Thyreophora + Xenarthra = Fuzzy Ankylosaurus?

Beautiful amateur artwork by yours truly. But why the fuzz?

For those of you who have been checking my YouTube page, you may have noticed that I posted a new video, this time about the anatomy of the Ankylosauria. In it I talk about everything ankylosaur, as well as include an image by yours truly showing some speculative interpretations of their hyoids. However, in that same image I also portrayed the ankylosaur, which was based on Ankylosaurus magniventris, with feather-like protofeathers (a.k.a. dinofuzz or whatever it's called these days). Some people might think that this is due to scientists' current preoccupation with taking every dinosaur and slapping some feathers onto it, shattering the hearts of so many JP fans. However, I actually have a very good reason why I believe that not just Ankylosaurus might have had feathers, but also that it might be likely that the entire Thyreophora might've had such fuzz, even more than other large-bodied dinosaur groups. It starts with observations of another living group of animals: the Xenarthra.

Xenarthra are a group of mammals exclusive to the Western Hemisphere. They include freaky animal families like Armadillos, Sloths, and Anteaters, as well as awesome extinct forms like Ground Sloths and Glyptodonts. They are bizarre on so many accounts relating to their anatomy, such as having vertebrae that articulate differently than any other mammal, their lack of tooth enamel, and don't even get me started about some of their unsavory habits, like the recently documented observations of Linnaeus's two-toed sloth drinking from human latrines (Heymann 2010). Yuck...

It suddenly seems a lot less cute after discovering about its drinking habits.
I wonder how Kristen Bell reacted to that discovery...

Anyways, these guys seem to share some aspects of their physiology with thyreophorans, and no I'm not just saying that because they both have members that are complete turtle-mimics. Xenarthra are interesting in that they have the lowest body temperature and metabolic rates of any mammal, being at 40 to 60% what you would expect for mammals of their size (McNab 1980). This low body temperature makes xenarthrans somewhat slow-moving animals, and they have to face problems many people would typically associate with reptiles, such as having to regulate their body temperature and suffer a slower growth rate than other mammals. The latter trait is particularly important, as it's a trait also shared by thyreophorans.

The thyreophoran taxa Scutellosaurus (Padian 2004), Stegosaurus (Redelstorff 2009), and numerous American ankylosaurs (Stein 2013) have all been shown to have slowed growth rates when compared to other dinosaurs. They weren't as slow as reptiles, but definitely not as fast-growing as mammals. This suggests that like xenarthrans, the Thyreophora had a metabolism slower than other dinosaur families. There is, however, the exception of Kentrosaurus, which might have had a faster growth rate (Redelstorff 2013). However, Kentrosaurus seems to have been an exception, and the majority of evidence points towards thyreophorans having lowered body temperatures like xenarthrans. Whether or not their actual body temperatures were at comparable levels to xenarthrans we don't know, but seeing as how many scientists now consider dinosaurs to be warm-blooded on the same level as modern mammals, similar thermoregulatory levels between thyreophorans and xenarthrans might be expected. I'd like to see someone try to test this idea in the future.

So, what does this all have to do with an Ankylosaurus having fuzz? Well, everything really. You see, the main argument made by people against the idea that large dinosaurs had feathers, or any type of fuzz really, is that they wouldn't need it, being large enough to be able to have a stable body temperature, as seen on modern large mammals. I have my own issues with this idea, since it assumes that protofeathers and hair are the same kind of structure (which they are not), and that dinosaurs have a similar physiology to mammals (which is unlikely). However, even assuming such ideas, we still know of large-bodied mammals that live in tropical regions which have thick, shaggy fur, and they're xenarthrans.

The Shasta Ground Sloth, a desert-dwelling sloth that was completely covered in shaggy fur.
Other members of the group were the size of an Elephant and just as fluffy.

Ground sloths are already so famous that they don't need an introduction. They are giant ground-dwelling sloths that were running around on the sides of their feet during the Ice Age, and alongside mammoths, were the largest animals in their environment. However, despite their size, they're often portrayed as being very fuzzy animals with shaggy fur. When in a discussion with someone the other day, they dismissed this as simply an assumption and asserted they were probably hairless like elephants. This is poppycock. We have mummified ground sloth skin from caves in Arizona, Nevada, Argentina, and Chile which show thick fur on the animals. We even have them preserved for Eremotherium, which was the size of a small elephant, and its likely that the fur is present in still larger members like Megatherium.

So this would seem contrary to what people say about large mammals shedding fur at larger sizes. Why did ground sloths keep their shaggy fur in tropical environments at such large sizes while other groups of mammals of equivalent size lost their fur? Our best answer is that this was due to their lowered metabolism, which means they had a harder time holding onto body heat than other mammals. Apparently holding onto that hair was also a pretty smart move, especially since ground sloths back then were perfectly able to extend their range up into temperate regions of North America and even as far north as Alaska during the last Ice Age, while living sloths have trouble tolerating even temperate latitudes. According to McNab, this was probably due to a combination of their thick fur, as well as greater size, larger muscle mass, and a constant food supply compared to their living relatives (McNab 1985).

So assuming that ankylosaur growth rates suggest that they had lowered body temperatures like xenarthrans, might we then expect that these guys were covered in shaggy fuzz, too?


Stegosaurus with tail spines made of porcupine-like filaments. Art by Mark Witton.
Looks like other scientists got the memo, or they're just slapping feathers on everything again...

Thus, even assuming that they had a physiology similar to mammals, and the falsified assumption that protofeathers were similar to hair, our knowledge of their possible xenarthran-like metabolism would still make them candidates for long filaments covering their entire body. We also know that ankylosaurs at least lived in temperate environments, such as Liaoning, and polar environments like Alaska, Australia, and New Zealand. These guys would've needed even longer fur in order to survive the cold snaps, if they were xenarthran-like in their metabolism.

When you take all this into account, it almost makes the picture I drew kind of conservative, showing only short fuzz compared to the longer hair common on ground sloths. Perhaps I'm diving too deep into speculation right now, but I think that my idea holds some merit. What are your thoughts? As always I'd love to hear everyone's opinion on this.

Stay sharp! And make sure you check out my brand new Ankylosaur video on YouTube:

References:

Heymann, E. W., Flores Amasifuén, C., Shahuano Tello, N., Tirado Herrera, E. T. & Stojan-Dolar, M. 2010. Disgusting appetite: Two-toed sloths feeding in human latrines. Mammalian Biology doi:10.1016/j.mambio.2010.03.003

McNab, Brian K. (November 1980). "Energetics and the limits to the temperate distribution in armadillos". Journal of Mammalogy (American Society of Mammalogists) 61 (4): 606–627. doi:10.2307/1380307. JSTOR 1380307.

McNab BK (1985). Energetics, population biology, and distribution of Xenarthrans, living and extinct. In: Montgomery GG (Editor), The Evolution and Ecology of Armadillos, Sloths and Vermilinguas. Smithsonian Institution Press, Washington and London, 219-232.

Padian K, Horner JR, Ricqlès A (2004) Growth in small dinosaurs and pterosaurs: the evolution of archosaurian growth strategies. J Vert Pal 24: 555–571. doi: 10.1671/0272-4634(2004)024[0555:gisdap]2.0.co;2.

Ragna Redelstorff & P. Martin Sander (2009) Long and girdle bone histology of Stegosaurus: implications for growth and life history, Journal of Vertebrate Paleontology, 29:4, 1087-1099, DOI: 10.1671/039.029.0420

Redelstorff, R., Hübner, T. R., Chinsamy, A. and Sander, P. M. (2013), Bone Histology of the Stegosaur Kentrosaurus aethiopicus (Ornithischia: Thyreophora) from the Upper Jurassic of Tanzania. Anat Rec, 296: 933–952. doi: 10.1002/ar.22701

Stein M, Hayashi S, Sander PM (2013) Long Bone Histology and Growth Patterns in Ankylosaurs: Implications for Life History and Evolution. PLoS ONE 8(7): e68590. doi:10.1371/journal.pone.0068590