Gangs of Feathered T-Rex


Packs of tyrannosaurs
Imagine, if you will, an early morning scene in the late Cretaceous. The air is quiet as the day warms. At the edge of a large forest a plain of ferns ripples in the light breeze (grass would not evolve for another 20 or so million years). Under the canopy of the ancient beeches and maples, there is movement. Nothing fast, just a hint. A flash of mottled colour against the background. A glint of light off an eye. A soft snuffling. A feather falls silently to the forest floor.

Among the nearby ferns, a pack of ceratopsians grazes, adults watching the woods carefully, nervously, herding the young towards the safety of the pack’s centre with prods of their heads, honking to get the young ones’ attention. They eat the flowers that dot the plain among the ferns and cycads, chew the horsetails that grow at the edge of the ponds and streams. A youngster sees a tasty patch of moss and, unnoticed, slips out between the elders to get it.

Suddenly the forest explodes. Leaves scatter and branches snap as the muscular forms crash through the cover and converge on the young triceratops, separated from the horned protection of the pack. Two large adults, a teenager and two younger tyrannosaurs running a well-coordinated hunt as they have done many times int he past. Their speed makes them a blur against the trees.

The ceratopsians bellow in fear and rage, and quickly form a circle, heads out, protecting the oldest and youngest within the centre. The pack of tyrannosaurs’ charge sounds like thunder, and they screech in anticipation as they race to surround the doomed youngster. They circle rapidly, darting to avoid the feeble attempts at defence from the surrounded dinosaur.

The herd can’t save it, and they move away, quickly, the outer ring still shuffling backwards to keep their ferocious, horned heads facing the danger. The tyrannosaur pack ignores the herd as it feeds, tearing off chunks of the living flesh as the youngster’s screams get fainter.

Their hunger slaked for the moment, the pack would soon retire to the forest to look for another easy target that might venture close by.

Triceratops vs tyrannosaur

Tyrannosaurs have long captured public imagination, even before Jurassic Park showed them as monstrously big, agile and canny killing machines (with a penchant for eating lawyers as hors d’oeuvres).

Tyrannosaurus Rex – King of the tyrant lizards, his name means, the largest of his family of theropod dinosaurs – was just one of a family of predators – the tyrannosauridae – that included (in size from larger to smaller) TarbosaurusAlbertosaurus, Daspletosaurus and Gorgosaurus.
Big dinosaurs
There were other theropod carnivores like the tyrannosaurs during the Mesozoic, some of them competitors, others related but distant ancestors, still others similar in shape and form but merely a case of parallel evolution: the Torvosaurs (including European giant, Torvosaurus gurneyi) and Allosaurs of the late Jurassic, the Carcharodontosaurids, Gigantosaurus (the largest of the carcharodontosaurids), and the largest of them all: Spinosaurus – also made famous by Steven Spielberg in the fights scene with the tyrannosaur in Jurassic Park 3 – questionable since Spinosaur’s head and teeth suggest its main diet was fish – not to mention it wouldn’t have happened historically because Spinosaurus lived 35 million years before tyrannosaurus).

But none have been as famous as T. Rex.

Paleontologists have argued about their behaviour of the tyrannosaurids for over a century; their lifestyle, their parenting, speed, senses and their diet. Some pictured them as lumbering, slow, carrion eaters – cold-blooded, giant scavengers, a sedentary lifestyle. Others – among them the famously controversial Robert Bakker – have argued for active, aggressive, smart, and hot-blooded.

The news out of BC this week suggests Bakker was right. The Globe & Mail reported tracks of several T. rex moving together as a pack:

…the footprints of other dinosaur species can be seen in the same formation. But while those tracks are oriented in random directions, the tyrannosaur prints run parallel to each other. The unlikelihood that the tracks of three similar predators, rare to being with, would point in the same direction by chance led the team to conclude the animals were together.

The find raises the hair-raising possibility that tyrannosaurs were pack hunters, working together to bring down large herbivores of similar or greater size.

This has earned  them the collective noun: a terror of tyrannosaurs. Bakker thinks this find shows the group was a family, related by genes like a pride of lions:

Robert Bakker, a paleontologist with the Houston Museum of Natural Science who specializes in dinosaur behaviour, said that if the tyrannosaurs were fellow travellers then there’s good reason to think they were part of a family group, not unlike a pride of lions on the Serengeti.

“Top predators are the most quarrelsome cannibalistic category in the ecosystem,” Dr. Bakker said. “It’s impossible that these [tyrannosaurs] would clump together in a common cause unless they were sharing genes.”

The find is consistent with other evidence that some tyrannosaurs spent at least part of their time together. In some cases, the bones of several individuals have been found in the same place, as though they were living as a group.

Fossil evidence suggests – but doesn’t prove – Tyrannosaurs were also cannibals, but we have to be careful when interpreting such finds. There’s no way to tell whether the bite marks were inflicted because they were cannibals: they might have been made by or during scavenging, during a fight, or simply killing potential competitors when packs met or during mating rituals.

It’s noted on Phys.org:

It was only after discovering the bite marks were from a T. rex that Longrich realized the bone itself also belonged to the behemoth. After searching through a few dozen T. rex bones from several different museum fossil collections, he discovered a total of three foot bones (including two toes) and one arm bone that showed evidence of T. rex cannibalism, representing a significant percentage.
“It’s surprising how frequent it appears to have been,” Longrich said. “We’re not exactly sure what that means.”
The marks are definitely the result of feeding, although scientists aren’t sure whether they are the result of scavengers or the end result of fighting, Longrich said, adding that if two T. rex fought to the death, the victor might have made a meal out of his adversary. “Modern big carnivores do this all the time,” he said. “It’s a convenient way to take out the competition and get a bit of food at the same time.”

T. rex may have been a cannibal when food was in short supply, or when another T. rex was killed in a fight, but we have no way to tell if this was normal behaviour, opportunistic or desperation. Or if it was a regional behaviour caused by local conditions, but not widespread among other tyrannosaurids on other continents. There’s so much we don’t know about their behaviour.

Thomas Holtz, director of the Earth, Life and Time Program at the University of Maryland, told Discovery News he thinks the new finds are “very compelling.”

“There have been face bite marks from tyrannosaurs before, but those were most likely from squabbling, not feeding,” Holtz said. “These could only have been made after the dinosaur was dead.”

Researcher Nicholas Longrich told Discovery Magazine:

This suspected fighting behavior, the cannibalism finding and the massive size of these dinosaurs all indicate T. rex was a solitary predator that did not hunt in packs. Some large modern carnivores, such as lions and hyenas, do hunt in organized groups, but T. rex individuals likely acted on their own.
“Personally, I suspect that a whole pack of full-grown T. rex would have a very hard time finding enough to eat,” Longrich said.

I’m not sure that’s valid. A lot of T. rex’s prey was equally large in comparison. A 12-tonne triceratops would have fed a lot of animals, even big predators like T. rex. (In this amusing page, the writer suggests a T. rex could survive well on 40,000 calories a day – or eating a human being every two days… not that humans were around to be snacked on by dinosaurs; we’re separated by 60-plus million years of time – but it could also subsist on about 80 McDonald’s hamburgers… a Cretaceous version of Super Size Me…).

T RexThe calorie-based argument has suggested that T. rex could survive solely as a scavenger. But that has since been discredited, as Wired Magazine pointed out in a story from 2011:

To see if T. rex could survive on a carrion diet, Carbone’s team developed a computer model that merged dinosaur abundance, based on fossils found in the same formations as T. rex, with modern Serengeti scavenging data. Because predator-to-prey ratios in the fossil record are similar to ratio in Africa’s scavenger-filled savannas, Carbone said the Serengeti was a good model to bring all of the dinosaur abundance and territory data into perspective.

They concluded that T. rex would have faced such fierce competition from smaller scavenging dinos, that carrion alone wouldn’t suffice as a primary food source…

Assuming T. rex could cover about one square mile of territory every day, it would find a 25-ton sauropod carcass once every 5 years, a triceratops-sized carcass once a year and a horse-sized carcass once every 2 months. Even an adult-human–sized carcass would be found only every 6 days.

Some estimates suggest an adult T. rex would bite off 500 lbs. (230kg) of flesh in a single bite. However, I don’t know if the narrow neck would allow it to swallow all that at once. It might need to use those 50 razor-sharp teeth to cut it into more manageable sizes.

In another news story:

The finding is a big clue into the obscure eating habits of these enormous predators. While today’s large carnivores often hunt together in packs, T. rex likely acted on their own, Longrich said. “These animals were some of the largest terrestrial carnivores of all time, and the way they approached eating was fundamentally different from modern species,” he said. “There’s a big mystery around what and how they ate, and this research helps to uncover one piece of the puzzle.”

That’s supposition. We don’t know they “approached” eating. We can only guess. Trying to determine their eating habits from 65-million-year-old fossils is very much like trying to deduce the eating habits of today’s teenagers from the abundant litter they leave behind (we know they drink beer and energy drinks, and sometimes eat burgers and fries, but have no idea what their intake of vegetables or fibre is… or who paid for the beer…)

As the Globe and Mail reports,

“There’s an advantage in numbers. You can take down large prey if you’ve got three versus one,” said Michael Ryan, curator of vertebrate paleontology at the Cleveland Museum of Natural History, who was not involved in the find.

I’m not a paleontologist, but the pack predator makes a lot more sense to me than the solo. A solo dino would expend a lot more energy hunting – tracking, running down and killing its prey – than the per-dinosaur expenditure of a pack. So a solo dino would need to eat more – therefore hunt more often, and likely scavenge more – than a pack.

The HuffPost notes:

The new tracks suggest that the tyrannosaurs may have hunted in packs to take down large prey, just as wolves do today.

“An individual wolf would not be able to take out a moose, but a pack of them would,” [Richard McCrea, a curator at the Peace Region Palaeontology Center in Canada] said.

Similarly, pack hunting could explain how tyrannosaurs could kill hadrosaurs, which are almost as large as the predators, without sustaining horrific injuries, he said.

That doesn’t mean tyrannosaurs would have been friendly to one another. In fact, other fossils reveal that the dinosaurs liked to head-bite each other. But the tyrannosaurs may have stuck together to hunt because it increased their odds of survival, McCrea said.

No one, of course, will ever know for sure, because we can only guess at behaviour and activity from this distance in time, and paleontologists will continue to argue over it for many years to come. Still, what would science be without these debates?

So what about the feathers? A recent story on IFL Science suggests that the abundance of non-avian dinosaurs with feathers means that all dinosaurs had them.

Over 30 species of non-avian dinosaurs have been confirmed to have feathers, either from direct fossilized evidence of feathers, or other indicators, such as quill knobs. Up until now, all of those dinosaurs were confirmed to be carnivorous theropods, like Velociraptor and the ancestors of birds. However, fossilized remains of a new type of herbivorous dinosaur indicate that all dinosaurs may have had feathers.

Okay, so they end with a caution that this is really just another hypothesis:

The researchers suspect that feathers were a common feature among dinosaurs, particularly smaller ones. They were likely used to insulate the animals, and evolved to aid in flight much later. Fossils have shown evidence of feathered dinosaurs over a 50 million year timespan, and it is possible that they first appeared 220 million years ago in the Triassic. Of course, it is a bit speculative to assume that every dinosaur could have had feathers, and a great deal of research will be needed to fully explore the prevalence of feathers among dinosaurs.

And while the fossils discovered recently were not tyrannosaurids, as National Geographic adds the find points to a wide possibility,

Over the past two decades, discoveries in China have produced at least five species of feathered dinosaurs. But they all belonged to the theropod group of “raptor” dinosaurs, ancestors of modern birds. (Related: “Dinosaur-Era Fossil Shows Birds’ Feathers Evolved Before Flight.”)

Now in a discovery reported by an international team in the journal Science, the new dinosaur species, Kulindadromeus zabaikalicus (KOO-lin-dah-DRO-mee-us ZAH-bike-kal-ik-kuss), suggests that feathers were all in the family. That’s because the newly unearthed 4.5-foot-long (1.5 meter) two-legged runner was an “ornithischian” beaked dinosaur, belonging to a group ancestrally distinct from past theropod discoveries.
“Probably that means the common ancestor of all dinosaurs had feathers,” says study lead author Pascal Godefroit of the Royal Belgian Institute of Natural Science in Brussels. “Feathers are not a characteristic [just] of birds but of all dinosaurs.”

It’s stirring to the imagination to picture a pack of feathered T. rex on a hunt, isn’t it?