‘Reaper of Death’ tyrannosaur discovered in Canada

The scar-faced dinosaur illuminates how T. rex and its relatives became top predators.

Monday, February 10, 2020,
By Maya Wei-Haas, Michael Greshko
To the untrained eye, many tyrannosaurs look like twins. But Thanatotheristes has several distinctive features that ...
To the untrained eye, many tyrannosaurs look like twins. But Thanatotheristes has several distinctive features that set it apart from the pack, including a set of prominent ridges that run down its snout.
Photograph by Illustration by Julius Csotonyi

Jared Voris is no stranger to death. By early 2018, the University of Calgary master’s student had spent more than a year poring over bones in museum collections, studying how tyrannosaurs matured from hatchlings into hulking terrors. During one visit to the collections of Alberta’s Royal Tyrrell Museum of Palaeontology, he noticed a cabinet with fossils he couldn’t quite place.

Now, after two years of careful research, Voris and his colleagues have identified the first new Canadian tyrannosaurid to be found in 50 years. Stretching 26 feet in length, the dinosaur is named Thanatotheristes, Greek for “reaper of death.”

Aged roughly 79.5 million years, Thanatotheristes degrootorum sits near the base of the tyrannosaurs’ ascent to ecological domination. The unearthed skull fragments—including upper and lower jawbones, teeth, and a partial cheekbone—sketch out the early pages of how tyrannosaurids, the tyrannosaur subgroup that includes T. rex, rose to power and became top predators.

“I tried to be really meticulous with identifying features that made it unique,” says Voris, who is now a Ph.D. student at the University of Calgary. “It’s interesting to have the opportunity to name a new species—and I’m hoping it isn’t all downhill from here.”

For now, skull fragments are all that remains of Thanatotheristes. While it is possible more was once preserved in the blue-grey cliffs that tower near its discovery site, recent flooding likely swept these pieces away.
Photograph by Image by Jared Voris

When tyrannosaurs first arose some 165 million years ago, they weren’t the tyrants that eventually reigned over the Cretaceous period in Asia and North America. Some were downright tiny—no bigger than five feet tall—and hunted in the shadows of the age’s more massive carnivores, including the bus-size allosauroids and the large-clawed megalosauroids.

About 80 million years ago, these other predators faded away, giving tyrannosaurs a chance to rise to the top of the food chain and grow into giants. By 66 million years ago right before its extinction, the infamous T. rex grew up to 40 feet long and weighed more than nine tonnes. But Thanatotheristes, unveiled in Cretaceous Research on January 23, doesn’t seem to have been as large or as hulking as T. rex, underscoring the diversity at the top of this period’s food chain. (Learn more about the world’s biggest T. rex yet found.)

“It seems like tyrannosaurs had a dynamic evolutionary history,” University of Edinburgh palaeontologist Steve Brusatte, who wasn’t involved with the study, says in an email. “They weren't all monstrous superpredators like T. rex, but there were many little subgroups that had their own domains and their own distinctive body types.”

Hunting the reaper

Tyrannosaurs were probably rare in life and even rarer as fossils. Regardless, filling in the group’s evolutionary picture from bony remains is a challenging task. Their plant-eating peers evolved a striking variety of large neck frills and head crests that helped the animals spot their species, rivals, and potential mates. But tyrannosaurs lacked these billboards.

It’s hard to tell when new species arrive in the fossil record, says College of Charleston palaeontologist Scott Persons, who was not part of the study team. “You get into this real nitty-gritty. You have to really fine-tune your taxonomic observations.”

Every scrap of tyrannosaur bone holds vital clues—even those found by chance, such as the Thanatotheristes remains. John and Sandra De Groot stumbled on the bones in 2010, as their family walked along the shoreline of southern Alberta’s Bow River. The pair contacted the Royal Tyrrell Museum, which sent palaeontologists to collect the fossils and search for more. To honour the family, Voris’ team gave Thanatotheristes the species name degrootorum.

“They’ve been an invaluable resource,” Voris says of the De Groots. “It just shows, you don’t have to be a palaeontologist to help out in palaeontology.”

Nearly a decade after the fossils were cleaned, catalogued, and stored, Voris and his colleagues began putting the palaeo-puzzle together. The team focused on the jawbones, which had uniquely prominent ridges that hinted at long-lost facial structures. The animal’s cheekbone also had an oval shape in cross-section, unlike other closely related tyrannosaurids.

Yet in other ways, Thanatotheristes was similar to its relatives, which by all accounts weren’t friendly creatures. Tyrannosaurs’ snouts are often crisscrossed with the marks of long-ago scuffles with other dinosaurs—including other tyrannosaurs. Thanatotheristes is no exception. A whitish scar, stretching four inches long, snakes along its right upper jaw. “It’s a Scarface,” Persons says.

Tyrannosaur territory

The fossil provides a deeper look into the diversity of North America’s tyrannosaurs, many of which lived and died along the western shorelines of a mighty inland ocean that stretched from the Arctic Ocean to the Gulf of Mexico.

Thanatotheristes hails from an understudied rock formation, and at 79.5 million years old, the fossils provide clues into how early tyrannosaurids evolved into giants. “It’s the oldest known tyrannosaurid from northerly North America,” says study co-author Darla Zelenitsky, a palaeontologist at the University of Calgary and Voris’ doctoral adviser.

With the addition of Thanatotheristes, tyrannosaurs from the western U.S. and Canada appear to form two distinct lineages: a northern group with long, deep snouts, and a southern group with shorter, bulldog-like snouts. Perhaps this division reflects two distinct feeding strategies, each shaped by the region’s varying prey and environment.

While it is unclear what’s driving this pattern in North America, one possibility is the size of their prey. The large-bodied Asian tyrannosaur group—that eventually yielded T. rex—lived alongside huge plant-eating dinosaurs, including the long-necked sauropods. It is possible that these tyrannosaurs reached momentous sizes to more effectively take down such titanic prey.

There is much left to learn about this tantalising pattern. Many gaps also linger in the tyrannosaur fossil record—especially for Thanatotheristes. Aside from one small jaw fragment found at another Albertan site in 2018, the Bow River bones are the only known fossils of this predator. Return expeditions to the site where the De Groots spotted Thanatotheristes didn’t turn up any more bones, and recent flooding may have washed away whatever else remained.

If Voris has his way, more specimens will join the ranks. He has plans of exploring the same rock formation in other parts of southern Alberta, in hopes of finding additional Thanatotheristes bones. Voris doesn’t fear the reaper. If anything, he loves it.

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