What Was Ankylosaurus? The Living Tank of the Late Cretaceous
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The last of the armoured dinosaurs was eight metres long, as wide as it was tall, covered from the eyelids to the tail in plates of bone set into the skin, and carried at the end of its tail a knob of fused bone the size of a car tyre on a handle of stiffened vertebrae. Ankylosaurus lived in the same floodplains as Tyrannosaurus and Triceratops in the last few million years of the Cretaceous, and everything about it was built for one purpose, which was not to be eaten. It is known from a handful of specimens and no complete skeleton, and the animal that stands in museums is assembled from its relatives as much as from itself.
The animal
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
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 club
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.
How it lived
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.
Why it matters
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 takeaway
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.