Stegosaurus, with its distinctive silhouette, remains one of the most recognizable dinosaurs from the Late Jurassic period, approximately 155 to 145 million years ago. This large, herbivorous quadruped is famously characterized by the prominent structures adorning its back and tail. These features have long fascinated paleontologists and the public, prompting inquiry into their identification and functions. Understanding these body parts provides insight into the life and behavior of this ancient reptile.
Naming the Stegosaurus’s Features
The large, bony, kite-shaped structures along the Stegosaurus’s back are known as dermal plates or scutes. These plates were not directly attached to the skeleton but were embedded within the animal’s skin, similar to osteoderms found in modern crocodiles. The largest plates could measure over 60 centimeters (24 inches) in both width and height. While early reconstructions depicted these plates lying flat or in a paired arrangement, articulated specimens indicate they stood vertically in two staggered, alternating rows along the dinosaur’s spine.
The spikes located at the end of the Stegosaurus’s tail possess an informal name: the “thagomizer.” This term originated from a 1982 Far Side comic strip by cartoonist Gary Larson, where a caveman lecturer humorously named the spikes “after the late Thag Simmons.” Despite its jocular origin, the term was adopted by paleontologists and is now widely used informally. Stegosaurus had four such spikes, each measuring between 60 to 90 centimeters (2 to 3 feet) long, protruding horizontally from the tail.
The Purpose of Plates and Spikes
The “thagomizer” on the Stegosaurus’s tail is widely believed to have served as a defensive weapon against predators. Fossil evidence supports this theory, including an Allosaurus tail vertebra discovered with a puncture wound that precisely matches a Stegosaurus tail spike, suggesting direct combat. The tail’s flexibility would have allowed Stegosaurus to swing its thagomizer with considerable force, delivering powerful blows to attackers. Approximately 10% of discovered Stegosaurus tail spikes show evidence of damage and subsequent healing, further indicating their use in defensive encounters.
The function of the large dermal plates on Stegosaurus’s back has been a subject of more varied scientific speculation. One prominent hypothesis suggests a role in display, either for attracting mates or intimidating rivals. The varied size, shape, and arrangement of plates across different stegosaur species support this idea, as such diversity often relates to visual signaling. The plates were likely covered in a keratinous sheath, which could have been brightly colored or able to change color, enhancing their visual impact.
Another significant theory proposes that the plates functioned in thermoregulation, helping the dinosaur control its body temperature. The plates contained numerous grooves and channels, indicating a rich network of blood vessels. By directing blood flow to these large, thin structures, Stegosaurus could have absorbed heat from the sun to warm up or dissipated excess heat to cool down, acting as a biological radiator or “solar panel.” Some researchers argue that the varied plate morphology among stegosaurs might not align with an exclusively thermoregulatory purpose, suggesting other factors were also at play.
A third hypothesis posits a defensive role for the plates. However, the plates were not rigidly attached to the skeleton and their placement left the dinosaur’s sides exposed, making them less effective as protective armor. The presence of many blood vessels also means significant bleeding would occur from even minor damage, further questioning their primary use for defense. Current scientific consensus largely favors display and thermoregulation as the more likely primary functions for these structures.