Ankylosaur tail clubs are odd structures, odder than they are usually given credit for. They represent substantial modifications to two different skeletal systems – the endoskeleton, in the form of the caudal vertebrae, and the dermal skeleton, in the form of the caudal osteoderms. The centra of the caudal vertebrae lengthen but stay robust, and the neural arches undergo huge changes, such that the prezygapophyses, postzygapophyses, and neural spine become a robust, V-shaped structure on the top of the centrum, and which creates a tightly interlocking vertebral series with almost no flexibility. We call this the handle of the tail club. The osteoderms at the tip of the tail smush together and two of them become huge: although the tail club knob is small in some species, there are colossal knobs exceeding 60 cm in width. The ankylosaur tail club represents one of the most extreme modifications to the tail in terrestrial tetrapods.
Look at that thing. That is a weird thing.
(This is UALVP 47273, a really nice club that I studied for my MSc work on tail club biomechanics.)
One of the questions I became interested in during my MSc research on ankylosaur tail club biomechanics was how the tail club evolved in the first place. Most ankylosaurs with tail clubs are known from a relatively narrow slice of time right at the end of the Cretaceous, but when and where did the tail club first evolve? Did the stiffening of the tail occur before the enlargement of the tail osteoderms, or vice versa? Or did both changes happen at about the same time? This was a fun question to address during my PhD research, once I had a fairly well resolved phylogeny of ankylosaurids, and once I had looked at tons of ankylosaurid fossils.
So, how did the ankylosaur get its tail club? Well, based on what we see in the fossil record, it looks like the changes to the vertebrae predate the changes to the osteoderms – in other words, the handle comes first and the knob comes later. There is at least one ankylosaur out there that seems to have a tail club handle but not a knob: Gobisaurus!
Hello Gobisaurus! Many many thanks to my friend and colleague Sydney Mohr for preparing this awesome illustration of Gobisaurus for me.
Gobisaurus, a shamosaurine ankylosaurid, has a really nice complete tail club handle that is indistinguishable from other ankylosaurid tail club handles, but does not have a knob. And it’s not just because the knob is broken off – it seems as though the last vertebrae in the tail are preserved, because they look very similar to the terminal vertebrae in a CT scan of a tail club from the University of Alberta collections. It’s likely that Gobisaurus had osteoderms along the sides of the tail like we see in most other ankylosaurs, but it doesn’t appear that there were osteoderms tightly enveloping the tip of the tail.
An even earlier ankylosaur seems to show some changes towards acquiring a tail club handle, as well. Liaoningosaurus, a basal ankylosaurid known only from a very small juvenile, has distal caudal vertebrae where the prezyapophyses extend about 50% the length of the adjacent vertebra. This is what we see in ankylosaurid tail clubs, but not in more basal taxa like Mymoorapelta where the prezygapophyses are much shorter. Liaoningosaurus is missing the tip of the tail and also lacks osteoderms on most of its body because it’s a juvenile, so it’s harder to say whether or not it had a tail club knob based just on the fossil alone.
I also did a cool and relatively simple thing with my phylogenetic tree to see if I could better understand the likelihood that some ankylosaurs without preserved tail material had a tail club handle or full tail club with a knob. Unsurprisingly, all shamosaurine and ankylosaurine ankylosaurids probably had a tail club handle. Liaoningosaurus is part of a basal polytomy of ankylosaurids, and it was a bit more equivocal whether or not any of these taxa was likely to have a tail club handle or not, partly because another basal ankylosaurid in this region of the tree, Chuanqilong, does not have modified distal caudal vertebrae.
All ankylosaurine ankylosaurids more derived than Pinacosaurus (so including things like Tsagantegia, Saichania, Euoplocephalus, etc.) almost certainly had a tail club knob, and shamosaurine ankylosaurids probably did not. Crichtonpelta, the most basal ankylosaurine, may or may not have had a tail club – we’ll need more data to know for sure. There is amounted skeleton of Crichtonpelta at the Sihetun visitor center in Liaoning, and it is shown with a tail club, but it isn’t clear whether or not this is sculpted or original material belonging to this specimen, and a full description of this material is necessary.
Gobisaurus and Liaoningosaurus both lived much earlier than the more familiar tail-clubbed ankylosaurs: Gobisaurus is no younger than 92 million years old, and Liaoningosaurus is about 122 million years old. The earliest ankylosaurid with a tail club in the fossil record is Pinacosaurus(from the Campanian), although there is a caveat to this: Talarurus, which is a bit older than Pinacosaurus, should have a full tail club based on its position in the phylogenetic tree, and while a tail club handle is known for this taxon, we haven’t found a tail club knob for Talarurus. Talarurus is in kind of a weird spot phylogenetically, since it’s from Mongolia but comes out as closely related to North American ankylosaurines, so I think it’s worth keeping an eye on this taxon in the future – perhaps Talarurus is another taxon with only a handle and not a knob, which would fit a bit better with its chronologic position if not its phylogenetic position.
Regardless, the changes to the vertebrae of ankylosaurs, starting with Liaoningosaurus at least 122 million years ago and continuing on towards Gobisaurus about 92 million years ago, seem to have occurred long before ankylosaurs evolved a huge osteodermal knob at the end of the tail. Was a stiff tail as good a weapon as a full tail club with a knob? What drove the evolution of the knob so long after the evolution of a stiff handle? And why did ankylosaurs even evolve a tail club at all? Now that I’ve had fun investigating how ankylosaurs might have used their tails, and how the tail club evolved, the next question feels like it should be ‘why’….so stay tuned for more tail club fun over the next year or so as I make an attempt at that question!
Read it for yourself! Arbour VM, Currie PJ. In press. Ankylosaurid dinosaur tail clubs evolved through stepwise acquisition of key features. Journal of Anatomy.