What Was Ankylosaurus? The Living Tank of the Late Cretaceous

Ankylosaurus magniventris, the fused lizard with the great belly, was named in 1908 by Barnum Brown from a specimen found in Montana, and it is the largest of the ankylosaurs, a family of low, wide, four-legged plant-eaters that appeared in the Jurassic and spread to every continent. It was about eight metres long, one and a half high and two wide, and weighed somewhere between five and eight tonnes; its skull was a wide triangle a metre across, with the nostrils opening forward, a beak, small leaf-shaped teeth for cropping soft plants, and a convoluted airway through the snout that may have warmed the air, improved the sense of smell or resonated its calls. Its legs were short and its belly, as the name says, was broad, and it moved at a walk.

The armour was made of osteoderms, plates of bone that grow in the skin, as they do in crocodiles, and were covered in life by a sheath of keratin that made them larger and sharper than the fossils:

• •Rows of oval plates across the back and flanks, from a few centimetres to over thirty across, with a keel along each, set in a mosaic of small bony pebbles that filled the gaps
• •Horns projecting from the back corners of the skull and the cheeks, and plates fused to the skull itself so that the whole head was armoured, including the eyelids
• •A neck protected by half-rings of fused plates
• •No armour on the belly, which is the one place the tank was soft and the reason a predator would have needed to flip it, which at six tonnes it could not

The tail's last vertebrae were fused into a stiff handle, wrapped in tendons that had turned to bone, and at its end sat the club, two large osteoderms fused with the vertebrae into a lump that in Ankylosaurus was about 60 centimetres wide. The handle let the base of the tail swing the club through an arc, and a 2009 study of its mechanics found that a full-sized club could be swung fast enough to break bone, and that the bone of the club was solid enough to survive doing so; a smaller club on a smaller animal would have bruised. The likeliest target is the shin of a tyrannosaur, which stood on two legs and would have gone down with one broken, and a 2022 study of another ankylosaur with healed damage to its own flank spikes suggested that the clubs were also used on each other, in contests between rivals, as the horns of Triceratops were.

The ankylosaurs were low browsers, cropping ferns, cycads and the flowering shrubs that grew along the rivers, and the wide body held a fermenting gut that processed tough plants slowly; the small teeth, worn on the sides rather than the tips, show that the jaws moved back and forth as well as up and down, which is more chewing than most dinosaurs did. Ankylosaurus is rare in the rocks compared with the duck-billed and horned dinosaurs it lived among, which suggests it lived inland or in the uplands rather than on the coastal plains where most fossils formed, and it was probably solitary. A relative found in Alberta in 2011, Borealopelta, was so well preserved that its skin, its keratin sheaths and the reddish-brown colour of its back, darker above and lighter below in the countershading of a hunted animal, survive, and the last meal in its stomach was mostly fern.

Ankylosaurus is the end point of a hundred and forty million years of dinosaurs growing armour against predators that grew teeth to match, and it is the extreme case of an animal that answered the problem of being eaten by refusing to be, at the price of speed and of everything else. It was among the last dinosaurs alive when the asteroid struck, and the club, the plates and the countershading of its cousin are among the best evidence there is of what the arms race between the Cretaceous's plant-eaters and its predators had reached. The complete skeleton has yet to be found.

The armoured dinosaurs are divided into two main branches, and pictures often blur them together. Ankylosaurus belongs to the ankylosaurids, which had broad, wide skulls with horns at the corners and the famous bony tail club. The other branch, the nodosaurids, had no club at all. Their tails simply tapered, their skulls were narrower and longer, and many carried large spikes projecting from the neck and shoulders instead. Borealopelta, the beautifully preserved armoured dinosaur from Alberta, was a nodosaurid, as were Edmontonia and Sauropelta from North America.

The two groups seem to have met predators in different ways. A clubbed ankylosaurid could strike at the legs of an attacker behind it, while a nodosaurid's shoulder spikes suggest it faced threats head-on or crouched low, protecting its unarmoured belly. Both groups belong within a larger family of armoured plant-eaters called thyreophorans, which also includes the stegosaurs with their plates and tail spikes. So when a toy or a film shows Ankylosaurus with long spikes jutting from its shoulders, it has probably borrowed features from a nodosaurid cousin rather than from the animal found in Montana.

The tail club did not appear all at once. Comparing ankylosaurids from different times, palaeontologists have found evidence that the stiff handle came first: some earlier members of the family had tails whose last vertebrae were already locked together by interlocking joints and bony tendons, but with no large knob at the end, or only a small one. The heavy club seems to have been added later, as the osteoderms at the tip grew and fused. Young animals tell a similar story, since juvenile ankylosaurids from Mongolia have stiffened tails but only small knob bones.

A tail weapon also evolved separately in a very different armoured dinosaur. Stegouros, described in 2021 from Chilean Patagonia, belonged to an early-branching southern group of ankylosaurs and had a short tail ending in a flat, frond-like weapon made of seven pairs of bony blades, which its describers compared to the Aztec obsidian-edged club called a macuahuitl. It was a separate experiment that reached a similar answer. Clubbed and bladed tails arose independently on opposite sides of the world, a sign that a strong swinging tail was a useful defence for a heavy, slow animal.

In 2011 a mechanical digger operator working in an oil sands mine in Alberta, Canada, struck a remarkable fossil: the front half of an armoured dinosaur preserved in three dimensions, with its plates and spikes still in place and traces of its skin. It belonged to a nodosaur, a close relative of Ankylosaurus without a tail club, later named Borealopelta.

The animal had apparently been washed out to sea and sank, landing on its back and being buried quickly. Preparing it took several years. In 2017 researchers reported chemical traces of pigment showing that it was reddish-brown on top and paler underneath, a pattern called countershading that camouflages animals today, which suggests even a heavily armoured plant eater still needed to hide from predators.

Ankylosaurus was an eight-metre, six-tonne armoured dinosaur of the last few million years of the Cretaceous, covered in keratin-sheathed bone plates from its eyelids to its flanks, with an unarmoured belly and a tail whose fused vertebrae formed a handle for a bone club heavy enough to break a tyrannosaur's leg and perhaps used against rivals too. It browsed low plants slowly, was rare and probably solitary, and it was the last and largest of a family that had been growing armour since the Jurassic.