The structures on many dinosaurs—commonly called “spikes”—represent a diverse collection of anatomical features that served multiple purposes. These projections differ in composition, location, and scientific names. They are generally composed of bone that forms within the skin or are extensions of the animal’s main skeleton. Understanding these differences requires exploring the specialized terminology paleontologists use to classify these features.
Bony Projections and Dermal Armor
The generalized term for bone structures embedded in the skin is osteoderm (literally “bone skin”). Osteoderms are bony deposits formed within the dermal layer of the skin, similar to the plates found in modern crocodiles or the armor of armadillos. These structures are not directly connected to the animal’s main skeleton, arising instead from the skin itself. They vary widely in shape and size, forming the basis for the defensive and ornamental architecture of many armored dinosaurs.
Ankylosaurs and Stegosaurs possessed these osteoderms. In Ankylosaurs, osteoderms manifested as small, flattened, often keeled plates called scutes, arranged in rows over the body to create a continuous shield. A scute is a bony external plate or scale typically overlaid with a thick layer of keratin. This combination created a highly effective form of body armor.
Stegosaurus featured large, distinctive structures along its back and tail, which are modified osteoderms. The kite-shaped structures running down the back are called plates, which were thin, highly vascularized, and stood upright in two staggered rows. These plates are anatomically distinct from the thick, defensive scutes of Ankylosaurs, having grown primarily outward. The pointed projections on the Stegosaurus tail, however, are true spikes and are also specialized osteoderms.
The internal structure of the large Stegosaurus plates features a thin wall of bone surrounding a large cancellous, or spongy, region. This construction makes them relatively light but suggests they were too fragile to be purely defensive armor. The outer surfaces of all these bony structures would have been covered by a sheath of keratin. This keratin sheath, the same material that forms human fingernails, would have extended the length and sharpness of any pointed projection.
Specialized Appendages and Unique Terminology
Beyond dermal armor, certain structures have acquired specialized names standard in paleontology. The array of tail spikes on the end of a Stegosaurus tail, usually four to ten in number, is known collectively as the thagomizer. This term originated not from a formal scientific description but from a 1982 comic strip by cartoonist Gary Larson. Coined as a joke, the term was later adopted by paleontologists and is now an informal but widely accepted anatomical term.
Ceratopsians, such as Triceratops and Styracosaurus, are known for impressive cranial ornamentation. The large bony shield extending over the neck is called the frill, formed from the parietal and squamosal bones of the skull. The frill is often edged with smaller, pointed bones called epoccipitals or epiossifications, which contribute to a serrated appearance. These projections are technically part of the skull structure.
The pointed structures on the face of a Ceratopsian are called horns, distinct from dermal osteoderms. These horns consist of a bony core that is an extension of the skull bone, covered by a thick, sharp keratin sheath in life. The diversity in horn and frill shapes across this group is remarkable, with species like Styracosaurus having numerous large horns projecting from its frill. The distinct anatomical origins of these features emphasize that “spike” covers many different evolutionary solutions.
Evolutionary Purpose of Dinosaur Spikes and Plates
The variety of spikes and plates suggests they evolved to fulfill several functions. Defense against predators is the most obvious function, especially for the tail weapons and body armor of ankylosaurs and stegosaurs. The tail spikes of Stegosaurus, the thagomizer, were likely swung at attackers, supported by evidence of puncture wounds found in the bones of predators like Allosaurus. The thick, fused armor and tail clubs of later ankylosaurs were designed to repel the bites of large theropods.
Elaborate structures, such as the large frills and complex horns of Ceratopsians, are thought to have evolved primarily for sexual or social display. The wide variation in frill and horn patterns among closely related species suggests their purpose was species recognition and attracting mates. These features would have advertised the dinosaur’s maturity and vigor to potential partners or helped intimidate rivals during intraspecific competition.
A third proposed function, particularly for the large, thin plates of Stegosaurus, is thermoregulation, or the regulation of body temperature. The internal structure of the plates shows they were highly vascularized, meaning they contained numerous blood vessels close to the surface. This design could have allowed the dinosaur to absorb heat quickly by orienting the plates toward the sun or to dissipate excess heat by moving blood through the plates. While the thermoregulation hypothesis is debated, it is possible that these structures served multiple functions simultaneously.