Ankylosauria

The name Ankylosauria was coined by Henry Fairfield Osborn in 1923 in his paper "Two Lower Cretaceous dinosaurs of Mongolia" (American Museum Novitates, 95: 1–10), in which he erected the suborder to accommodate the armored dinosaurs allied with Ankylosauridae. However, the first ankylosaur known to science predates Osborn's naming by nearly a century. In 1832, Gideon Mantell discovered the remains of an armored dinosaur in the Tilgate Forest of Sussex, England, which he described the following year as Hylaeosaurus armatus (Mantell, 1833). This specimen was one of the three genera that Richard Owen used in 1842 to establish the concept of Dinosauria.

Barnum Brown erected the family Ankylosauridae in 1908 based on material from the Hell Creek Formation of Montana. The modern two-family classification of Ankylosauria—separating Ankylosauridae from Nodosauridae—was formalized by Walter P. Coombs Jr. in his 1978 monograph in Palaeontology, which identified suites of cranial and postcranial synapomorphies distinguishing the two families. This dichotomy has remained the standard framework, though subsequent cladistic analyses have refined internal relationships considerably.

Under phylogenetic nomenclature, Ankylosauria is defined as the maximum clade containing Ankylosaurus magniventris Brown, 1908, but not Stegosaurus stenops Marsh, 1887 (Madzia et al., 2021, PeerJ). This places Ankylosauria as the sister group of Stegosauria within the clade Eurypoda, which is itself nested within Thyreophora (shield-bearers), a major ornithischian lineage.

Within Ankylosauria, the traditional subdivision recognizes two major families. Ankylosauridae (Brown, 1908) is defined as the maximum clade containing Ankylosaurus magniventris but not Nodosaurus textilis. Members are characterized by broad, heavily ornamented skulls, relatively wide muzzles, and the presence of a bony tail club formed by modified distal caudal vertebrae and enlarged terminal osteoderms. Notable genera include Ankylosaurus, Euoplocephalus, Zuul, Pinacosaurus, Tarchia, and Saichania. The family is further subdivided into Ankylosaurinae and Shamosaurinae, with tribes such as Ankylosaurini recognized within Ankylosaurinae.

Nodosauridae (Marsh, 1890) is defined as the maximum clade containing Nodosaurus textilis but not Ankylosaurus magniventris. Nodosaurids lack tail clubs but frequently possess prominent lateral spikes on the shoulders and flanks, and tend to have narrower skulls and muzzles. Representative genera include Nodosaurus, Edmontonia, Sauropelta, Borealopelta, Panoplosaurus, and Polacanthus.

In 2021, Soto-Acuña and colleagues described Stegouros elengassen from the Late Cretaceous of Magallanes, Chile, and proposed a new clade, Parankylosauria, to accommodate basal Southern Hemisphere ankylosaurs. This clade is defined as the first ancestor of Stegouros that is not also an ancestor of Ankylosaurus, and all descendants of that ancestor. It includes other Gondwanan taxa such as Antarctopelta, Kunbarrasaurus, and possibly Minmi. Parankylosauria is recovered as the sister group to Euankylosauria (the combined clade of Ankylosauridae + Nodosauridae), suggesting a deep biogeographic split within Ankylosauria dating back to the breakup of Pangaea during the Middle to Late Jurassic.

The most diagnostic feature of ankylosaurs is their extensive dermal armor. Osteoderms embedded in the skin range from small, pebble-like ossicles to large plates and conical spikes. The armor typically includes cervical half-rings protecting the neck, thoracic plates and lateral spikes along the trunk, and, in ankylosaurids, a terminal tail club. In some taxa, such as Euoplocephalus, even the eyelids bore bony coverings, providing what Britannica describes as essentially impregnable armor coverage.

The skull is low, broad, and box-like. Dermal osteoderms are frequently co-ossified with the underlying cranial bones, obscuring sutures and covering the temporal fenestrae. The jaw apparatus is relatively weak, with small predentary bones, limited jaw muscle attachment sites, and small, loosely spaced, leaf-shaped teeth that are reminiscent of those seen in the most basal ornithischians. This suite of features suggests a diet of soft, pulpy vegetation.

The postcranial skeleton reflects a graviportal body plan. The limbs are short and stout, with the humerus and femur being longer than the radius-ulna and tibia-fibula, respectively. The metapodials are stubby, and the terminal phalanges are broad and hoof-like. The hind limbs are somewhat longer than the forelimbs, but not to the extreme extent seen in stegosaurs. Body size ranges from relatively small forms—Stegouros at approximately 2 meters in length—to the large Ankylosaurus magniventris, estimated at 6–8 meters long and weighing 4,500–6,000 kilograms.

The ankylosaurid tail club is one of the most distinctive weapons in the dinosaurian fossil record. It consists of tightly interlocking distal caudal vertebrae that form a rigid "handle," terminating in enlarged osteoderms that create a heavy, blunt knob. Biomechanical studies by Arbour (2009, PLoS ONE) demonstrated that these clubs could deliver substantial impact forces.

The traditional interpretation has been that tail clubs evolved as defensive weapons against large theropod predators. However, a landmark study by Arbour, Zanno, and Evans (2022, Biology Letters) presented evidence from the holotype of Zuul crurivastator (ROM 75860) that challenges this view. The specimen, which preserves nearly complete in situ integument, exhibits multiple pathological osteoderms localized to the flanks in the hip region rather than distributed randomly across the body. Statistical analysis confirmed this non-random distribution. The researchers argued that the localized pattern of injuries is consistent with lateral tail strikes from conspecifics in ritualized combat, analogous to flank-butting behavior seen in bison or necking in giraffes.

The study also noted that large-bodied theropods appeared in the fossil record well before tail clubs evolved in ankylosaurids, that nodosaurids sharing ecosystems with the same predators never evolved tail clubs, and that tail club knob size does not correlate with predator body mass. These findings support the hypothesis that sexual selection, rather than predation pressure, was the primary driver of tail club evolution, suggesting that ankylosaurids were behaviorally complex animals that likely engaged in ritualized intraspecific combat for social dominance.

The oldest known ankylosaur is Spicomellus afer from the Middle Jurassic (approximately 165–168 million years ago) El Mers Formation of Morocco. Initially described in 2021 from a single partial rib with fused spikes, additional material described in a 2025 Nature paper revealed this animal to be approximately 4 meters long, bearing an extraordinary bony collar of giant spikes around its neck, each extending roughly one meter outward. This discovery establishes North Africa as a center of early ankylosaur evolution and demonstrates that the group had already achieved remarkable morphological specialization by the Middle Jurassic.

From the Late Jurassic (approximately 150 million years ago), reasonably complete basal ankylosaurs are known from North America, including Gargoyleosaurus and Mymoorapelta from the Morrison Formation. The divergence of Ankylosauridae and Nodosauridae appears to have occurred during the Early Cretaceous, after which both families diversified and became prominent components of herbivorous dinosaur faunas across Laurasia until the end-Cretaceous extinction at the K-Pg boundary.

Ankylosaurs were low-browsing herbivores, likely feeding on ferns, cycads, low-growing shrubs, and early angiosperms. Their low head position, weak jaws, and small teeth suggest that they consumed relatively soft plant material without extensive oral processing. The Natural History Museum, London, estimates that a large-bodied ankylosaur such as Ankylosaurus would have needed to consume approximately 60 kilograms of plant matter daily, comparable to a modern elephant.

Remarkable preservation in Borealopelta markmitchelli (Brown et al., 2017, Current Biology) provided direct evidence of ankylosaur diet through fossilized stomach contents, which revealed a preference for particular fern species, indicating some degree of selective feeding. The same specimen preserved evidence of countershading—a reddish-brown dorsal surface with a lighter ventral surface—suggesting that even large, heavily armored ankylosaurs were subject to significant predation pressure, likely from large theropods.

Ankylosaurs have traditionally been regarded as Laurasian dinosaurs, with the richest fossil records from western North America (from the Morrison Formation through the Hell Creek Formation), Europe (England, France, Romania, Spain), and Asia (Mongolia, China, Japan). However, discoveries from the Southern Hemisphere have significantly expanded their known range. Australia has produced Minmi and Kunbarrasaurus from Early Cretaceous deposits in Queensland. The Antarctic Peninsula has yielded Antarctopelta from Late Cretaceous strata. South America, particularly Patagonia, has contributed Stegouros and other fragmentary material.

The geographic distribution of ankylosaurs appears to reflect the breakup history of Pangaea. The basal Parankylosauria, if upheld as a valid clade, originated in southern Gondwana and remained morphologically more conservative than their Laurasian relatives. In contrast, Euankylosauria (Ankylosauridae + Nodosauridae) diversified primarily in Laurasia, achieving their greatest diversity during the Late Cretaceous in North America and Asia.

Ankylosaur research has accelerated significantly since the 2010s. Key milestones include Arbour and Currie's (2016) comprehensive phylogenetic analysis of Ankylosauridae, the proposal of Parankylosauria by Soto-Acuña et al. (2021), the intraspecific combat hypothesis of Arbour et al. (2022), and the expanded description of Spicomellus afer in 2025. Madzia et al. (2021) formalized the phylogenetic definitions of major ankylosaur clades under the International Code of Phylogenetic Nomenclature (PhyloCode), providing a standardized nomenclatural framework.

Nevertheless, several major questions remain unresolved. The basal phylogenetic relationships within Ankylosauria remain poorly resolved, particularly the placement of enigmatic taxa such as Polacanthus (which has been variously assigned to Ankylosauridae, Nodosauridae, or left as a basal ankylosaur). The validity and composition of Parankylosauria require further testing with expanded taxon sampling and additional Gondwanan discoveries. The timing and geographic context of the Ankylosauridae-Nodosauridae split are still debated. Additionally, the rarity of ankylosaurs in most ecosystems (they typically constitute a small fraction of dinosaur faunas) poses ongoing challenges for understanding their true diversity and paleoecological roles.