After writing "The Evolution of
Ankylosaurus" in my Dinosaur Home blog, I discovered that the Carnegie
Ankylosaurus is realistic enough to illustrate my points. It may be even better for this purpose than the pictures appearing in the other blog.
Seen in dorsal view, the head of
Ankylosaurus (above) displays many caputegulae or "head tiles." Among the most important were the loreal caputegulae. They appear in anterolateral positions i.e. on the sides of the snout near the front. The loreal caputegulae overlie the external nares, not visible in dorsal view but positioned laterally on the snout (whereas in most other ankylosaurs they were in front).
Seen in lateral view, the right external naris is, in the words of Arbour, "roofed over by cranial ornamentation" or specifically the right loreal caputegulum. It is clearly a robust osteoderm, which most likely evolved to protect the nostril from
T. rex. The nares are prime targets for carnivores, which often try to suffocate prey by crushing or fusing them. By overlying the nostrils, the loreal caputegulae shielded them from the deadliest, or maxillary, teeth of
T. rex.
Enhanced defense was also the reason for the lateral shift in the nostril's position. The front of the snout (or premaxillae) could not accomodate enough armor to adequately protect the nostrils. The nostrils themselves would've weakened the defense.
Although the premaxillae were armored in earlier ankylosaurs, such as
Saichania and
Tarchia, this was no longer sufficient by the time of
T. rex.
Saichania and
Tarchia faced
Tarbosaurus which, although large, was not as powerful as the later (late Maastrichtian)
Tyrannosaurus.
T. rex had unmatched jaw musculature conferred in part by its posteriorly expanded skull (i.e. providing increased area for muscle attachment). In addition to jaw power,
T. rex had the most robust teeth. Less vulnerable to breakage and more capable of crushing or penetration, such teeth were ideal for use against armored quarry. Without doubt,
T. rex was the most dangerous nemesis of ankylosaurs (as well as other taxa). It is not surprising that the ankylosaurid contemporary of
T. rex required, besides the largest size, the best narial defense to survive.
