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School bus-size 'Reaper of death' was an apex predator. Here's why that matters.

This artistic representation shows a Tyrannosaurus rex hunting Gallimimus dinosaurs during the Cretaceous period. (Image credit: Science Photo Library - LEONELLO CALVETTI/Getty Images)

The top predator of the Jurassic and Cretaceous landscapes was usually a species of meat-eating dinosaur (opens in new tab). These predators walked on two legs, had powerful jaws lined with sharp teeth and included species from groups known as tyrannosaursspinosaurs and carcharodontosaurs.

Tyrannosaurus rex, the goat-eating, jeep-chasing tyrannosaur from the movie Jurassic Park, was the apex predator of North America just before dinosaurs went extinct at the end of the Cretaceous Period (opens in new tab). Although iconic, T. rex was only one species of many large, meat-eating dinosaurs that dominated various ecosystems at different times over the 130 million years of dinosaur reign.

During the Cretaceous Period, most species of top predator that evolved in North America and Asia were either carcharodontosaurs (shark-toothed dinosaurs) or tyrannosaurs (tyrant dinosaurs). The earlier part of the Cretaceous was ruled by carcharodontosaurs, after which tyrannosaurs replaced them as the top predators until the end of the Cretaceous.

New species

Recently two new species of these large Cretaceous predators were discovered — a tyrannosaur from Canada (opens in new tab) and a carcharodontosaur from Uzbekistan (opens in new tab). I was lucky enough to be involved in the study of both. These two discoveries, although unrelated, have some interesting parallels.

In 2019, paleontologists Jared Voris and Kohei Tanaka — both who had trained in my lab at the University of Calgary (opens in new tab) — visited museums to look at fossils housed in collections. Voris went to the Royal Tyrrell Museum in Drumheller, Alta., and Tanaka to the State Geological Museum in Uzbekistan.

Each found a fossil specimen they thought may have been important, although overlooked. Both fossils had been found in Cretaceous age rocks of their respective region, and had sat in the museum collections for at least a decade without much notice.

After many months of study, each of these fossils turned out to be an entirely new species of meat-eating dinosaur, previously unknown to science. This meant that we would need to formally describe them, and each would be given its own species name.

We named the new tyrannosaur species Thanatotheristes degrootorum, which means “reaper of death.” The name draws inspiration from its predatory role in the 80-million-year-old ecosystem and for the first discoverer of the fossil bones, an Alberta rancher called John DeGroot.

On the other hand, we named the carcharodontosaur species Ulughbegsaurus uzbekistanensis after Ulugh Beg (opens in new tab), a historical figure and early astronomer in Uzbekistan.

An illustration of the enormous carcharodontosaur Ulughbegsaurus with the smaller tyrannosaur Timurlengia. (Image credit: Julius Csotonyi)

Top predators

The two species are known from only a few skull bones, with the remainder of their skeletons completely unknown. The most recognizable bones are from the jaws — the upper and lower jaw of Thanatotheristes and the upper jaw of Ulughbegsaurus.

From the jaws, it was apparent both species were a respectable and similar size. We were able to figure out their body size from these preserved bones. Measuring from the tip of the snout to the end of the tail, both species would have been around eight metres long — the length of the average school bus.

In these two studies, we discovered that Thanatotheristes and Ulughbegsaurus were each, by far, the largest predator of their ecosystems. The previous absence of a large predatory species in either ecosystem before was puzzling, as populations of large plant-eating dinosaurs would likely have grown unchecked, as in living herbivores (opens in new tab).

An illustration of Thanatotheristes degrootorum, an apex predator from what is now Canada. (Image credit: Julius Csotonyi)

Most other known predatory species from these ecosystems were small, typically less than three metres long. In fact, the older Uzbekistan ecosystem was also home to a small tyrannosaur species (opens in new tab) that was dwarfed by the large Ulughbegsaurus (opens in new tab).

The rise and demise of top predators

Around 90 million years ago, all carcharodontosaur species went extinct (opens in new tab) – Ulughbegsaurus was among the last of its kind. Their extinction left a vacancy in North American and Asian ecosystems for new, large predators to evolve and take over. The tyrannosaurs, which for the most part, were knee-high to a carcharodontosaur for tens of millions of years prior, finally made their play.

Somewhere between 90 and 80 million years ago, tyrannosaur species began to evolve towards a larger body size (opens in new tab)Thanatotheristes was one of the earliest species of these large tyrannosaurs, living around 80 million years ago in Alberta’s prehistoric past.

Thanatotheristes and its kin were among the ancestors that led to even larger tyrannosaur species, like the 12 metre long Tyrannosaurus rex. These large species went on to rule Cretaceous ecosystems of North America and Asia for the last 10 million years before the mass extinction event that wiped out the dinosaurs.

This article is republished from The Conversation (opens in new tab) under a Creative Commons license. Read the original article (opens in new tab).

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Darla K. Zelenitsky
Darla K. Zelenitsky

Darla is an associate professor of dinosaur paleobiology at the University of Calgary in Canada. Darla’s research is primarily in the area of dinosaur paleobiology, with particular interest in theropod dinosaurs as well as the evolutionary relationship between dinosaurs and birds. Darla was a primary researcher describing the first dinosaurs preserved with feathers from the Americas, found in a species of ostrich-mimic dinosaur from Alberta, Canada, and has published a number of scientific papers related to eggs, babies and nests of dinosaurs in order to help understand behavioral and other biological implications of these fossils.