Ankylosauria

Ankylosauria is one of two main subgroups of Thyreophora, alongside Stegosauria. Its members represent the most heavily armoured land animals in the fossil record. Osteoderms (bone embedded in the skin) covered their entire dorsal surface, ranging from small studs to large plates and spikes. Some species even featured bony armour on their eyelids.

The clade consists of three major lineages: Nodosauridae, which lacked tail clubs and often bore prominent shoulder spikes (e.g., Borealopelta, Edmontonia, Sauropelta); Ankylosauridae, which possessed tail clubs, broader skulls, and generally smaller spikes (e.g., Ankylosaurus, Euoplocephalus, Tarchia, Pinacosaurus); and the more recently named Parankylosauria, a Gondwanan group that retained flexible armour and an unfused tail (e.g., Stegouros, Antarctopelta).

Their skulls were reinforced with co-ossified bone, housing brains that were small relative to their body size. Small, leaf-shaped teeth were restricted to the rear of the jaw, indicating they primarily ground up low-lying vegetation. Nodosaurids had broad beaks, but ankylosaurids had even wider ones, suggesting a grazing habit similar to modern cattle.

The tail club found in Ankylosauridae proper was a formidable defensive tool formed by a fused mass of caudal vertebrae and osteoderms. It was capable of delivering bone-breaking blows to the legs of predators. Calculations suggest that Ankylosaurus's tail club could have shattered the tibia of a T. rex.

Ankylosaurs were low-feeding herbivores across their global range. In Borealopelta (Brown et al. 2020), preserved stomach contents consisting mainly of fern leaves and charcoal suggest these animals browsed in post-fire regrowth landscapes. Mechanical studies by Arbour & Snively (2009) support the use of tail clubs against predators, though they may have also been used in combat within the species. Evidence from bonebeds, such as the juvenile Pinacosaurus sites in Mongolia, indicates that young ankylosaurs lived in groups, whereas adults appear to have been more solitary.

• Borealopelta markmitchelli (TMP 2011.033.0001) — Royal Tyrrell Museum; among the best-preserved dinosaur fossils ever found, with skin, scales, gut contents, and even reddish-brown countershading pigment confirmed by melanosome analysis.
• Stegouros elengassen holotype — University of Chile; defines Parankylosauria.
• Euoplocephalus tutus specimens — multiple AMNH and Royal Tyrrell mounts.
• Pinacosaurus grangeri juvenile bonebeds — Mongolian Academy of Sciences and Polish-Mongolian collections.

Parankylosauria as a clade (Soto-Acuña et al. 2021) is recent and largely accepted but reshapes ankylosaurian biogeography. Tail club origin and function: tail club evolution is now well-traced (Arbour & Currie 2016) but precise function (predator defence vs intraspecific) is debated. Ancestral integument: whether basal thyreophorans bore filamentous integument similar to Tianyulong is unresolved.

Ankylosaurus is the iconic ankylosaur and appears in Jurassic World (2015), Walking with Dinosaurs (1999), and Prehistoric Planet (2022). The 2017 Borealopelta unveiling at the Royal Tyrrell — looking like a "dinosaur sculpture" rather than a fossil — went viral globally.

• ankylosaurus
• euoplocephalus
• nodosaurus
• stegosauria
• ornithischia

• Carpenter, K. (ed.) (2001). The Armored Dinosaurs. Indiana University Press.
• Arbour, V. M., & Currie, P. J. (2016). Systematics, phylogeny and palaeobiogeography of the ankylosaurid dinosaurs. Journal of Systematic Palaeontology, 14, 385–444.
• Brown, C. M., et al. (2020). Dietary palaeoecology of an Early Cretaceous armoured dinosaur (Ornithischia; Nodosauridae). Royal Society Open Science, 7, 200305.
• Soto-Acuña, S., et al. (2021). Bizarre tail weaponry in a transitional ankylosaur from subantarctic Chile. Nature, 600, 259–263.
• Arbour, V. M., & Snively, E. (2009). Finite element analyses of ankylosaurid dinosaur tail club impacts. Anatomical Record, 292, 1412–1426.