September 30, 2009
By now it should not surprise anyone that birds and theropod dinosaurs were closely related. Numerous discoveries have revealed that many “bird” characteristics, like feathers, first evolved in dinosaurs and were passed on to the avian descendants of one group of theropods called coelurosaurs. Tyrannosaurus was a coelurosaur, and while the question of whether it had feathers during any time of its life is still open for debate, a new study published in PLoS One shows that the “tyrant king” suffered from a disease that still plagues modern birds.
Conducted by paleontologists Ewan Wolff, Steven Salisbury, Jack Horner and David Varricchio, the study took a closer look at a series of holes commonly seen in Tyrannosaurus lower jaws. These holes have previously been thought to be bite marks from another Tyrannosaurus or the result of some sort of bacterial infection in the bone. To test these hypotheses, the team looked at 61 tyrannosaurid skulls, approximately 15 percent of which displayed this jaw pathology.
The holes were inconsistent with damage that a bite from another predatory dinosaur would have inflicted. Bones are not static but living, and when they are damaged by physical trauma (like a bite) they react. If all these tyrannosaurids were bitten, it would be expected that the bones would show some inflamation, signs of bacterial infection and indications of healing after the event. Instead, the holes are smooth-edged and show no signs that they were made by the teeth of another dinosaur.
With dinosaur-on-dinosaur combat ruled out, the researchers looked at diseases that had been proposed to account for the damage. None of those previously suggested fit, but the bone pathology was consistent with damage done by a microorganism called Trichomonas gallinae that infects some modern birds. While some forms are virtually harmless to their host, others cause severe ulcers in the mouth and upper digestive tract of pigeons and birds of prey. Some of the tyrannosaurids appear to have suffered from the same sort of microbial infestation.
While paleontologists cannot be sure that the damage done to Tyrannosaurus was caused by the same species of Trichomonas, at the very least the study suggests that they were susceptible to a close relative of the modern organism and that this “avian” disease was already present over 65 million years ago. If this hypothesis is true, it is yet another line of evidence that links coelurosaurs closely to birds, even if Tyrannosaurus was more evolutionarily distant from birds than some of its “raptor” relatives.
Yet the paleontologists took their studies a step further. How could this harmful microorganism have been transmitted from dinosaur to dinosaur? In living birds Trichomonas gallinae is often taken up in drinking water by birds like pigeons, which are then preyed upon by hawks and other birds of prey. That tyrannosaurids drank water containing this kind of microorganism cannot be demonstrated, but their role as large predators would be consistent with disease transmission.
Rather than drinking up infected water, tyrannosaurids would transmit the microorganism through fights with each other or even eating the carcasses of infected individuals. While the holes in the lower jaw were not caused by such events, there is evidence that tyrannosaurids sometimes bit each other on the face. This would have allowed the microorganisms from the mouth of one dinosaur to enter the wounds opened up on another. The microorganisms would then infest the mouth and throat of the dinosaur, opening lesions in the flesh and bone. The Tyrannosaurus known as “Sue” presents one of the most severe cases; there were so many lesions in her jaw that she may have had trouble eating. With the infestation at such an advanced stage, the researchers suggest, the largest Tyrannosaurus yet discovered may have starved to death.
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