Stegosauria

Stegosauria is one of the two primary subgroups of Thyreophora, the armoured ornithischians, with Ankylosauria being the other. They are defined by two parallel rows of dermal plates or spikes along their backs. These structures were supported by isolated dermal bone rather than being connected directly to the skeleton. Most members also possessed powerful tail armament. This cluster of spikes is informally known as a "thagomizer," a term coined in a 1982 Far Side cartoon and later adopted by the scientific community.

The stegosaurian body plan was notably asymmetric. Their hindlimbs were much longer than their forelimbs, resulting in a posture with a low front and high hips. They had small, narrow skulls and beaks, with leaf-shaped teeth limited to the rear of the jaw. Compared to the sauropods they lived alongside, stegosaurs were relatively small. The clade is divided into the basal Huayangosauridae, such as Huayangosaurus, and the more derived Stegosauridae, which includes Stegosaurus, Kentrosaurus, Tuojiangosaurus, Dacentrurus, Hesperosaurus, and Miragaia.

The shape of their plates varied significantly across species. Stegosaurus had large, kite-shaped plates, while Kentrosaurus possessed a mix of plates and long spikes. Tuojiangosaurus was primarily covered in spikes. One member, Miragaia, was unusual for its exceptionally long neck, which contained 17 cervical vertebrae.

Studies of plate histology (Main et al. 2005) reveal extensive vascularisation. This suggests the plates served for display or potentially for regulating body temperature. In contrast, the tail spikes were clearly used for defense. Fossils of Allosaurus have been found with wounds that match the shape of stegosaur spikes, including one famous specimen with a tail vertebra pierced by a thagomizer. Their high-backed, low-headed anatomy was well-suited for browsing on ferns, horsetails, and cycads near the ground.

In their ecosystems, stegosaurs were generally less common than sauropods. Their diversity declined sharply during the Early Cretaceous. The last known member of the group, Mongolostegus, lived during the Aptian–Albian ages.

• Sophie (NHMUK PV R36730, Stegosaurus stenops) — Natural History Museum, London; most complete Stegosaurus.
• Dacentrurus armatus Holotype — multiple European specimens.
• Hesperosaurus mjosi holotype (HMNS 14) — Houston Museum of Natural Science.
• Mounted Kentrosaurus (MB.R.1934, MB.R.4800) — Museum für Naturkunde, Berlin.

Plate function is the canonical stegosaurian debate: display, species recognition, thermoregulation, defence — all have advocates, and recent consensus favours display + possible secondary thermoregulation. Plate arrangement: Whether the plates were paired or alternating in life is settled (alternating, in Stegosaurus at least). Tail spike count and posture: the canonical four-spike thagomizer in Stegosaurus stenops is now well-supported. Body posture: whether stegosaurs could rear up bipedally to feed (Bakker 1986) is contested but mechanically plausible for Stegosaurus.

Stegosaurus's climactic battle with Allosaurus in Fantasia (1940) and a similar scene in King Kong (1933) made it a cinematic icon. It remains one of the four or five most-recognisable dinosaurs in popular culture.

• stegosaurus
• kentrosaurus
• tuojiangosaurus
• ankylosauria
• ornithischia

• Galton, P. M., & Upchurch, P. (2004). Stegosauria. In The Dinosauria (2nd ed.).
• Maidment, S. C. R. (2010). Stegosauria: a historical review of the body fossil record and phylogenetic relationships. Swiss Journal of Geosciences, 103, 199–210.
• Main, R. P., et al. (2005). The evolution and function of thyreophoran dinosaur scutes. Paleobiology, 31, 291–314.
• Carpenter, K. (ed.) (2001). The Armored Dinosaurs. Indiana University Press.