Tyrannosaurus Rex: Scary. Smart. Social?

A new fossil site in Utah reopens an old debate about whether T. rex and its ancestors lived in complex social groups. 

By Eric Betz|Friday, April 12, 2019
RELATED TAGS: PALEONTOLOGY
Teratophoneus1
Teratophoneus1

Before Tyrannosaurus rex, numerous other tyrannosaurs called North America home, including Teratophoneus curriei (shown). A recently found group of fossils may belong to the species.

Illustration by Mark Witton

A truck barrels down a dusty desert road in southern Utah’s vast Grand Staircase-Escalante National Monument, speeding through canyons and along cliffs toward a towering sandstone formation known as the Kaiparowits Plateau. Alan Titus, a Bureau of Land Management paleontologist here for nearly 20 years, is behind the wheel, his shaggy hair flapping in the July wind as Led Zeppelin blasts from the speakers.

Titus stops the truck on a shrubby hill. With his passenger, University of North Florida paleontologist Barry Albright, he sets off on a small foot path into the high desert.

The two men, longtime friends and collaborators, are heading to the scene of a catastrophe — and, possibly, the biggest find of Titus’ decades-long career.

In 2014, Titus was prospecting for dinosaurs when he kicked over a bit of bone beside a scraggly old juniper tree. It was a piece of skull from a tyrannosaur — animals that include Tyrannosaurus rex and dozens of related and ancestral species. Since that chance find, Titus and his team have recovered remains from four tyrannosaurs — an adult, a teenager and two juveniles — from a site about 1,000 square feet in size. Titus thinks these tyrannosaurs died together some 76 million years ago during the Cretaceous Period. If he’s right, it could confirm a controversial idea about one of the most iconic dinosaurs that ever lived.

Tyrannosaurs have been found from the Arctic Circle to the subtropics. And for decades, paleontologists have debated whether these top predators were social creatures. But fossil evidence has been hard to find. “That’s why this is exciting,” Titus says. He believes that “these animals are the direct ancestors of Tyrannosaurus rex living in a complex social group.”

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Paleontologist Alan Titus surveys the vast landscape of southern Utah from the Kaiparowits Plateau, part of the Grand Staircase-Escalante National Monument. The area is rich in fossil remains from the Late Cretaceous Period — including what may be a family group of tyrannosaurs.

Jake Bacon

But not everyone is convinced. “It’s hard to tell just from bones,” says Albright, who helped work the site. He pauses and adds, “Their brains are too small to hunt in packs.”

Titus isn’t dissuaded. Even the site’s nickname is a nod to his enthusiastic optimism. Team members call it “Rainbows and Unicorns.”

Pop culture usually portrays tyrannosaurs as loners ravaging the landscape (remember Jurassic Park?), not animals with complex social relationships. But a handful of recent discoveries may be prompting a revision. In 2014, for example, paleontologists announced a trio of tracks in British Columbia from adult tyrannosaurs, possibly Albertosaurus, walking together in the mud. The following year, a study in the journal PeerJ described fossil evidence that suggested tyrannosaurs often bit at each other’s faces, likely during competition. Titus and some of his colleagues believe that these finds suggest the predators may have moved in packs, jockeyed for social hierarchy and perhaps even cared for their young through adolescence.

Many modern predators exhibit these behaviors, but the long-standing dinosaur stereotype is that their brains just weren’t big enough for such sophisticated relationships.

“A lot of that is simple bias against reptiles,” says University of Maryland paleontologist and tyrannosaur expert Thomas Holtz Jr. “Crocodilians engage in play, deception and problem solving, things that people once thought they didn’t have the brains to do.” And because tyrannosaurs were brainier for their body size than any crocodile, it’s reasonable to expect they’d be capable of even more complex behaviors, Holtz says. It’s possible, in fact, that tyrannosaur brains explain their rise to dominance in the first place.

TreeofTyranny
TreeofTyranny

The Age of Tyrannosaurs 
The first animals to be considered tyrannosaurs show up in the fossil record 170 million years ago. But if you were placing bets on a future superpredator, you wouldn’t have put your money on them. Proceratosaurus, shorter than a human and not particularly terrifying, was discovered in England a century ago but only identified as a tyrannosaur in 2010.

“The reason [tyrannosaurs] took over as the dominant predator is still a question,” says Joseph Sertich of the Denver Museum of Nature & Science, who worked with Titus on Rainbows and Unicorns. “They seemed to be in the right place at the right time.”

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Alan Titus examines a fossilized partial turtle shell, which provides clues to the Cretaceous ecosystem.

Jake Bacon

That right time began about 100 million years ago, when a period of intense volcanic activity caused a mini-mass extinction in the Northern Hemisphere, killing many of the larger dinosaurs, including allosaurs, the apex predators of their day.

There’s a gap in the tyrannosaur fossil record of many million of years, but it appears that, after this event, tyrannosaurs began their climb to the top of the food chain — and, some researchers speculate, their smarts may have helped them do it.

In 2016, paleontologists described a well-preserved skull from a tyrannosaur that lived in Uzbekistan 90 million years ago. The fossils suggested that this horse-sized tyrannosaur, Timurlengia euotica, had keen senses. Its large inner ears would have allowed it to hear low-frequency sounds, a common trait in predators that hunt larger animals over long distances. The researchers believe it’s evidence that the tyrannosaur brain allowed it to become an apex predator long before the animals reached their peak in size: By the time an asteroid wiped out the non-avian dinosaurs 66 million years ago, an adult T. rex could weigh 10 tons and stretch some 40 feet from nose to tail, one of the largest predators ever to walk the Earth.

MappingMesozoic
MappingMesozoic
Mapping the Mesozoic
During the Mesozoic Era (252 million to 66 million years ago), seaways intermittently flooded southern Utah (far right). The region’s rock layers preserve fossils from throughout the Mesozoic and earlier Permian Period.

Pack Mentality 
Through much of the Cretaceous, a vast body of water called the Western Interior Seaway divided North America into two distinct landmasses. In the west, a long, thin strip of a continent named Laramidia extended from Alaska toward Central America. And at that time, the region of Utah now encompassing Grand Staircase-Escalante National Monument was sandwiched between the seaway and a mountain range comparable to the modern Andes.

The subtropical region received more than 60 inches of rain during an annual wet season. Rivers meandered past redwoods and palms. Enormous herds of plant-eating dinosaurs grazed along the muddy banks, including duck-billed hadrosaurs, horned ceratopsians and the tanklike armored ankylosaurs.

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Titus's team found these four toe bones from what they suspect are four tyrannosaurs of different sizes. 

Jake Bacon

Ecosystems back then were not all that different than ecosystems today,” says University of Alberta paleontologist Phil Currie, who has studied tyrannosaur bone beds in western North America and Asia. Thousands of fossilized duck-billed hadrosaur footprints were found in Alaska’s Denali National Park & Preserve in 2014, showing the animals moved like elephants in large herds of both young and old. And by analyzing other herbivores’ teeth for elements that can provide a record of location, scientists say they found evidence that some dinosaurs migrated seasonally.

These plant-eater herds would have been well protected against a lone predator, Currie says. But a group of tyrannosaurs could break up the herd to take on a single large animal.

“In today’s ecosystems, where you have big groups of herbivores, you have big groups of carnivores,” Currie says. We shouldn’t expect animals to behave so differently back then, he says.

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Titus shows off an adult tyrannosaur tooth. 

Jake Bacon

In 1996, Currie was at the American Museum of Natural History in New York, rummaging through the collections of Barnum Brown, the famed dinosaur hunter who discovered the first T. rex skeleton back in 1902. When Currie opened a cabinet drawer, he found the left foot of the smaller tyrannosaur Albertosaurus, which Brown had brought back from a field expedition in 1910.

“I opened the next drawer, and there’s another foot,” Currie says. By the time his team finished going through the cabinets, they had found fossils from nine individuals. Currie’s luck wasn’t up yet. The archives had pictures of a dig site on a distinctive ridge at Dry Island Buffalo Jump Provincial Park in Alberta, Canada. He set off to find it the following year.

“It ended up being one of the hottest days of the year, so when I found the ridge, I was at heat exhaustion,” Currie says. “I didn’t think of looking around my feet. The next day we came back, and there was Albertosaurusbone everywhere.”

Back in 1910, Brown worked only a 65-by-65-foot section. The larger site that Currie’s team excavated held bones belonging to possibly 20 or more tyrannosaurs killed in a single event some 70 million years ago. Growth rings on the bones show the animals’ ages ranged from 2 to 24 years old — possibly parents and offspring.

How young tyrannosaurs made their way in the world has long been a hotbed of speculation. At 2 years old, an infant T. rex, for example, was the size of a golden retriever, with teeth not much bigger. Some paleontologists believe sexual maturity came around 18 years old. Along the way, their proportions morphed from sleek-bodied, narrow-skulled youth into school bus-sized adults with broad skulls and teeth as big as bananas.

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Lab manager Katje Knoll prepares a large bone found at the Rainbows and Unicorns site.

Jake Bacon

Based on the size range of animals at the Dry Island site, Currie wonders if the younger, faster tyrannosaurs might have run into herds, flushing out prey for the adults, suggesting a division of labor within a hunting pack.

“As a scientist, you’ve got to be cautious,” Carthage College paleontologist Thomas Carr says of the Albertosaurus mass grave and some of the speculation about it. Other animals, such as Komodo dragons, for example, gather in groups to feed but lack complex social relationships. “We’ve just got to wait to see if scientists find more bone beds [like it] to see if that holds up. It’s just one site.”

Back in Utah, Rainbows and Unicorns could back up Currie’s Dry Island hypothesis. But the finds from the Kaiparowits Plateau have yet to be published.

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At his BLM lab in Kanab, Utah, Titus examines an adult tyrannosaur skull that he and a volunteer are painstakingly working to extract from the rock.

Jake Bacon

Thousands of Puzzle Pieces 
At Titus’ lab in Kanab, Utah, in a room the size of a large garage, bones are scattered across every flat surface. Half a dozen volunteers — mostly local retirees — huddle over workstations, using compressed air scribes that look like tattoo guns to pick minuscule chunks of rock from tyrannosaur fossils in plaster jackets. It’s tedious work.

Studying the site has proven to be a monumental task, in part because the rock is exceptionally hard. The fossils themselves are mostly what Titus calls “kibbles and bits,” thousands of jumbled pieces from dozens of different puzzles. Each fragment must be cataloged, studied and, if possible, reassembled.

“Rainbows and Unicorns is a very unusual bone bed,” says Sertich, the Denver Museum of Nature & Science paleontologist. “I’ve never seen anything even remotely like it. It’s like a spaghetti noodle cluster of bones.”

How these creatures died, however, may prove to be less of a puzzle. The tyrannosaur remains were found near each other, but the larger dig site also holds half a dozen 3-foot-plus turtle shells, along with remnants of giant fish called gars, parts of a 20-foot crocodile and other bones. Other than the tyrannosaurs, the other fossils are mostly aquatic creatures.

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BLM Lab volunteer Deb Shanahan uses a compressed air tool called an air scribe to reveal an infant tyrannosaur skull. 

Jake Bacon

That’s because the site was once a roughly mile-wide lake. Titus believes the tyrannosaurs drowned there together, possibly washed in by a flood. The lake eventually dried up in a subsequent drought, killing off the fish and other aquatic animals.

In a corner of the lab, volunteer Deb Shanahan wears a face mask and lab coat as she chisels away at an infant tyrannosaur skull encased in rock. She started on the skull back in September 2016. With each new section of fossil exposed, Shanahan becomes the first to see an animal that’s been locked up in rock for 76 million years.

On a large table nearby, Titus and another volunteer are revealing the menacing grimace of an adult tyrannosaur’s massive skull and teeth. The team suspects their dinosaurs could belong to a species called Teratophoneus curriei, or “monstrous murderer,” named after Currie. The species lived in this region but is known only from a partial specimen. Having four more individuals could reveal details about the species’ development from infancy to adulthood.

However, unlike T. curriei and other local tyrannosaurs, all four of the new specimens have a flat lacrimal bone, which sits above the eye socket. That could indicate it’s a new species altogether. The team won’t know for some time. They’ve been chipping away material from the adult’s giant skull for more than two years, but months of work remain.

Titus isn’t in a rush. “I’ll only get one shot in my career to work on something this cool,” he says. “It’s a disservice to my co-authors and to science to not take my time.”

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