Ankylosaurs: The dinosaurs with built-in armour

But what purpose did this armour serve and what sets ankylosaurs apart from other armoured dinosaurs such as stegosaurs?

Our researcher and armoured dinosaur expert Susannah Maidment is here to tell us more.

What is an armoured dinosaur?

Ankylosaurs are members of Thyreophora, a diverse group of dinosaurs characterised by their body armour. The name Thyreophora comes from the Greek for ‘shield-bearer’. Within this group, ankylosaurs form one of the two main subgroups, the other being Stegosauria. These dinosaurs lived from the Mid Jurassic Period through to the end of the Cretaceous Period.

“Armoured dinosaurs are a group of dinosaurs that are characterised by having bony plates or bony ossifications, which are generally embedded in the skin,” explains Susannah.

“Imagine a crocodile or alligator that has bony plates and scutes on their backs. This would have been the same with armoured dinosaurs.”

What was their armour made of?

“Their armour is bone and that’s why we have it preserved, otherwise it would’ve rotted away. There would’ve been some sort of keratinous covering – something like the material our fingernails are made of – covering those bones,” Susannah explains.

“We know this because if you look at the plates, you can actually see where blood vessels coursed across the surface. Presumably, because there was a blood supply going towards this proteinaceous covering, it allowed it to grow.”

Although the covering of the plates isn’t normally preserved, one ankylosaur specimen called Borealopelta has keratinous scales overlying the bony armour osteoderms preserved. Not only this, but its stomach contents were also found, revealing details of its last meal.

What did ankylosaurs use their armour for?

We don’t know exactly why ankylosaurs evolved their armour, but there are several theories. In living animals, bony plates often serve multiple purposes. Alligators, for example, use them for protection, to help stabilise their spine on land, as calcium stores for producing eggshells and even as a way of regulating their temperature.

“I think the reason that armoured dinosaurs evolved this armour is most probably for defence. But it could have been multifunctional. They might have used these plates for some sort of display and as a way to make themselves look bigger and more scary. It could be a visual deterrent for predators or a way of attracting a mate,” says Susannah.

As for their tail club, for a long time this was assumed to be purely a defensive weapon. The discovery of some fossils of the genus Zuul, however, suggested that ankylosaurs may also have used their tail clubs in combat with one another. Predator attacks typically leave wounds scattered randomly across the body, whereas the injuries on Zuul were concentrated along the sides. This led scientists to believe that another ankylosaur may have caused the wounds.

What did ankylosaurs eat?

Ankylosaurs were herbivores. We know this because they have small, triangular teeth instead of big, sharp teeth for slicing through meat.

Susannah explains, “some ankylosaurs also evolved quite complex jaw mechanics that allowed them to move their jaws side to side or forward and backwards, and that would’ve helped grind plant material in their mouths.”

How easy is it to distinguish between ankylosaurs and stegosaurs?

Ankylosaurs and stegosaurs both belong to the clade Thyreophora, but there’s some distinct differences between the two groups.

“Within the armoured dinosaurs there’s two groups” explains Susannah. “There are the ankylosaurs, which are wide bodied, short limbed and covered in armour, which is all over their backs, down parts of their limbs and their skulls. The other group, stegosaurs, have two rows of plates or spikes down their backs, and they’ve also got spikes at the end of their tails.”

“Anatomically, they’re very different from each other. Generally, you can pick up the bone of an ankylosaur and tell it’s from an ankylosaur. Some of their armour is a bit similar, so sometimes we find tail spikes of stegosaurs, and some ankylosaurs have very similar spikes, and they can be a bit difficult to tell apart, but generally their limb bones are very distinctive from each other. Their vertebrae are relatively distinctive from each other as well, so we can usually tell these apart quite easily.”

Did ankylosaurs and stegosaurs live at the same time?

We know that stegosaurs and ankylosaurs shared a common ancestor about 170 million years ago. There’s an early record of an armoured dinosaur called Scelidosaurus, that sits on the evolutionary tree before stegosaurs and ankylosaurs split from each other.

“The earliest members of both of those groups are from the Mid Jurassic Period, and then they diversified. Stegosaurs became common in Jurassic ecosystems, particularly in the northern hemisphere. They’re best known from North America, Europe and China,” says Susannah.

“Ankylosaurs didn’t get going until the Cretaceous Period, and this coincides with the stegosaurs starting to decline in diversity. The stegosaurs went extinct by the end of the Early Cretaceous Period, about 100 million years ago, and the ankylosaurs then really took off.”

In the Late Cretaceous Period, ankylosaurs in North America were living alongside dinosaurs such as Tyrannosaurus and Triceratops. They lived right through to dinosaurs going extinct, which was at the end of the Cretaceous Period, 66 million years ago.

What ankylosaurs have been discovered?

Three ankylosaurs from the Early Cretaceous Period have been found in the UK, in Surrey, Sussex and the Isle of Wight. One of these, Hylaeosaurus, which was discovered in West Sussex, was the first ankylosaur to ever be found. This specimen was one of three fossils on which Sir Richard Owen based his new group Dinosauria in 1842.

In 1906, the first bones of the species Ankylosaurus were discovered in Montana, USA. Ankylosaurus is arguably the most famous ankylosaur – its name comes from the Greek for ‘fused lizard’.

The oldest ankylosaur ever found is Spicomellus afer. It was discovered in 2021 by Susannah and her team in Morocco. Remarkably, unlike other ankylosaurs that have been found, the spikes on the back of Spicomellus were actually fused to its ribs, instead of being embedded in its skin. It had a collar of spikes around its neck, with some being an extraordinary 87 centimetres long. This was also the first ankylosaur to be found in Africa.

What ankylosaurs can you see here at the Museum?

We’re home to the world’s most diverse collection of armoured dinosaurs. In our Dinosaurs Gallery, you can see a specimen of the ankylosaur Scolosaurus. It was discovered in Alberta, Canada, and it’s between 83 and 76 million years old.

Behind the scenes, as part of the collection we care for, we have 10 ankylosaur specimens, including Spicomellus afer and Polacanthus foxii – one of the ankylosaurs that was discovered in England.

These specimens continue to help researchers like Susannah to study and understand these incredible dinosaurs.