Mystery of missing apex predators in Cretaceous North American and Asian ecosystems solved

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The main predator of Jurassic and Cretaceous landscapes was usually a species of carnivorous dinosaur. These predators walked on two legs, had powerful jaws lined with sharp teeth, and included species groups known as tyrannosaurs, spinosaurs, and carcharodontosaurs.

Thanatotheristes degrootorum. Image credit: Julius Scotonyi, Royal Tyrrell Museum.

tyrannosaurus rexthe goat-eating, jeep-hunting tyrannosaur from the movie Jurassic Park, was North America’s apex predator just before the dinosaurs went extinct at the end of the Cretaceous.

Although iconic, T. rex was just one of many species of large carnivorous dinosaurs that dominated various ecosystems at different times during the 130 million year reign of the dinosaurs.

During the Cretaceous period, most top predator species that evolved in North America and Asia were either carcharodontosaurs (shark-toothed dinosaurs) or tyrannosaurs (tyrant dinosaurs).

The early part of the Cretaceous was ruled by carcharodontosaurs, after which tyrannosaurs replaced them as the main predators until the end of the Cretaceous.

Two new species

Recently, two new species of these great Cretaceous predators have been discovered: a tyrannosaurus from Canada and a carcharodontosaur from Uzbekistan.

I was fortunate to participate in the study of both. These two findings, while unrelated, have interesting parallels.

In 2019, paleontologists Jared Voris and Kohei Tanaka visited museums to observe fossils held in collections. Voris went to the Royal Tyrrell Museum in Drumheller, Alberta, and Tanaka to the State Geological Museum in Uzbekistan.

Each found a fossil specimen that they thought had been important, albeit overlooked.

Both fossils had been found in Cretaceous rocks in their respective regions and had remained in museum collections for at least a decade without notice.

After several months of study, each of these fossils turned out to be an entirely new species of carnivorous dinosaur, hitherto 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 species of tyrannosaurus Thanatotheristes degrootorum, which means ‘reaper of death.’ The name is inspired by its predatory role in the 80 million year old ecosystem and the first discoverer of fossil bones, an Alberta rancher named John DeGroot.

On the other hand, we have named the species of carcharodontosaurus Ulughbegsaurus uzbekistanensis after Ulugh Beg, historical figure and first astronomer in Uzbekistan.

Life reconstruction of Ulughbegsaurus uzbekistanensis and a smaller tyrannosaur.  Image credit: Julius T. Csotonyi.

Reconstruction of the life of Ulughbegsaurus uzbekistanensis and a small tyrannosauroid. Image credit: Julius T. Csotonyi.

Main predators

Both species are only known for a few skull bones, with the rest of their skeletons completely unknown.

The most recognizable bones come from the jaws – the upper and lower jaw of Thanatotherists and the upper jaw of Ulughbegsaurus.

From the jaws, it was obvious that both species were respectable and similar in size.

We were able to determine their body size from these preserved bones.

Measuring from the tip of the snout to the tip of the tail, both species would have been about eight meters long, or the length of an average school bus.

In these two studies, we found that Thanatotherists and Ulughbegsaurus were each, by far, the largest predator in their ecosystems.

The previous absence of a large predatory species in either ecosystem was puzzling, as populations of large herbivorous dinosaurs would likely have increased unchecked, as in living herbivores.

Most other known predatory species in these ecosystems were small, typically less than three meters in length.

In fact, the ancient ecosystem of Uzbekistan was also home to a small species of tyrannosaur which was dwarfed by the large Ulughbegsaurus.

Rise and Fall of Top Predators

About 90 million years ago, all species of carcharodontosaurs became extinct – Ulughbegsaurus was among the last of its kind.

Their extinction left a void in North American and Asian ecosystems for new top predators to evolve and take over.

Tyrannosaurs, which for the most part were knee-high to a carcharodontosaurus for tens of millions of years before, finally played their part.

Somewhere between 90 and 80 million years ago, tyrannosaur species began to evolve to larger body sizes.

Thanatotherists was one of the earliest species of these large tyrannosaurs, living about 80 million years ago in Alberta’s prehistoric past.

Thanatotherists and its parents were among the ancestors that led to even larger tyrannosaur species, such as the 12 m long tyrannosaurus rex.

These large species then ruled the Cretaceous ecosystems of North America and Asia for the last 10 million years before the mass extinction that wiped out the dinosaurs.

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Jared T. Voris et al. A new tyrannosaurine (Theropoda:Tyrannosauridae) from the Campanian Foremost Formation of Alberta, Canada, provides insight into the evolution and biogeography of tyrannosaurids. Cretaceous research, posted on January 23, 2020; doi: 10.1016/j.cretres.2020.104388

Tanaka Kohei et al. 2021. A new carcharodontosaurian theropod dinosaur occupies an apex predator niche in the early Late Cretaceous of Uzbekistan. R. Soc. open science 8(9):210923; doi:10.1098/rsos.210923

Author: Darla K. ZelenitskyAssociate Professor, Dinosaur Paleobiology, University of Calgary.

This article was originally published on The conversation.

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