The Armoured Dinosaurs--The Ankylosaurs

In my last article I discussed the group of armored dinosaurs known as the Nodosaurs, in this aricle, I will be discussing the Ankylosaurs. The Ankylosaurs first appeared during the mid Cretaceous Period, becoming more abundant toward the late Cretaceous Period (140 to 65 million years ago), and went extinct at the end of the Cretaceous Period. They mostly replaced their relatives the Nodosaurs (who first appeared during the mid to late Jurassic Period (195 to 140 million years ago) and survived until the late Cretaceous Period) in western North America and East Asia.

The name Ankylosaur means "fused reptile," and refers to the fact that they had bones in their skin that were fused together into shield-like pieces of armor plating. Like the Nodosaurs, the Ankylosaurs were heavily armored along their backs with thick plates and spikes. The armor on their heads was more extensively developed than that of the Nodosaurs, and was welded into large slabs of bone. They had a large ball of fused bone supported by ossified tendons at the tip of their tails, which could likely be swung from side to side like a club as defense against an attacking carnivore with potentially lethal effect. In the later species, their armor became consolidated into an inflexible carapace.

To support their heavy clubbed tails, the Ankylosaurs were shorter and stockier than the Nodosaurs, with massive bones in their hips and hind-legs. Their hips were fused to their backbones with at least eight sacral vertebrae. To support their tails, their hip bones changed greatly from the classic ornithischian dinosaur format.

The best known of the Ankylosaurs is Ankylosaurus, whose name means "fused reptile", the Family Ankylosauridae name is derived from this species. Ankylosaurus lived during the late Cretaceous Period in North America; at 33 feet (10 meters) in length and weighing 4 US tons (3.6 tonnes), it is the largest known Ankylosaur, and was one of the last Ankylosaurs to survive.

Euoplocephalus is the only other known North American species of Ankylosaur, its name means "well-armoured head." It lived during the late Cretaceous Period, and reached 18 feet (5.5 meters) in length. It is one of the best studied species of Ankylosaur. Euoplocephalus was heavily armoured, even to the point of having armoured accessory eyelids to protect its eyes. Euoplocephalus had a horny, toothless beak, leading many paleontologists to believe that it was a generalist herbivore, cropping vegetation indiscriminately, while other species of Ankylosaur were more selective feeders.

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