Ankylosaurus’s Nostril Placement

[suspsy]

https://www.dinosaurhome.com/the-evolution-of-ankylosaurus-15158.html

Interesting stuff. From an evolutionary standpoint, it makes perfect sense that the placement of the nostrils on Ankylosaurus would decrease the risk of a T. rex getting in a lucky bite.

[Neosodon]

Very interesting. Also sheds light on tyranosaur hunting strategies.

[ZoPteryx]

Hmm, certainly possible.  But why didn't other ankylosaurs like Saichania, Pinacosaurus, or Euplocephalus develop the same approach?  They all coexisted with their own big tyrannosaurids, after all.

[Newt]

Doesn't make sense to me. If tyrannosaurs were trying to suffocate their prey felid-style, they wouldn't be doing it by plugging up the nostrils, they would be crushing the whole nasal passage - in which case the nostril postion is irrelevant. I also don't know of any evidence that that is how big tyrannosaurs hunted (I'm admitting ignorance here - please chime in if you know otherwise). Intuitively I am skeptical - tyrannosaur jaws do not seem suited for that sort of work (if you want to do crushing work, you want a short massive jaw, like the aforementioned cats, hyenas, parrots, etc.), besides which a big tyrannosaur that had gotten a secure hold on an ankylosaur's head would be quite strong enough to just yank on it, causing spinal trauma and killing or incapacitating the animal with minimal handling time. Increased cranial armament would have been aimed at preventing predators from getting a good grip in the first place, not preventing suffocation.


I would also point out that the writer's idea that tyrannosaurs would grab ankylosaur snouts from the front seems unlikely - a tyrannosaur would have been taller than any ankylosaur small enough for it to dare to attack; and the ankylosaur would have been pivoting around its vertical and horizontal axes, trying to keep its head down and away from the predator and its tail up and swinging at the predator, making it extra difficult to bite from the front. The tyrannosaur would be much more likely to bite from the side and top - unless it had some secret means of hypnotizing the ankylosaur to stand stock still, head up and towards its killer, in which case snout-crushing hardly seems necessary.


So what is the reason for Ankylosaurus' unusual snout morphology? I don't know, and I find Arbour and Mallon's suggestions unconvincing (to be fair, they air them only as possibilities). I think any hypotheses must keep in mind also the changed dental characters (more, smaller teeth than in other ankylosaurines) and the architecture and function of the complex nasal passages, which would have been somewhat re-arranged in Ankylosaurus compared to its relatives. The nostril movement may have been only a sequela or side-effect of nasal passage or jaw changes, not a selected character in its own right.


If I might air another hand-wavy hypothesis: we know that the ankylosaur Liaoningosaurus probably ate fish, based on thoracic contents. Arbour and Mallon mention this as a possible explanation for Ankylosaurus' unusual snout morphology, that it was adapted for fish hunting (which I find implausible - this is not an animal made for wading, or for grabbing slippery fish). But, I do think it's likely that Ankylosaurus and other ankylosaurs scavenged carcasses of fish and other animals - not for ordinary nutritive requirements, but for calcium. Ankylosaurs grew a lot of bone, and that requires a lot of calcium, which is difficult to acquire from plants alone. So, perhaps many ankylosaurs stalked shorelines, shoveling up dead fish, shells, and any other calcium-rich detritus they could find.


Which still doesn't explain why Ankylosaurus would be different from its relatives - here's where the hand-waving comes in. Suppose that Carpenter was right that Ankylosaurus was an upland animal (as far as I know, his only evidence for this is its relative rarity in the coastal plain deposits where it has been found). It would not have had easy access to the seashore, a convenient place to seek calcium supplements. So where would it satisfy its mineral needs? The bones of other large animals contain a trove of calcium, but Ankylosaurus did not have the necessary feeding apparatus to crush bones like a hyena or to gnaw them like a rodent. Smaller bones it would no doubt ingest when it could find them, but that is a relatively rare resource and unlikely to sustain it. Perhaps it could also ingest small bits of limestone or other calcium-rich minerals - but these stones are often massive and prone to decay by dissolution rather than fracturing, so that bite-size chunks are hard to find, and may be more difficult to digest than bone (I'm guessing), and for all I know may have been rare or absent as outcrops in the range of Ankylosaurus. That leaves - freshwater shellfish! Modern streams, ponds, and swamps are often packed with snails, clams, mussels, and crustaceans, and Late Cretaceous streams certainly were too. Perhaps Ankylosaurus made a habit of rooting around in shallow water seeking out mineralized invertebrates to satisfy its bone-building needs. Its rearward-facing, posteriorly-displaced nostrils would have allowed it to breathe while engaging in this behavior. 


I realize this says nothing about the small teeth or re-arranged nasal passages I just said any hypothesis must consider. Oh well, it's the best I've got.

[Bowhead Whale]

You know what I personnally think? I think the reason why the nostrils of the Ankylosaurus were located so close to the mouth simply was because the muzzle needed space for the armor protection. The armor protection needing to be thicker, the nostrils just had to change position to make room for the armor. As simple as that. What do you think?