Many plant-eating dinosaurs, known as thyreophorans, were adorned with spikes, plates, and armor. These bony adornments came in many forms, including large back plates, sharp tail spikes, and full-body armor plating. The structures were integral to their survival in a world populated by large predators.
The Stegosaurids: Plates and Tail Spikes
The Stegosaurid family thrived during the Late Jurassic period, from about 155 to 145 million years ago. Its most well-known member, Stegosaurus, was a large dinosaur reaching up to 9 meters (30 feet) in length. It possessed a double row of large, kite-shaped plates that ran vertically along its back. These plates were not directly attached to the skeleton but were embedded in the skin.
The tail of Stegosaurus was equipped with four long, sharp spikes called a “thagomizer.” This weapon was held high, a posture enabled by its short forelimbs and long hind limbs. While Stegosaurus is the most iconic, other members like Kentrosaurus from Tanzania show variations. Kentrosaurus was smaller and had spikes running from its mid-back to its tail, plus a spike on each shoulder.
The Ankylosaurids: Nature’s Armored Tanks
Living during the Cretaceous Period, ankylosaurids took a different approach to defense. These low-slung, wide-bodied herbivores were covered from head to tail in bony armor plates called osteoderms. The most famous of this group, Ankylosaurus, lived between 68 and 66 million years ago in North America. Its skull bones were fused for increased strength, a feature that gives the genus its name.
The armor of Ankylosaurus included bony half-rings covering the neck and a mosaic of plates across its back. Its most distinct feature was a heavy club at the end of its tail, formed from fused osteoderms. This club was supported by a stiff handle of interlocked tail vertebrae, allowing it to be swung with force. The group was diverse, with relatives like Nodosaurus having extensive armor but lacking the tail club.
The Function of Spikes and Armor
The primary function of this armor was defense against large predators. Fossil evidence confirms a Stegosaurus could use its spiked tail to wound an attacker like Allosaurus. Similarly, an Ankylosaurus’s heavy armor made it difficult to attack, while its tail club could deliver bone-shattering blows.
Beyond defense, these features likely served other purposes. The distinct shapes of stegosaur plates suggest they were used for species recognition. Another theory for the large, blood-vessel-filled plates of Stegosaurus is thermoregulation, acting as radiators or solar panels. Some evidence also suggests ankylosaur tail clubs were used in combat between individuals for mates or territory.