Ankylosaurus Narial Defense
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.
posted by starman at 5:05 AM
15 Comments:
Your post about the narial defenses of Ankylosaurus is quite interesting. Like other parts of the body, the nostrils needed to be protected. Tyrannosaurus rex was the apex predator of the Cretaceous. Although it was not the largest theropod of the Mesozoic era, T. rex had the most formidable teeth and a bite force that was very exceptional.
Agreed. :) Ankylosaurus had to be coadapted to the archpredator.
January 21, 2018
The fossil record indicates that Ankylosaurus made it to the end of the Cretaceous. Its tail club must have been an important factor in self defense. As we know, that tail club was absent in most nodosaurids. That is a point of difference between ankylosaurids and nodosaurids.
Yes indeed, it is possible that nodosaurs disappeared in Laramidia before the K-Pg. Back in 1986, Carpenter suggested they did, based on stratigraphic data.
No doubt, the nodosaurid survival strategy was effective throughout much of the Cretaceous, but T.rex ultimately proved too much for it.
January 26, 2018
You're right; nodosaurids lasted for quite a while. The remains of Sauropelta edwardsorum date to the Albian, which was early Cretaceous. But as you said, the nodosaurids were no match for T.rex.
Do you remember my DH post "Nodosaur Extinction Revisited"? That post only mentioned additional evidence for nodosaurs occurring low stratigraphically in the Lance/Hell Creek, suggesting extinction before the K-Pg. It didn't suggest a cause. But a drop in diversity of large dinosaurs appears to coincide with the appearance of T. rex. Denversaurus (or whatever nodosaur taxon was present) may have been one of the casualties, albeit not as early as one or more others.
January 26, 2018
I remember that post. You're right; the evidence indicates that nodosaurs may have died out before the K-T extinction. Even if the nodosaurs were not completely extinct, they were extremely close to extinction, and would have died even if the K-T extinction event had not occurred.
Russell suggested the nodosaurs didn’t go extinct but just migrated eastward, away from the Hell Creek depositional area, due to regression, as they seem to have had a preference for near-marine habitats. I doubt that, however. There was in fact transgression, not regression, near the very end of the Maastrichtian in Laramidia. Nodosaurs wouldn’t have abandoned the area or if they did they would’ve returned before the end. Therefore their absence in upper Lance/Hell Creek strata was likely due to extinction.
January 27, 2018
As we've discussed, T. rex was the apex predator. It must have been quite numerous, so that the T. rex population was dense in the area of the Hell Creek strata. That would have made it very difficult for nodosaurs to hide.
Crocodylomorphs were also predators in the Cretaceous, but they were not on a par with heavyweight theropods, such as T.rex. So carnivorous theropods (especially tyrannosaurs)were the main threat to nodosaurs.
Yes of course. In Campanian times the big allogatoroid, Deinosuchus, may have ambushed dinosaurs at the water’s edge, including an occasional nodosaur. But tyrannosaurs were far and away the main predators of these armored taxa. One nodosaur specimen from the lower Lance, nicknamed “tank,” has pathologies. The specimen has yet to be published but I syuspect a T. rex was responsible.
February 14, 2018
I also think that a T. rex was most likely the culprit. As we've discussed, tyrannosaurs were the main threat to nodosaurs. Of course, the presence of additional predators did not help the situation for nodosaurs. There was no safe refuge from tyrannosaurs. Fleeing to the water was not a safe thing to do. In freshwater, crocodylomorphs were a threat. Going into the sea would have also invited catastrophe, since there were marine predators. The mosasaurs were especially dangerous aquatic carnivores, though sharks were also a threat.
Of course. Nodosaurs did not evolve to flee. In theory, their armor and spikey tails were sufficient to repel tyrannosaurs, so fleeing was unnecessary.
February 15, 2018
I agree; their tail weapons and armor were intended to fend off predators. Although it worked against some carnivores, those defensive features were not sufficient for defense against the alpha predator, i.e. T. rex.
Well we can’t really be sure. I suspect Denversaurus—or whatever taxon it is—succumbed to T. rex, but more evidence is needed. It may help if the nodosaur discovered by the Japanese is finally published. I don’t know what caused the reported pathologies.
February 16, 2018
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