What Is an Ankylosaur? Armour, Low Build and a Sideways Approach to Everything
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
A group of armoured plant-eating dinosaurs solved the problem of being hunted by becoming almost impossible to attack. Everything about them follows from that, including their gut, their nostrils and the way they held their heads.
The body plan
These animals were low, wide and heavily built, with short limbs, a broad body and an armoured covering of bony plates embedded in the skin across the back, flanks and even the eyelids. The head was armoured too, with plates fused to the skull surface producing a solid helmet. The stance was quadrupedal and the limbs were held under the body but were short, which kept the centre of mass low and made the animal difficult to overturn, and the width did the same. The group divides into two main branches, one carrying a bony club at the end of a stiffened tail and the other carrying prominent shoulder spikes and lacking the club. Sizes ranged from a couple of metres to around eight, and the largest were built like nothing alive.
The unusual features
Several details are peculiar even by the standards of extinct animals:
- •Convoluted nasal passages, looping through the skull in a complex path, which have been reconstructed digitally from scans
- •A broad barrel-shaped body implying an enormous gut, which suits fermenting low-quality plant material
- •A wide beak in some species and a narrow one in others, indicating different feeding strategies within the group
- •A brain that is small relative to body size even among dinosaurs, though the olfactory region is well developed
- •Armour that varies between species in arrangement and is diagnostic, though it scatters after death
- •Teeth that are small and leaf-shaped, suited to cropping rather than to processing, which raises the question of how they handled their food
What the nose was for
The looping nasal passages have attracted specific research and the answers are well supported. Airflow modelling through digital reconstructions indicates that the extended path acted as a heat exchanger, warming and humidifying inhaled air and recovering heat and moisture from exhaled air, which is efficient and which matters for a large animal. The same arrangement would have cooled blood travelling to the brain, which is a specific benefit given that a large body generates heat the brain cannot tolerate. An acoustic function has been proposed, with the passages acting as resonating chambers that would have given the animals distinctive calls, which is plausible and harder to test. And the well-developed olfactory region suggests smell mattered, though that does not require the looping itself. The likely answer combines heat exchange with whatever else the arrangement permitted.
How the armour was built
The covering is more sophisticated than a layer of plates. Individual osteoderms vary in size and shape across the body, with large keeled plates over vulnerable regions, smaller ossicles filling the spaces between them, and spikes at the shoulders and along the flanks in the groups that had them. In several species the plates over the hips fuse into a single rigid shield. Internally the plates are not solid bone but contain a network of collagen fibres arranged in layers at angles to each other, which is a composite structure resembling engineered materials designed to stop cracks propagating, and testing of fossil material has confirmed that the arrangement resists penetration far better than plain bone of the same thickness would. That structure is the most direct evidence available that the armour was genuinely defensive, since no other function requires crack resistance.
Living behind armour
The whole arrangement represents a strategy with identifiable costs. Carrying that much bone is expensive to grow and to move, and the animals were correspondingly slow, which means armour replaced escape entirely rather than supplementing it. A low wide body cannot reach high vegetation, so the diet was restricted to whatever grew near the ground, and the large gut and the fermenting strategy follow from making a living on low-quality material. The defensive posture was presumably to crouch and present the armoured surface, which works against a predator and works against nothing else. Specimens are frequently found upside down, which has been attributed to the shape causing carcasses to roll over in water, and that accident of preservation means the armoured back is what is usually exposed for collection. The strategy evidently worked, since the group persisted to the end of the Cretaceous.
The takeaway
Low, wide, short-limbed and armoured across the back, flanks and eyelids, with plates fused to the skull. Convoluted nasal passages acted as a heat exchanger and probably cooled blood going to the brain. Armour replaced escape rather than supplementing it, which forced a low-growing diet and a large fermenting gut. Carcasses frequently rolled and are found upside down.