Introducing ‘Zuul,’ an Ankylosaur That Could Really Make Your Ankles Sore

A finely preserved fossil sheds new light on the curious tail of armored dinos

An illustration of the spiky new dinosaur Zuul. (Danielle Dufault)
smithsonian.com

It didn’t take paleontologist Victoria Arbour long to come up with a name for the latest armored dinosaur to plod onto the scientific scene. It was obvious: The ’saur just had to be named Zuul.

Between the armored dinosaur’s prominent horns and blunt snout, Arbour immediately saw a resemblance to the monstrous gatekeeper from 1984’s Ghostbusters. “I half-jokingly said that it really ought to be Zuul,” she says, recalling batting around names for the newly-discovered, 75 million year old ankylosaur with Royal Ontario Museum colleague David Evans. Evans immediately agreed, and now science welcomes a creature with one of nerdiest names ever selected for the ranks of Dinosauria. 

But there’s far more to Zuul than a cheeky name. The dinosaur was spectacularly preserved, including its skull, part of its skeleton and its iconic club tail, Arbour and Evans report today in the journal Royal Society Open Science. Thanks to that preservation, Zuul offers paleontologists a closer look at how armored dinosaurs may have arrayed their spiky ornamentation for defense.

Zuul ranks among the ankylosaurids—prickly, club-tailed dinosaurs that experts like to call “living tanks.” Their tubby, low-slung frames were dotted by distinctive patterns of pointed bones—called osteoderms—that acted as ornamentation as well as possible defense against the tyrannosaurs of their time. These animals were so well-protected that, in some species, even the eyelids were armored. 

Finding a fossilized skull alone would have made Zuul easy enough to tell apart from its relatives. While the new dinosaur—discovered in the Judith River Formation of northern Montana—shares some similarities in skull shape with ankylosaurs from Montana and Alberta, Arbour says, “Zuul would be easy to recognize based on the shape of the horns at the back of the skull and by the rough, peaked ornamentation along the snout and in between the eyes on the forehead.”

But the new finding included much more than just the skull: It was attached to a partial skeleton and tail club, making it the most complete dinosaur of its kind yet discovered in North America. “In North America, ankylosaurids form only about five percent of the dinosaur fauna” Evans says. The discovery of new species like Zuul, therefore, offers key pieces to the puzzle of how the continent’s dinosaur communities evolved.

Even better, this fossil preserves skin impressions and some of those armor plates held in their original place. “Because the armor plates are in the skin, they often fall away from the skeleton after death as the animal is decomposing,” Arbour says. In Zuul they stayed put, with fossils of skin and the keratin sheaths that covered the bony armor to boot—offering a rare glimpse at what this dinosaur really looked like. 

Despite being an herbivore, Zuul would have had cut a rather imposing figure. Ankylosaurids had highly modified tails to carry the hefty tail clubs at the end, which researchers thinks they swung at the limbs of attackers with painful precision. (Arbour has gone so far as to catalog depictions of ankylosaurs swatting their foes, from children’s books to Jurassic World.) The interlocking vertebrae near the end of the tail had a thin V-shape, buttressed along their length by ossified tendons to create what paleontologists call the “handle” for the tail club. 

Like other ankylosaurids known from exceptional tails, Zuul also had additional armor running almost all the way to the club. Broad triangular spikes just out from both sides of the tail, giving this dinosaur a rather sharp look. No wonder that Arbour and Evans decided to give Zuul the species name crurivastator—meaning “shin destroyer”—in recognition of the dinosaur’s potential to be a literal pain to the tyrannosaurs of its time. 

“I’ve been itching to name an ankylosaur ‘ankle breaker’ for years, but was waiting for a specimen that included a nice tail club,” Arbour says, “and they don’t get much nicer than this.”

But did Zuul actually use its sledgehammer tail to break the bones of its attackers? We aren’t sure. In a previous study, Arbour surveyed ankylosaur skeletons for signs of healed fractures that would support the idea that these dinosaurs were using their unusual anatomy to defend themselves. Of the few injuries that turned up, none conclusively supported the tail-clubbing hypothesis. If ankylosaurs were whacking other dinosaurs, their skeletons haven’t turned up the crucial evidence yet.

Nevertheless, Arbour says, there’s still good reason to think that all that ornate armor had a practical use. Some tyrannosaur skeletons show healed shin fractures, Arbour says, which could be signs of reprimands delivered with sledgehammer force by the ankylosaurs they lived alongside. 

Then again, maybe ankylosaurs were more worried about each other than predators. “When we think about animals alive today,” Arbour says, “most animals with specialized weapons like horns or antlers use them for fighting members of their own species, so it’s also possible that Zuul turned its tail club on other ankylosaurs when fighting for mates or territory.” 

Whether paleontologists will ever know for certain what ankylosaurs used their tail clubs for is a secret still held by the fossil record. All the same, Zuul offers one of the closest looks at an ankylosaur as a living animal and not a jumble of bones. Arbour and Evans report that the dinosaur’s torso was wrapped up in a block of stone weighing over 15 metric tonnes. From what paleontologists may still learn from this skeleton, Zuul may become a new gatekeeper for our understanding of the most ornate dinosaurs to walk the Earth.

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