Tyrannosaurs dominated their Cretaceous ecosystems | Science

tyrannosaurus rex may be the most perfect dinosaur name ever invented. What else would you call a carnivorous, bipedal reptile that could grow to over 40 feet in length and weigh over nine tons? The size and apparent ferocity of T. rex was obvious from the start, but, paleontologists have learned, this Cretaceous carnivore and its relatives truly had a tyrannical hold over the ecosystems in which they lived. The differences between adult and adolescent tyrannosaurs were so great that the animals lived almost like different species, pushing the mid-sized carnivores back into a prehistoric takeover. This is among the key findings of a study published earlier this year in the Canadian Journal of Earth Sciencesin which François Therrien, a researcher at the Royal Tyrrell Museum of Paleontology, and his colleagues discovered that young tyrannosaurs behaved and bit their prey differently from adults.

The idea that T. rex and related dinosaurs such as Gorgosaurus had an outsized influence on the habitats they stalked comes not from a single discovery, but from years of chance finds of fossils, analyzes of dinosaur anatomy, and putting those clues into place. an ecological context. A host of new papers have come to the same conclusion – the way large tyrannosaurs grew allowed them to dramatically influence their ecosystems in ways no other predator had before. Young, small, and slender tyrannosaurs had different predation abilities than adults and pursued smaller meals. It was not until adolescence, during a spectacular growth spurt, that these dinosaurs took a liking to big game. This dinosaurian chance allowed the tyrannosaurs to ward off other carnivores, creating unprecedented ecosystems dominated by a single large predator.

Even though tyrannosaurs have a very long history, with the first of their legendary family evolving around 170 million years ago, these carnivores remained small for tens of millions of years. With few exceptions, it wasn’t until about 80 million years ago that a subset of tyrannosaurs – the tyrannosaurids – began to grow to giant sizes, their success guaranteed by terrible crushing bites that made it possible to both hunt and dismember carrion with ease. And it was 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 would encounter big, lanky tyrannosaurs like Albertosaurus and Gorgosaurus. These dinosaurs were the main predators of their ecosystems, just like their most famous relative T. rex. Although juvenile specimens of the large T. rex are rare, note Therrien and his co-authors, experts have discovered several juveniles of Albertosaurus and Gorgosaurus. This has allowed paleontologists to piece together a more complete picture of how these tyrants grew up. In the case of the new study, Therrien and his co-authors found that during their first decade of life Albertosaurus and Gorgosaurus had relatively weak bites and fine blade-like teeth. Around age 11, however, when their lower jaws were about 23 inches long, the teeth that constantly replaced each other in the dinosaurs’ jaws began to change. Instead of being thin and blade-like, the teeth took on a more circular shape that was resistant to breaking and capable of delivering more punishing bites to struggling prey. But that was not all. During their teenage growth spurt, between the ages of 11 and 20, the skulls of these tyrannosaurs grew deeper and better able to distribute the stresses of powerful bites.

Even if an 11 year old child Gorgosaurus was far from being an adult, it was still an impressive animal. At that age, Therrien says, these tyrannosaurs would have measured over 18 feet long and over 1,500 pounds. This is large enough to hunt young duck-billed dinosaurs, as well as some of the medium-sized prey in the same habitat, such as “ostrich-like” dinosaurs called ornithomimids. From there, the tyrannosaur menu kept expanding. While a 10 year old child Gorgosaurus was a slasher that attacked small prey and had only about 13% of the maximum bite force of an adult, 10 more years would make the same animal a deep-skulled carnivore capable of breaking bones. In fact, notes Therrien, tyrannosaurs evolved to place more emphasis on devastating bites than other large carnivorous dinosaurs that retained large claws and arms to subdue their prey. “My results show that the bite force of an adult T. rex was about 15 times that of an alligator, while that of the largest Giganotosaurus– a non-tyrannosaur carnivore – “was only about 4 times that of an alligator”.

Paleontologists have found a similar pattern in T. rex himself. A study published in June in Paleontology and evolutionary sciencecarried out by paleontologist Joseph Peterson of the University of Wisconsin Oshkosh and his colleagues, discovered that a teenager of about 13 years T. rex named Jane had a bite capable of piercing through bone. The dinosaur probably wasn’t hunting large duckbills on its own, but was just beginning to acquire some of the most powerful adult abilities when the young tyrannosaur perished. This agrees with Therrien’s research; he found that body size was the strongest predictor of bite force in tyrannosaurs. A T. rex and one Gorgosaurus of the same size would have roughly the same bite force, indicating that getting bigger was a major strategy for these apex predators.

The model, Peterson notes, follows what naturalists have seen in living alligators and crocodiles. Juveniles are not just miniature versions of adults, and they have different diets. “Newborns and juvenile crocodilians have different prey, but there is also overlap, and this is true from juveniles to subadults and from subadults to adults,” says Peterson. Juvenile tyrannosaurs did not eat one type of food and changed their food as they aged as much as they expanded what they could munch on as they grew.

The fact that young tyrannosaurs must have developed their bone-crunching abilities might have bigger implications than the life story of these Cretaceous celebrities. Paleontologists have often wondered why medium-sized carnivores seem to be lacking in places where large tyrannosaurs were common. Haven’t we found them yet, or is there something else going on?

It’s possible, Therrien notes, that earlier carnivore extinctions allowed tyrannosaurs to take over and settle between 80 and 66 million years ago. The times before the rise of the giant tyrannosaurs are still little known, and changes may have occurred during this period that favored the tyrant dinosaurs. According to 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 the horned and duck-billed dinosaurs while the giant carnivores that ruled before such as Allosaurus, disappeared, leaving a carnivorous void that tyrannosaurs could take advantage of. Even so, notes Schroeder, tyrannosaurs seem to have taken growth changes to an extreme and this might have allowed them to conquer more ecological space once they established themselves as the top carnivores.

A comparison with another famous era helps. In the Upper Jurassic, about 150 million years ago, lived a whole range of carnivorous dinosaurs, from small to giant. Turkey-sized tyrannosaurs such as Stokesosaurusmedium-sized hunters like Ceratosaurusto real giants like Allosaurus and Torvosaurus, a gradient of carnivorous species existed. But in the Hell Creek ecosystem T. rex traveled 68 to 66 million years ago, the picture is very different. There were small carnivorous raptors such as paronychodona rare medium-sized raptor, then T. rexthe tyrannosaurus being just as common in formation as its prey edmontosaurus.

This change makes sense given that all dinosaurs, including tyrannosaurs, started life at a relatively small size. These dinosaurs hatched from eggs about the size of a large grapefruit and took years to mature. This means that baby tyrants competed for food and space with other species of carnivores, and these species seem to have given way to tyrannosaurs.

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

The various paleontological considerations are not limited to tyrannosaurs and their biology. Understanding how these dinosaurs lived is only one aspect of understanding the world in which they roamed. “The more pieces of the puzzle we can unravel regarding dinosaur physiology and behavior,” says Schroeder, “the closer we will get to a comprehensive understanding of dinosaur ecosystems as a whole, which will then open the door to questions that are much about their evolution, dominance and eventual extinction.