Around 76 million years ago, in what’s now Alberta, Canada, there lived an ankylosaur
called Dyoplosaurus.
Today this area is rocky and dry, but when this animal lived there, it was a lush, coastal
environment striped with rivers -- perfect for big, plant-eating dinosaurs.
Dyoplosaurus was one of those large herbivores, measuring around 4.5 meters long, with a wide
body covered in thick, bony armor.
And at the end of its tail, there was something that would make it a very special discovery,
millions of years later: a well preserved club.
Now you might think that this piece of equipment is pretty typical for an ankylosaur. But when
this dinosaur was first discovered in 1919, nothing like it had ever been seen before.
Because! Dyoplosaurus was the first ankylosaur discovered with evidence of a tail club.
And even though we’ve known about them for more than a hundred years, we’ve only started
to get to the bottom of how they worked and how they developed in the
first place.
Strangely enough, it looks like the earliest ankylosaur tail clubs actually started out
… clubless.
Tail clubs are pretty rare in the animal kingdom -- no animals have them today -- but they’ve
popped up several times within the evolutionary tree of tetrapods.
For example, some glyptodonts had clubs. These giant relatives of today’s armadillos thrived
in the Americas from the Miocene to the Pleistocene Epochs.
Some very weird extinct relatives of turtles also developed tail clubs, as did a few groups
of dinosaurs.
The sauropods Mamenchisaurus and Shunosaurus had small clubs, but the most famous group
by far to have this unusual structure is the ankylosaurs.
Which are my personal favorites because they’re just totally metal, and 8 year old me just
wants to throw a saddle on one and ride it into battle.
Ankylosaur tail clubs were the biggest, at least proportionally, and they also required quite
a few changes to the dinosaur’s anatomy.
And while clubs are practically synonymous with ankylosaurs, most of these dinosaurs
didn’t actually have them.
True tail clubs were only around for the last 20 million years of the more than 100-million-year
reign of these armored dinos.
Now, the ankylosaur family tree can be split into two main groups, the ankylosaurids and
the nodosaurids.
All of these dinos had impressive armor, but none of the nodosaurids had clubs.
And, among the ankylosaurids, only a small, derived subset, the ankylosaurines, had them.
So what were they used for?
Pretty much from the beginning, it was assumed that the club was used as a weapon - either
for defending against predators or for fighting with other members of the species.
Some other hypotheses have been suggested over the years, like the idea that the club
functioned as a “false head” to confuse would-be predators, or that it could have
been used as a sexual display for potential mates.
But the weapon hypothesis always seemed to be the most plausible, based on both the formidable
anatomy of the clubs and the fact that other weaponized tails were known from dinosaurs
like Stegosaurus.
But no one actually tested this idea for nearly 85 years.
Then, in 2009, a paleontologist named Victoria Arbour studied the muscles in ankylosaur tails,
what they looked like, and how much force they could generate.
And, maybe not surprisingly, those muscles turned out to be pretty extreme.
Arbour studied the tail anatomy of ankylosaurs and compared it to that found in living archosaurs,
like crocodilians.
And she found that the muscles involved in swinging the tail ran the length of the mobile
portion of the tail, at its base.
And these were supported by other muscles that were anchored to the dinosaur’s wide
hips and thigh bones, and attached to the tops and bottoms of the tail vertebrae.
Altogether, these muscles gave the tail a massive cross section, which increased the
force that the muscles could produce.
Ankylosaurs also had tendons that had effectively turned into bone! These are known as ossified
tendons and they helped support the tail.
So by taking into account this musculature, the range of motion allowed by the tail vertebrae,
and the size and weight of the club itself, Arbour was able to calculate the impact force
that the club could produce.
We could finally answer with certainty whether ankylosaur tail clubs could’ve been used
as a weapon.
And the answer was a bone-shattering yes. Like literally.
The tail club of an adult Euoplocephalus could potentially generate impact forces up to 14,360
Newtons - it was capable of not only breaking, but pulverising bone.
And in fact, further work showed that larger clubs could, theoretically, generate such a forceful
impact that they were at risk of fracturing themselves.
The chonkiest of these bois were not messing around.
So, the tail clubs were powerful weapons, although we still don’t know whether they
were used for defense, to compete for mates or territory, all of the above, or maybe something
else.
But! While Arbour was working on how these clubs worked, she realized that their evolutionary
origin was also a mystery, and it was a mystery with two parts.
Because, the clubs of the ankylosaurines are made up of two components: a “handle”
and a “knob.”
The handle is formed from highly modified and interlocking or fused vertebrae that create
a rigid structure at the end of the tail.
The knob is made up of enlarged, fused osteoderms, or bones found in the dinosaur’s skin, that
also form the group’s signature body armor.
And this arrangement is really interesting, because it combines both the normal, internal
skeleton of the ankylosaur with the protective dermal skeleton to make a single structure.
This is part of what I find so fascinating about these guys. I mean, look at them. They’re
like tanks!
So, the question Arbour set out to answer was: Which part of the club evolved first?
The handle, the knob, or both at the same time?
If the handle evolved first, the first ankylosaurs that began to develop this structure would’ve
had a section of modified, rigid vertebrae at the end of their tails.
But if the knob evolved first, the tail would’ve been flexible - like it was in ankylosaurs
that didnt have clubs - and there would be some large growth at the tip.
And! If the handle and knob evolved together, then the first ankylosaurs with tail clubs
would’ve had evidence of both forming at the same time.
The only way for Arbour to figure out which of these hypotheses was correct was to scour
the fossil record for ankylosaurs with tails that matched one of the three options.
And, luckily, it turned out there were a couple that were already in museum collections.
For example, Gobisaurus was an early ankylosaurid that lived around 92 million years ago in
what is now China.
It was big, about six meters long and weighing several tonnes. And it had a very interesting
tail indeed.
It had all of the structural support for a tail club, with a stiff handle made of modified
vertebrae that looked pretty much the same as the handles of ankylosaurs that had full
clubs.
But, there was no knob at the end.
And the entire tail was preserved, including the very tip, so we know that it hadn’t
just lost its knob, say, during the process of fossilisation.
So Gobisaurus shows that the first hypothesis was correct: ankylosaurs developed the handles
first. And there looks to be more evidence even further
back in the ankylosaurid family tree, in another Chinese ankylosaur called Liaoningosaurus.
This dinosaur lived around 122 million years ago and also has evidence of that stiffening
of the tail that formed the handle, though to a lesser extent than Gobisaurus.
The parts of the vertebrae that interlock were much longer than those seen in earlier
ankylosaurs, but there was no sign of fusion between the vertebrae.
However, the only known specimen of this dinosaur with a tail is a tiny juvenile and the very
end is missing.
So it’s hard to say for sure whether it really didn’t have a knob on its tail or
if it just got lost.
Or it may not have grown in yet. Because, we know that the bony armor and tail clubs
of ankylosaurs developed and expanded as the animal grew.
But, given its position in the ankylosaurid tree and the appearance of its tail vertebrae,
it looks to be more early evidence that the handle evolved first.
And, according to Arbour, it seems that even a stiffened tail alone could still have delivered
a painful blow despite the lack of a knob.
It’s likely that enlarged osteoderms at the tip of the tail made that blow more effective.
So over time they were selected for, leading to what we now think of as the typical ankylosaur
tail.
So that’s how my favorite dinosaur got its club.
The fossil record is full of transitional forms, like Gobisaurus, that can help us solve
evolutionary mysteries like this, if we just think to ask the right questions.
In this case, we just had to ask, which came first: the handle or the knob?
Hey! Before you go, PBS Digital Studios and Eons want to hear from YOU. We do a survey
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Eons is your favorite!
You want to know what my favorite day is? National Fossil Day, Wednesday October 14th
--it's one of the most important holiday for paleo fans everywhere!
And to celebrate, the hosts of PBS Eons -- Kallie Moore, our new host Michelle Barboza-Ramirez,
and myself -- will be competing in a virtual scavenger hunt at the Smithsonian National
Museum of Natural History Deep Time exhibit.
Link to the livestream is in the description.
Big thanks to this month’s Eontologists for being part of the club:
Kallie...KALLIE!
Sean Dennis, Jake Hart, Annie & Eric Higgins, John Davison Ng, and Patrick Seifert!
We’d also like to take this time and give a shout out to the one and only STEVE who
was one of our first Eontologists in 2018. Thank you Steve for all your support through
the years. We’ll miss you!
Eonites get perks like submitting a joke for us to read like I’m going to do right now!
This one is from Elise Baker and I'm reading it cold. Here we go.
Ah a limerick:
There once was a T-Rex in shale... Who had been quite a dazzling male... His cute dino fuzz... Set the ladies abuzz... Not to mention the size of his tail.
Can we say this on YouTube?
And as always thank you for joining me in the Konstantin Haase studio. Subscribe at
youtube.com/eons for more adventures in deep time!