At 13 metres in length, Tyrannosaurus rex had little to fear from other predators. But it was occasionally attacked by an enemy far smaller than itself. In a wonderful piece of forensic palaeontology, Ewan Wolff from the University of Wisconsin has shown that the tyrant lizard king was often infected by a microscopic parasite, whose relatives still infect the birds of today. Potentially transmitted through bites from other tyrannosaurs, the parasite could have starved the infected animals to death.
Many of the large meat-eating dinosaurs have wounds on their heads that were clearly inflicted during fights with their own kind. Bite marks and long gouges caused by teeth raking bone are both fairly common, but both types of injuries show signs of healing – unlike the marks found on prey, these bites weren’t inflicted to kill. But tyrannosaurs also show a second type of injury – smooth-edged pits and holes, particularly in the jaw, where the bone has been eaten away. Some are small; others are centimetres across. They weren’t made by any tooth and they don’t match the shape of any mouth.
They do, however, bear a striking resemblance to injuries found in the beaks of modern birds, particularly falcons, pigeons and chickens. In birds, these injuries are the result of trichomonosis, a disease spread by a parasite called Trichomonas gallinae. The parasite creates ulcers throughout the bird’s mouth and throat, and erodes its jawbones.
Based on the strikingly similar size, shape and locations of the Tyrannosaurus pits and holes, Wolff thinks that the prehistoric predator was afflicted by a very similar contagion and even mounted a similar immune response. It’s yet further evidence of the close relationships between modern birds and their dinosaur ancestors. They may even have been attacked by the very same parasite, although based solely on the scars left behind, that’s impossible to determine.
Wolff examined the remains of 61 tyrannosaurs and discovered these trichomonosis-style holes on 15% of them. Even the famous ‘Sue’ was affected. Wolff did a differential diagnosis of the lesions (House, eat your heart out), ruling out a variety of other diseases that affect modern birds and crocodiles. Infections caused by bacteria, viruses, fungi and parasitic worms, and even vitamin C and D deficiency, were struck off the list. All affect facial bone, but all of them produce very different injuries. Only trichomonosis fit the bill.
Modern birds of prey can spread Trichomonas from one to another in many ways. Eating infected prey is one of these but Wolff thinks it’s unlikely that Tyrannosaurus ran a similar risk. After all, not a single one of the plant-eating dinosaurs that it fed on has ever been found with a lesion that looks like trichomonosis. Other possibilities include drinking contaminated water, but Wolff thinks that the many animals became infected after biting each other during fights.
Tyrannosaurs certainly bit each other on the head on a regular basis. The reasons are unclear – perhaps combat, courtship or cannibalism – but such behaviour was probably very common for around half of all individuals carry the resulting scars. Among those that Wolff examined, the presence of bite wounds and trichomonosis lesions go largely hand in hand.
Modern birds can pass on the infection by touching each other on the bill – Tyrannosaurus seems to have done something similar, although in its own inimitable style. Strangely enough, a similar fate befalls today’s Tasmanian devils. They bite each other on the face regularly and, in doing so, they spread a debilitating type of contagious cancer. The tumours grow so large that the devils can’t eat and starve to death. Their population is crashing as a result. Their own behaviour is killing them.
Wolff suggests that tyrannosaur populations could have been weakened in a similar way, for the size of some of the pits shows that many infections were very serious. If the disease progressed as it does in modern birds, by the time the parasite had actually started to erode the jawbone, it would also have thoroughly invaded the animal’s gums and riddled its mouth and throat with ulcers (the yellow lumps in the reconstruction painting). With such painful sores, feeding would have been difficult.
As Wolff says, “it is very probable that [Sue] and other more seriously affected individuals succumbed to starvation… A disease like modern-day avian trichomonosis may have been the scourge of tyrannosaurids, thanks in part to their antagonistic behavior.”