Join the Club – How Ankylosaurs Evolved Their Formidable Tails

Everyone knows Ankylosaurus. It’s the epitome of the titular group of highly-armored herbivores to which it belonged, and there’s even a case to be made that it’s the best dinosaur. The living tank’s tail club certainly plays into that. It’s difficult to look at that knob of bone and not imagine it smacking into the leg of an attacking tyrannosaur or battering the side of another ankylosaur in a territorial dispute. But how did this famous piece of the dinosaur armament evolve?

North Carolina Museum of Natural Sciences paleontologist Victoria Arbour has quickly become one of the world’s foremost experts on ankylosaurs. In the past she’s helped sort out the number of ankylosaur species and looked at the biomechanical capabilities of those intriguing tail clubs. Now, along with Philip Currie, Arbour has retraced how ankylosaurs went from having flexible tails to stiffened handles supporting great protrusions of bone.

Not all ankylosaurs had tail clubs. The earliest of their kind, as well as the spiky forms called nodosaurids, lacked the famous feature. It was only among a subgroup called ankylosaurids that tail clubs evolved, and, as Arbour and Currie point out, this special appendage had two parts. There was the handle – a series of interlocking vertebrae at the end of the tail – and the knob, which is the actual bony business end.

A timeline of ankylosaur tail weaponry. Courtesy NCSU.
A timeline of ankylosaur tail weaponry. Courtesy NCSU.

Up until now, paleontologists have been unsure how the handle and knob evolved. Did the handle come first? The knob? Did they evolve together? A pair of ankylosaurs from Asia solve the puzzle. These two dinosaurs, named Liaoningosaurus paradoxus and Zhongyuansaurus luoyangensis, both had interlocking tail vertebrae that formed early versions of the handle but lacked tail knobs. What’s more, Arbour and Currie note,  Liaoningosaurus lived about 122 million years ago and Zhongyuansaurus was shuffling around about 92 million years ago, while the first ankylosaurs with fully-formed tail clubs evolved around 75 million years ago. The handle came first.

The handle-first model makes biomechanical sense. If ankylosaurs evolved a heavy club on a flexible tail, Arbour and Currie write, then it would have been more like a flail than a club, and more likely to injure the ankylosaur wielding it. It was only after the tail became a stiff, bat-like appendage did the terminal club knobs start to grow. Yet, as the paleontologists remind us, ankylosaurs had large osteoderms running along the sides of their tails all the way back to the earliest of their ilk, like Scelidosaurus. Ankylosaurs may have been waving their tails around for defense or display since their earliest days, and we’re just starting to understand the impressive variety of ways they did so.

For more, read Arbour’s post on her new paper at Pseudoplocephalus.

Reference:

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

5 thoughts on “Join the Club – How Ankylosaurs Evolved Their Formidable Tails

  1. I developed an interest into paleontology and its associated sedimentary geology while living in semi-retirement in Grande Cache, Alberta, Canada.
    It surely is fascinating.

  2. Fascinating article. I love seeing the evolutionary backstory of a organisms and their adaptation laid out and explained like this.

    Something I’ve been curious about ever since I learned that paleontologists had adopted Gary Larson’s “thagomizer” to describe stegosaur tail spikes: has anyone proposed a similar scientific term for ankylosaurid clubs? My brother Jay invented the word “oblagonge” (pronounced “OBE-la-GONJ”) as his personal term for the tail clubs. He admits that he was going for a word that was catchy more than etymologically significant, though he did choose the prefix “obla-” to invoke the word “obliterate” (bringing to mind the power and destructive potential of the ankylosaurid tail as a defensive weapon). However, I have noted (through some cursory etymological Google research) that the components of the word are reminiscent of the Latin words “oblongus” (“longish”), which seems appropriate for describing the shape of many ankylosaurid clubs, and “gōnia” (“angle”), which could be interpreted as a reference to the rigid shape of the tail and the fact that it was swung at an angle to the body (or even the perpendicular attachment of the knob to the “handle”). In any case, I just thought I’d throw Jay’s submission out there (with his permission). I’d love to hear if anyone else has any ideas.

  3. Oblagonge is a super fun word for a tail club! I’ve generally followed Walter Coombs’ terminology used in his 1995 paper on tail clubs – hence the ‘handle’ for the modified vertebrae, and ‘knob’ for the osteoderms at the tip of the tail. William Blows also proposed the term ‘caudorbitos’ for the knob in a 2001 paper.

  4. The ankylosaurs and the club handle are also a nice example of convergent evolution: the Glyptodonts with a club on the tail such as Doedicurus, Panochthus and Hoplophorus have a caudal tube which stiffens the distal end of the tail and is equivalent to the club handle in Ankylosaurs.
    The Meiolanid turtles lack the equivalent stiffening device on the tail and seem to only have spikes on the end of the tail and not a true club.

    LeeB.

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