Tyrannosaurus – The Truth Behind The Legend

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Tyrannosaurus rex, the tyrant lizard king. Since its discovery and publication by palaeontologist Henry Osborn in 1905, this prehistoric predator has been immortalised as the quintessential dinosaur. A fearsome predator, shaking the ground as it chases down its prey and rattling the air with its mighty roar.

But how much of T. rex’s Hollywood PR holds up to the fossil evidence? Thanks to its popularity, Tyrannosaurus is among the most heavily studied of all dinosaur species, and the results of those studies paint a picture of an animal more complex and fascinating than any depiction on the silver screen.

Senses

One of the most pervasive ideas about T. rex that Hollywood has spread is that it suffered a strange visual impairment, unable to see objects that remained motionless. Beyond the obvious flaw that such a condition would stop the animal from seeing its surroundings, the structure of T. rex’s skull suggests that it really had outstanding vision.

T. rex’s skull is very wide at the back and very narrow down the snout. This gave its eyes an overlapping field of view, providing binocular vision which is essential for a hunter who needs to judge distance. The eyes themselves were huge! Around 13 cm in diameter, compared to the average 2.3 cm of the human eye. Grapefruit sized eyes would have let in a lot of light, which would give T. rex strong visual acuity and low-light vision.

Dinosaur skulls also contain a braincase, a hollow cavity which holds the brain. The shape of the braincase can give us an idea of the shape of the brain, and it seems that Tyrannosaurus had very large and well developed olfactory bulbs, which process smell, and cochlear ducts, which process sound. All this together presents us with an animal with incredibly keen senses, which it would need as a predator seeking out prey.

Hunting and Feeding

The debate over whether T. rex was a hunter or a scavenger tends to sit somewhere in the middle these days. Indeed most vertebrate carnivores today will both hunt and scavenge depending on the opportunity. Only vultures are exclusively scavengers, because they have the advantage of being able to detect a carcass from a distance and descend upon it without using much energy.

Being so large and heavy, Tyrannosaurus’s top running speed was likely around 15-20 kph. More of a power-walker than a sprinter, it had a trick up its socks to make up for it. The bones in its ankles were heavily compressed. Known as an arctometatarsalian condition, this fusing of the foot bones assists in absorbing and retaining energy while travelling over long distances. Rather than being a speedy ambush predator like big cats, Tyrannosaurus may have been a persistence hunter like wild dogs; keeping pace behind its prey for miles on end until they gave in to exhaustion.

The fossil evidence for T. rex’s hunting habits is quite striking. Bones of other dinosaurs, including duck-billed Hadrosaurs and horned Ceratopsids, have been found with holes punched through, chunks torn off and even teeth stuck inside them. Incredibly, some of these bones have healed, showing that the animal was alive when it was attacked. It must have been a miraculous escape, since biomechanical studies of the skull structure and muscle attachments of Tyrannosaurus suggest it had a bite force of around 35,000 newtons! More than enough to pulverise whatever it got its railroad spike sized teeth into; teeth which were shed & replaced throughout its lifetime.

Ecology and Growth

But how can we be sure that the culprit behind those bite marks was Tyrannosaurus? Afterall many different predators live alongside one another in many places. In Africa, Lions share their territories with Leopards, Cheetahs, Wild Dogs and Hyena. The same is true of dinosaurs. For example in the Jurassic Morrison formation of the American midwest around 150 million years ago, many large predators including Allosaurus, Ceratosaurus, Torvosaurus and Saurophaganax shared the environment.

However, Tyrannosaurus seems to have had an unusual impact on its local food chain. There were plenty of large prey animals to go around, but the next largest predator in T. rex’s habitat was a dromaeosaur called Dakotaraptor (which might yet turn out to be more than one species, but that’s a whole other issue). This dinosaur was a relative of Velociraptor, and weighed around 300 kg. Compared to the 7-10 ton Tyrannosaurus, this would be like if the savannah’s second largest predator after the Lion was a Fox. So where are all the other medium to large predators in this ecosystem?

The prevailing idea is that those spaces in the food chain are being taken up by young Tyrannosaurs. Skeletons of juvenile T. rex show that, as well as being more lightweight than the adults, they had proportionately longer legs and more lightly built skulls. This means they could have chased smaller, faster prey such as the ostrich-like Ornithomimosaurs and dome-headed Pachycephalosaurs, occupying the ecological space normally taken up by other predatory species.

They didn’t stay small for long. Cutting through the thigh bones of Tyrannosaurus reveals growth rings, just like tree trunks, and it looks like the tyrant lizard grew astonishingly quickly. During their teenage years T. rex went through a sharp growth spurt, reaching full adult size before their 20th birthday. For an animal that likely weighed less than 5 kg when it hatched, that’s an increase of 1,600 times or more. Equivalent to a human growing to the size of an elephant!

Injuries

Being an apex predator is inevitably a tough lifestyle, and the skeletons of Tyrannosaurs are often riddled with illnesses and injuries. Broken and healed ribs are common, as well as bite marks around the head and neck. Without other large predators to compete with, it’s likely that Tyrannosaurs fought one another over food, territory or breeding rights, inflicting vicious injuries on one another.

Some pathologies are truly horrifying. Specimen BHI 3033, nicknamed Stan, has a broken and healed neck, with vertebrae fused together which would have caused him pain and stiffness for the rest of his life. AWMM-IL 2022.21, aka Barbara, suffered a crippling foot injury, which might have kept her from hunting and ultimately led to her death. Perhaps most shockingly is the specimen dubbed Wyrex, who had half its tail bitten off! There are allegedly signs of infection on the remaining damaged vertebra, suggesting the poor creature survived the injury, if only for a little while. Dinosaur tails supported muscles which are essential for walking, so this brutal amputation would have been debilitating.

There is so much more that has been learned and theorised about Tyrannosaurus, from its social habits to whether or not it had feathers. Every year new discoveries are made and new ideas are tested, proving that more than a simple-minded killing machine, Tyrannosaurus was a complex and fascinating animal, that occupied multiple trophic levels during its lifetime and could endure punishment that would floor most mammals. Truly unlike anything alive today and with many more secrets yet to be revealed.

Further Reading

The Tyrannosaur Chronicles – David Hone

Age & growth dynamics of Tyrannosaurus rex – John R. Horner & Kevin Padain

Estimating maximum bite performance in Tyrannosaurus rex using multi-body dynamics – K. T. Bates & P. L. Falkingham