The image of a massive herbivore with striking, bony structures running down its spine is instantly recognizable. This iconic silhouette represents one of the most unique body plans in the entire dinosaur lineage. The creature, known for its rows of plates and spikes, is perhaps the most famous armored dinosaur that ever lived. Its distinctive features raise questions about its purpose and life in the ancient world.
Identifying the Stegosaur
The dinosaur most strongly associated with large, prominent spikes and plates on its back is Stegosaurus (meaning “roof lizard”). This heavily built, four-legged herbivore belonged to the order Ornithischia and the family Stegosauridae, a group defined by their armor plating. Fossil evidence places Stegosaurus firmly in the Late Jurassic period, approximately 155 to 145 million years ago. Its remains have been primarily recovered from the Morrison Formation in the western United States, though specimens have also been found in Portugal. The sheer abundance of fossil material makes Stegosaurus one of the most recognizable dinosaurs from this era.
Description of Plates and Thagomizer
The dorsal features of Stegosaurus are composed of two distinct types of bony structures: the plates and the tail spikes. The plates are large, flattened, kite-shaped growths known as osteoderms, which are bony deposits that form within the skin. These structures were not directly attached to the spinal column but were anchored within the thick hide of the animal. They ran in two rows down the neck, back, and hips, arranged in an alternating, staggered pattern rather than parallel pairs.
At the end of the long, muscular tail are the notorious tail spikes, often referred to as the thagomizer, a term popularized by a cartoon. Stegosaurus typically possessed two pairs of these sharp, elongated spikes extending horizontally from the tail’s tip. These spikes were also highly modified osteoderms and were likely covered in a sheath of keratin, the same tough protein found in fingernails and horns.
Scientific Theories on Function
Thermoregulation
The purpose of the large dorsal plates has been debated among paleontologists for decades. One prominent theory suggests the plates functioned as a sophisticated thermoregulation system. Fossil evidence shows the plates were highly vascularized, meaning they contained numerous channels for blood vessels to pass through the bone. This extensive blood flow would have allowed the dinosaur to absorb heat from the sun to warm up, or dissipate excess heat by circulating blood near the surface, acting as large biological radiators.
Display and Recognition
A second widely accepted hypothesis posits that the plates served as a species recognition and display mechanism. The varied size and unique shape of the plates across different stegosaur species support the idea that they were used as visual signals. They could have been used to attract mates or intimidate rivals. It is also possible that the keratin sheath covering the plates was brightly colored or capable of changing color through flushing of blood, enhancing their use in display.
Defensive Weaponry
In contrast to the plates, the function of the tail spikes is more clearly understood to be defensive. Evidence from a fossilized Allosaurus tail vertebra shows a puncture wound perfectly matching a Stegosaurus spike, strongly suggests their use as a weapon. The animal’s tail was stiffened but flexible enough to be swung powerfully, allowing the thagomizer to inflict serious damage upon predators like Allosaurus.
Other Armored Dinosaurs
While Stegosaurus is the most famous spiky-backed dinosaur, other armored species also featured similar defensive structures. The Stegosauridae family itself includes other genera, such as Kentrosaurus, which lived in what is now Africa during the Late Jurassic. Kentrosaurus featured smaller plates near its neck and back that transitioned into much longer, more pronounced spikes extending along its tail and even on its shoulders.
The Ankylosauridae family represents a different lineage of armored dinosaurs and also had extensive armor, though it was generally flatter and more shield-like. Dinosaurs like Ankylosaurus and Euoplocephalus were covered in hundreds of bony scutes and smaller spikes, offering passive defense across their entire backs and flanks. These creatures often lacked the tall, upright plates of Stegosaurus and instead relied on a low-slung, impenetrable shield of fused bone, sometimes culminating in a massive, bony club at the end of the tail for active defense.