A Tale of Tails: The Evolution Driving Ankylosaur Tails & Armor

Peloroplites cedrimontanus, a nodosaurid from the Cedar Mountain Formation.
Image taken by yours truly at the USU Eastern Prehistoric Museum in Price, Utah.

Ankylosaurs are some of my favorite dinosaurs, which is saying a lot considering I love all dinosaurs pretty much the same. Perhaps it's the mental image of such a powerful tank of an animal walking across the landscape seemingly impervious to attack, or just my own personal connection to them through studying what we know of their internal and external workings. Either way, I'm hooked on these beasts and eagerly anticipate reading whatever I can on their biology, anatomy, and behaviors. I covered most of the reasons I love these animals in a video a while back, so I'll keep things rather focused in this post to some new stuff that's just come out.

Rewind back to a couple of years ago to SVP 2013 in Los Angeles. I attended the event and sat in a seminar by Victoria Arbour, in which she spoke of a study she was working on attempting to track the evolution of the ankylosaurid "tail club." Her results seemed to have concluded that ankylosaurids may have gone through something of a step system throughout the evolution of their tail club, gaining certain traits over roughly 40 million years. Fast-forward back to the present day, and it seems her paper just came out in the most recent Journal of Anatomy, coauthored by Philip Currie. Together they come up with some rather interesting new conclusions which has widespread impact on how we view the evolution of said tails in the group.

Arbour and Currie come to the conclusion that ankylosaurid tail clubs evolved in a series of evolutionary stages. These went from the long and flexible whip-like tails of the earliest ankylosaurs and ankylosaurids, to a number of early Cretaceous ankylosaurids which Arbour and Currie deem "bat-tailed," they name them so for having caudal vertebrae with lengthened prezygapophyses, which conjoin the vertebrae, while also lacking a tail club. Finally in the late Cretaceous appear derived, so-called "club-tailed" ankylosaurids, which possess both lengthened prezygapophyses and dramatically enlarged osteoderms forming the so-called tail club. I prefer calling these ankylosaurids "mace-tailed" personally, as club is something of a highly generalized term that encompasses a lot of very different types of weapons, but that isn't really important. What is important is finding out what all this means about ankylosaur biology. Can learning how they evolved tell us how these tails were used?

Non-ankylosaurid ankylosaurs might provide some help. Polacanthids (or simply just polacanthinae, depending upon whom you ask) had long flexible tails without any extreme specializations with one exception: Some polacanthid species, such as Gastonia burgei, had extremely large and ominous-looking osteoderms along the sides of an otherwise flexible tail, leading to suggestions that they might be used in defense. The documentary series Jurassic Fight Club (also called Dinosaur Secrets for those of you in the UK) went so far as to call this form of armor a "prehistoric chainsaw," and although this might seem over-the-top, the tails of these ankylosaurs do remind me of a certain TV Tropes weapon. (I have more to say about JFC in a future post.)

Gastonia burgei, also from the Cedar Mountain Formation, but part of the earlier Yellow Cat Member.
Skeletal by Gregory S. Paul from his book The Princeton Field Guide to Dinosaurs.

Nodosaurids, on the other hand, tended to have much simpler tails. Examples like Sauropelta and Edmontonia both possess either simple tiny osteoderms down the tail, or no osteoderms at all. The tails were, however, extremely long and whip-like, and could've still been used to give a predator a good whack to the face when necessary. It's interesting to note, however, that many nodosaurids do possess extensive armor plating on and around the head and front of the torso. Perhaps this suggests that nodosaurids were much more head-on than other ankylosaurids with their defensive approach. Edmontonia especially has forward-projecting shoulder spines which are bifurcated like deer antler, which has led to the suggestion that it is some form of weapon that could be used to charge predators or lock with rivals.

But what about ankylosaurids proper? Why take the first step towards "bat-tailed" forms with tightly-locked tail vertebrae? Sexual selection might actually play something of a role here. A "bat-like" tail of fused vertebrae delivers more force than a whip does, so the development of a more robust and immobile tail might help in sparring matches where two ankylosaurids line up side-by-side to smack each others flanks. This kind of behavior would then eventually lead to the evolution of thicker and sturdier tails, and would also encourage the evolution of the tail club. The same idea has also been suggested for South American glyptodonts and ancient meiolaniid turtles, which are also coincidentally huge, well-armored herbivores with wide girths and extensive osteoderm coverings. Perhaps this kind of competitive behavior is something which develops uniquely with this type of body design.

Or we could go with the more traditional view of a defensive evolutionary drive for the evolution of tail clubs. In this view, earlier ankylosaurids first developed the elongate prezygapophyses in order to create a more active and dynamic form of defense against predators outside of the possibly more sit-and-wait strategies of other ankylosaurs. The "bat-tails" could be used as a whacking weapon to smack at the heads and legs of hungry theropods, or to sound more awesome, to slice them up. Note that although the club part had yet to evolve, some earlier ankylosaurids had smaller osteoderms on these so-called "bat-tails." Gobisaurus possessed a "bat-tail" with elongate prezygapophyses and no enlarged tail club made of osteoderms, but it did seem to possess decent-sized smaller osteoderms along either side of the tail like Gastonia. This means it had a robust, bat-like tail, with bladed sides. Perhaps ankylosaurids like it should be dubbed "sword-tailed" or "tebutje-tailed." Of course, neither idea directly debunks the other, and multiple purposes for such a structure seems entirely likely.

A tebutje, a Polynesian form of war club with bladed edges.
Kinda like basal ankylosaurid tails, although this one's made out of shark teeth.

In the latest Cretaceous, true club- (or mace)-tailed ankylosaurids appeared, and they immediately took off with hugely varied designs for the club. The earliest ankylosaurid with a big knob at its tail tip is Pinacosaurus (or possibly Talarurus) which had a rather modest-sized tail club, and from humble beginings we get whole variety of designs ranging from the abnormally narrow knob of Dyoplosaurus, to the abnormally massive and robust tail knob of Ankylosaurus, and even the extremely massive and pointy knob of Anodontosaurus. All-in-all, this wide variety of tail clubs suggests something else: Could they have each been used in different ways? I personally think so. Having a massive, heavy tail club at the end of your tail is extremely powerful to possess, but it can be cumbersome to "wield," and because of its weight, rather slow to swing. On the other hand, having a smaller tail club is much easier and faster to swing, but against some rather large predators might be lacking in sufficient strength to do any significant damage.

Could the types of predators and their behaviors in a given ecosystem have influenced the development of these clubs? Well, some ankylosaurids with smaller knobbed tail tips, such as Pinacosaurus, seemed to live in an environment where smaller dromaeosaurids were the most common form of predator. Paul even suggested in his book The Princeton Field Guide to Dinosaurs that, with its smaller tail club and lighter build, Pinacosaurus could easily hit and threaten all kinds of smaller predators. On the other end of the spectrum, a number of ankylosaurids with truly massive tail clubs were contemporaries of massive tyrannosaurids, in which even larger and more powerful knobs (the ones required to break a rex's ankles) were required.  This is just me delving into some speculative science though, and I don't know if anyone else has found any such correlation. Might be something interesting to look into.

And I think I'll stop there. Ankylosaurs are definitely a very interesting bunch of animals, and I will be sure to discuss them again in the future. If you want more ankylosaur-goodness though, I recommend checking up on Victoria Arbour's blog Pseudoplocephalus. She has a great post out now on her new study, so make sure to read that, too. Cheers!

Citation

Arbour, V. M. and Currie, P. J. (2015), Ankylosaurid dinosaur tail clubs evolved through stepwise acquisition of key features. Journal of Anatomy. doi: 10.1111/joa.12363

Paul, G.S., 2010, The Princeton Field Guide to Dinosaurs, Princeton University Press p. 232