The term “spike” is a general description for the bony and keratinous protrusions on many dinosaurs, but it is not a precise scientific name. These structures served various purposes, including defense, species recognition, and potentially thermoregulation. Because they evolved independently in different dinosaur groups, these features have specific anatomical names that reflect their unique composition and location. These specialized names distinguish between armor embedded in the skin, weaponized tail appendages, and cranial display structures.
Dermal Armor: Osteoderms and Scutes
Many features commonly referred to as dinosaur “spikes” are actually variations of bony deposits embedded in the skin, known scientifically as osteoderms. The word osteoderm literally translates to “bone skin,” describing these structures that develop within the dermis layer of the skin, much like the armor of modern crocodiles or armadillos. Osteoderms can range in form from simple, small ossicles to large, fused plates and prominent spikes that jut out from the body.
The most famous examples of osteoderms forming extensive armor are found in the Ankylosaurs, the “armored lizards.” These dinosaurs were covered from head to tail in a mosaic of bony knobs and plates, which sometimes included sharp, cone-shaped spikes protruding from their flanks. The entire arrangement functioned as a heavy, passive defense against predators.
Often, the bony osteoderm was covered by a layer of keratin, the same tough protein that makes up human fingernails and hair. This keratinous covering is properly called a scute. The scute would have made the exterior of the spike or plate even larger and sharper than the fossilized bone core suggests. Ankylosaur spikes were a combination of a bony osteoderm core and a keratinous scute sheath, creating a formidable defensive shell.
Specialized Defensive Appendages
Some of the most iconic “spikes” are specialized structures at the end of the tail, primarily used as active weapons. The four to eight long, pointed tail spikes found on Stegosaurs, such as Stegosaurus, are collectively known as the Thagomizer. These modified osteoderms were highly weaponized and believed to have been swung at predators.
The term Thagomizer has a unique origin, as it was not initially a scientific term. It was coined in 1982 by cartoonist Gary Larson in a Far Side comic strip, where a caveman professor names the spikes “after the late Thag Simmons.” Despite its humorous beginning, the term was adopted by paleontologists and is now universally accepted to describe this specific Stegosaur tail structure.
Evidence suggests the Thagomizer was a highly effective weapon. Studies of tail spikes show a high incidence of trauma-related damage, and a fossilized Allosaurus tail vertebra was found with a puncture wound that perfectly matches a Stegosaurus spike.
Another specialized tail weapon is the Tail Club, found in Ankylosaurid dinosaurs. This massive structure is a bony knob composed of enlarged and fused terminal osteoderms. The club was supported by a rigid “handle” formed by the stiff, interlocking vertebrae of the distal tail. Studies have shown that a large Ankylosaurid tail club could generate enough impact force to potentially break the leg bones of a large predator like Tyrannosaurus rex. This weapon appears to have been used both for defense against predators and possibly for intraspecific combat, such as fighting rivals for mating rights or territory.
Keratinous Structures: Horns and Crests
Cranial protrusions, like the famous weaponry of Triceratops, represent a different category of dinosaur “spikes.” These structures, commonly called horns or crests, are bony outgrowths of the skull itself, distinct from the osteoderms embedded in the skin elsewhere on the body. They are most prominent in the Ceratopsians, or “horned dinosaurs.”
The bony core of a dinosaur horn or crest was covered by a thick sheath of keratin, similar to the horns of modern cattle or sheep. A rough or rugose texture on the fossilized bone surface indicates that it anchored this external keratin sheath. This keratin layer would have significantly increased the effective length and sharpness of the structure.
Because keratin rarely fossilizes, the preserved bone only represents the base of the structure, meaning the horns and frills of Ceratopsians were much larger and more elaborate in life. These large cranial modifications were likely used for defense, but they also served as important visual signals for communication and attracting mates. The shape and size of these ornaments often differed between species, highlighting their role in display and rivalry.