Stegosaurus

Stegosaurus is the type genus of Stegosauridae and the namesake of Stegosauria. The standard anatomy: 17 large kite-shaped dorsal plates arranged in alternating left-right pairs along the back, four tail spikes at the tip (the "thagomizer"), a small narrow head with a long beak and weak leaf-shaped cheek teeth, hindlimbs much longer than forelimbs (so the hips sit high), and a barrel chest housing a substantial fermenting gut.

Three species are accepted from the Morrison Formation. S. stenops is the type, with the famous plate arrangement. S. ungulatus is longer with proportionally smaller plates and possibly more tail spikes. S. sulcatus is the third. Specimens from Portugal have been assigned to S. stenops or to S. ungulatus depending on the author.

Plate morphology varies along the back: small at the neck, large kite-shapes at mid-back, smaller again toward the tail. The histology shows heavy vascularisation, which fits a display function (and possibly thermoregulation). The plates were not strong enough for direct combat. The tail spikes were another matter. At ~60 cm long they were straightforwardly defensive, and we have the puncture wounds on Late Jurassic predators to prove it.

Stegosaurus shared the Morrison Formation with Allosaurus, Diplodocus, Apatosaurus, Brachiosaurus, and Camarasaurus. We have an Allosaurus tail vertebra punctured by a Stegosaurus spike, and a Stegosaurus throat puncture that matches an Allosaurus premaxilla. These are among the best documented predator-prey interactions in the fossil record. Tooth wear and snout shape suggest Stegosaurus was a low-to-mid browser, possibly capable of brief bipedal rearing to reach higher foliage (Bakker 1986; biomechanically plausible).

The brain was famously tiny: about 80 g, roughly one-fifth of one percent of body mass. The folkloric "second brain" in the hip turned out to be a glycogen body, an enlarged neural canal feature also seen in modern birds, not a separate brain.

• Sophie (NHMUK PV R36730) — Natural History Museum, London; most complete Stegosaurus stenops known, ~85% complete.
• Spike, Sara, and Stephen — multiple Sauriermuseum Aathal Stegosaurs, Switzerland.
• USNM 4934 — long-time Smithsonian Stegosaurus stenops mount.
• Hesperosaurus mjosi (close relative) — HMNS 14, Houston Museum.

Plate arrangement is settled for S. stenops: alternating in pairs, replacing earlier paired and single-row reconstructions. Plate function is mostly settled too, with display and species-recognition leading and thermoregulation as a plausible secondary role. Bipedal rearing is mechanically possible (Mallison 2011) and behaviourally plausible. Saitta (2015) proposed sexual dimorphism in plate shape, but later work has pushed back.

Stegosaurus is one of the most recognisable dinosaurs anywhere. It appears in King Kong (1933), Fantasia (1940), and dozens of films since. The Natural History Museum's acquisition of "Sophie" in 2014 was a major popular science event in the UK.

• stegosauria
• kentrosaurus
• tuojiangosaurus
• allosaurus

• Carpenter, K. (ed.) (2001). The Armored Dinosaurs. Indiana University Press.
• Maidment, S. C. R., et al. (2015). Locomotor biomechanics of the armoured dinosaur Stegosaurus stenops. Royal Society Open Science, 2, 150447.
• Saitta, E. T. (2015). Evidence for sexual dimorphism in the plated dinosaur Stegosaurus mjosi. PLOS ONE, 10, e0123503.
• Main, R. P., et al. (2005). The evolution and function of thyreophoran dinosaur scutes. Paleobiology, 31, 291–314.
• Mallison, H. (2011). Rearing giants. Paleontologia Electronica, 14, 4A.