligator, whereas that of the larger Giganotosaurus”—a non-tyrannosaur carnivore—“was only about 4 times that of an alligator.”

Paleontologists have found a similar pattern in T. rex itself. A study published in June in Paleontology and Evolutionary Science, carried out by University of Wisconsin Oshkosh paleontologist Joseph Peterson and colleagues, found that a roughly 13-year-old T. rex named Jane had a bite capable of piercing bone. The dinosaur probably wasn’t hunting down big duckbills on its own, but was just starting to get some of the more powerful adult abilities when the young tyrannosaur perished. This dovetails with Therrien’s research; he found that body size was the most important predictor of bite force among tyrannosaurs. A T. rex and a Gorgosaurus of the same size would have about the same bite force, indicating that getting bigger was a major strategy for these apex predators.

The pattern, Peterson notes, follows what naturalists have seen in living alligators and crocodiles. The youngsters aren’t just miniature versions of the adults, and they have different diets. “Hatchling and juvenile crocodylians have some different prey, but there is also overlap, and that’s true for juveniles to subadults, and subadults to adults,” Peterson says. Juvenile tyrannosaurs didn’t eat one type of food and switch over as they aged as much as they broadened what they could chomp on as they got bigger.

The fact that young tyrannosaurs had to grow into their bone-crunching abilities might have greater implications than the life history of these Cretaceous celebrities. Paleontologists have often wondered why medium-sized carnivores seem to be missing in places where large tyrannosaurs were common. Is it that we just haven’t found them yet, or is there something else going on?

It’s possible, Therrien notes, that extinctions of earlier carnivores allow tyrannosaurs to take over and settle in between 80 and 66 million years ago. The epochs prior to the rise of the giant tyrannosaurs are still little-known, and shifts may have occurred in that time period that favored the tyrant dinosaurs. It’s possible that the evolution of new large herbivores and the extinction of previously-dominant carnivores offered tyrannosaurs an evolutionary opportunity to live large says University of New Mexico paleontologist Kat Schroeder, who published a study in Science this February on large predatory dinosaurs. The Late Cretaceous saw the rise of horned and duckbilled dinosaurs while giant meat-eaters that previously ruled, like Allosaurus, disappeared, leaving a carnivorous gap tyrannosaurs could take advantage of. Even so, Schroeder notes, tyrannosaurs seem to have taken growth changes to the extreme and this might have allowed them to take over more ecological space once they became established as top carnivores.

A comparison to another famous time helps. During the Late Jurassic, about 150 million years ago, there lived an entire array of meat-eating dinosaurs from small to giant. From turkey-sized tyrannosaurs such as Stokesosaurus, to medium-sized hunters like Ceratosaurus, to true giants such as Allosaurus and Torvosaurus, a gradient of carnivorous species existed. But in the Hell Creek ecosystem T. rex roamed 68 to 66 million years ago, the picture is very different. There were small, carnivorous raptors such as Paronychodon, a rare mid-sized raptor, and then T. rex, with the tyrannosaur being just as common in the formation as its prey species Edmontosaurus.

The shift makes sense given that all dinosaurs, tyrannosaurs included, started life at relatively small size. These dinosaurs hatched out of eggs roughly the size of a large grapefruit and took years to mature. That means baby tyrants were in competition for food and space with other carnivore species, and those species seem to have made way for the tyrannosaurs.

The types of prey that were available may have played a role, too. In the Jurassic example, for instance, the most abundant herbivores were long-necked sauropod dinosaurs, some of which could grow to be over 100 feet long. In the Hell Creek ecosystem, by contrast, large herbivores were primarily duckbilled and horned dinosaurs—still big, but not quite as gigantic. There may be a connection here, as pointed out by University of Maryland paleontologist Thomas Holtz, Jr. in another tyrannosaur paper published in June. The evolution of predators is affected by the evolution of prey, of course, and so shifts in who was around to be eaten might have affected what predators prowled the particular landscape.

The various paleontological considerations are about more than just tyrannosaurs and their biology. Understanding how these dinosaurs lived is just one aspect of understanding the world they ambled around. “The more pieces of the puzzle we can tease out regarding dinosaur physiology and behavior,” Schroeder says, “the closer we will be to a comprehensive understanding of dinosaur ecosystems as a whole, which then opens the door to much broader questions concerning their evolution, dominance and eventual extinction.”

Riley Black | | READ MORE

Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.