The Complicated Social Life of a Tyrannosaurus Rex
Fossil evidence has shifted our view of T. rex from a complete loner tyrant into a more nuanced carnivore.
Every time I’m in New York, I have to visit her. She’s always waiting in the same place, standing on the fourth floor of the American Museum of Natural History, grinning so wide that her teeth gleam under the bright lights. Experts know her as AMNH 5027. I know her as the first Tyrannosaurus rex I ever met.
Before the discovery of “Sue,” “Stan,” “Scotty,” and many other famous skeletons of Earth’s last great carnivorous dinosaur, AMNH 5027 was the icon. The partial skeleton was the first T. rex to include a complete skull – only slightly smushed. It left no doubt that the dinosaur was an impressive flesh-ripper, reconstructed with jaws agape when the museum’s paleontologists assembled real bones and casts in the world’s first great introduction to the dinosaur. (And, when Michael Crichton’s Jurassic Park needed a skeletal dinosaur for the cover, AMNH 5027 was the model.) I first encountered the dinosaur in 1988, when it was posed like Godzilla and I was not much larger than a Mesozoic mammal. I could practically see the drool hanging from those jaws.
Like me, AMNH 5027 has undergone some significant adjustments since we first met. Now the T. rex has its spine parallel to the ground and appears to be strolling, as if stalking along an Edmontosaurus scent trail. She’s less monstrous and seems at ease, calm despite all the bite-sized mammals running around her feet every day. Sometimes, when the hall is quiet and I can race up the stairs ahead of the school groups, I like to imagine what she would have sounded like: huffs and rumbles in the Cretaceous lowlands. So, when I decided to tell the life story of T. rex in my new book Tyrant Lizard Queen: The Love, Life, and Terror of Earth’s Greatest Carnivore, it was AMNH 5027 whose bones I wrapped in flesh to imagine the story’s protagonist, Snaggletooth.
Though Snaggletooth is a unique character, everything about her is framed by more than a century of research about the most famous dinosaur ever discovered. We know more about T. rex than perhaps any other dinosaur, and we’ve changed our view of her from a slow and menacing scavenger to a hot-blooded carnivore with a complicated social life.
When the first T. rex skeletons were pulled from the ground, people envisioned the reptiles as vicious loners. Discovered and excavated by Barnum Brown in 1908, AMNH 5027 was found alone. In those early days, it was easy to characterize T. rex as a regal recluse that ruled over its ancient landscape. Such dinosaurs, paleontologist William Diller Matthew wrote in 1915, “walked or ran upon the hind legs; in many of them the forelimbs are quite unfitted for support of the body and must have been used solely in fighting or tearing their prey.” The dinosaur existed solely to eat, and any meeting of tyrannosaurs was expected to be a clash.
Based on the abundance of T. rex fossils in rocks from 68 and 66 million years ago, and insight into the dinosaur’s ecological needs, paleontologists estimate that the total T. rex population at any one time was around 20,000 individual animals. That’s an impressive number of giant dinosaurs spread between what’s now southern Canada and Texas. Only recently have we begun to understand why T. rex met each other, and what happened when they did.
More than a century after T. rex was discovered, paleontologists have assembled a case that, yes, they mostly lived alone, but they also battled, successfully wooed each other, and raised hatchlings for over two million years, until an asteroid crashed the dinosaurian party. When paleontologists stopped thinking of T. rex as an ever-hungry monster, they could appreciate it as an animal with a complicated social life.
First, let’s look at the evidence that outlines their solitary life. Our best look at how T. rexes socialized doesn’t come directly from tooth and bone. It comes from tracks and traces. A series of three-toed footprints, tooth marks left on skeletons, and even tyrannosaur poop — all of these are examples of trace fossils. This diverse array of fossils marks what prehistoric animals left on their world. In other words, trace fossils record prehistoric behavior. A set of five T. rex tracks found in North Dakota record a leisurely four-mile-per-hour stroll for the carnivore.
The tracks that have been discovered do suggest that T. rex often walked alone. Scientists have found only a few T. rex footprints so far. The first, found in New Mexico, was described in 1994. It was a large, single footprint, one of only about five ever found. Only this year, more than 120 years after T. rex was named, did experts unveil a series of T. rex tracks. Along with the fact that most T. rex skeletons are single individuals, the sparse track record hints that the adult animals may have been like big cats, valuing their alone time. Earlier tyrannosaur species grouped together now and then, but no evidence of T. rex flocking has been found.
Perhaps a somewhat solitary life suited adult T. rex well. The dinosaurs were enormous. Some adults exceeded 40 feet in length and weighed more than 9 tons. Their punishing and pulverizing bites were easily capable of puncturing and breaking up bone – tattered muscle and bone shards found in T. rex scat underscoring just how thoroughly they could dismantle a carcass. Between small dinosaurs, juveniles of duckbills and horned dinosaurs, and scavenging on carcasses of herbivores too big to hunt, adult T. rex could feed themselves without needing a cooperative strategy. And with such a large belly to fill, perhaps tyrannosaurs were reluctant to share.
Damaged bones, another type of trace fossil, provide evidence of these carnivores’ social side by showing what happens when they met. Scrapes on T. rex bones convey the chilling fact that these dinosaurs had no qualms about feeding on each other. About half of all large tyrannosaur skulls bear healed puncture wounds. T. rex and related dinosaurs chomped each other on the face often enough that dozens of damaged skulls have been found so far.
The wounds' placement makes sense. T. rex and related dinosaurs evolved large heads threaded with powerful jaw-closing muscles. To keep their balance, generations of evolutionary change led to smaller arms held close to the body. So, when two T. rex came into conflict, perhaps after encountering the same rotting Triceratops, the dinosaurs’ maws were the weapons of choice for resolving the dispute. T. rex evolved a bite capable of crushing bone, after all, and so the dinosaur was just as vulnerable to the bite of its own species as little herbivores were. Healed bones indicate T. rex often survived these encounters, but swollen, infected tissue in their noses and jaws must have been incredibly painful.
Not every social interaction between two tyrannosaurs was hostile. Even as nests, eggs, and hatchlings of T. rex have proven even more elusive than the dinosaur’s footprints, the behaviors of other theropod dinosaurs, modern birds, and living crocodiles give me and some paleontologists every reason to think that T. rex tended their nests and could have watched over their offspring for a time, just as alligators do.
(Did tyrannosaurs live in groups? Experts discuss new fossil clues.)
When direct fossil evidence is hard to find—as with T. rex nesting behavior—paleontologists often rely on an approach called phylogenetic bracketing. Here's the logic: Birds are the only living dinosaurs left; alligators and crocodiles are the closest living non-avian relatives of dinosaurs as a group; and if a trait is shared in common between birds and crocodylians, it was likely present in many dinosaurs as well.
Bird parents of many species are attentive to their nests and hatchlings. Alligators and crocodiles do the same, chasing scavengers away from their nests and caring for their young for a year or more. Current evidence suggests T. rex was similar, with parent tyrannosaurs staying near their nests and offspring long enough for the little ones to gain enough strength to find their own food. To build those egg-filled nests in the first place, T. rex had to get along with each other at least long enough.
Whatever interactions T. rex had in their day-to-day lives, at least one scenario demanded social interaction. T. rex, like all dinosaurs, laid eggs. And those eggs were the result of courtship and mating that required at least some fleeting social graces from the tyrant dinosaurs.
Up until recently, paleontologists had no idea what dinosaur courtship and mating were like. The critical reveal came in 2016. Around 100 million years ago, in what’s now Colorado, large dinosaurs similar to T. rex gathered to scrape at the ground. The depressions look almost nest-like, perhaps suggesting the performer’s ability to make nests. It was a Cretaceous lek, a display arena where dinosaurs with sharp-clawed feet showed off. Scientists have found similar scrape sites in Cretaceous rocks in Canada and South Korea, hinting that this was a common way for theropods to show off.
(Can You Draw a T. Rex Correctly?)
So far, experts haven’t found such scrapes in the same rocks as T. rex. But now they know what to look for. The fact that both large and small carnivorous dinosaurs made these nest scrapes suggests they were a common form of dinosaur courtship. We can even see it today. Ground-nesting birds like puffins, shorebirds, and even parrots scratch false nests in the ground the same way dinosaurs did long ago. The scratches suggest that ancient dinosaurs strutted around and tried to impress each other, much as many birds do, and T. rex must have done so in some fashion, too.
We can’t say for sure whether T. rex puffed themselves up and scratched at the ground to make their potential mates feel a little more hot-blooded. But that’s how I like to imagine AMNH 5027 when I visit her. Flesh-ripping carnivory was part of her life. But so was sleeping, drinking water, watching the world go by, and, presumably, sharing affectionate moments with another T. rex every now and then. It’s easy to fixate on the rows of teeth. Those jaws also fed an entire animal that did much more than hunt her next meal. I can picture her there, my tyrant lizard queen, on a warm Cretaceous evening, part of a small gathering of rumbling, scratching reptiles preparing to make the Age of Dinosaurs last a little bit longer.
