Stegouros (/ˌstɛɡəˈjʊərɒs/, meaning "roofed tail") is a genus of ankylosaurian dinosaur from the Late Cretaceous Dorotea Formation of southern Chile.

[1] In February 2018, the skeleton of a small ankylosaur was recovered by a team of Texan researchers near the Río de las Chinas valley of Ultima Esperanza province in the Magallanes y la Antártica Chilena region of southern Chile.

The specific name elengassen is derived from an armoured creature in the mythology of the Aónik’enk, the indigenous inhabitants of the region where the holotype was discovered.

It lacks the skull roof, the rear lower jaws, the shoulder blades, the right humerus and the pubic bones.

Unlike all other known Ankylosauria, the tail is short with no more than twenty-six caudal vertebrae, the last twelve of which are covered by seven pairs of large osteoderms, the last five of which again are fused together to form a flat connected structure.

[1] Stegouros differs from its close relative Antarctopelta in its smaller body size, relatively larger neural canal, longer dorsosacral vertebrae, higher and narrower vertebral bodies of the sacral vertebrae, absence of ossified tendons on the tail, teeth with six serrations on the anterior margin instead of seven or eight, and cingula, thickened tooth crown bases, without vertical grooves.

Stegouros differs from Kunbarrasaurus by having a curved instead of straight ulna and radius and a process from the maxilla running towards the lacrimal bone that is narrower and sloping backwards.

The praemaxillae of the front snout are toothless, short, high and narrow, and completely fused at the midline.

The row of teeth starts slightly in front of the lacrimal bone and continues below the eye socket.

The vertebral centra are wider than they are long, a typical ankylosaur feature, but they have hollow sides like Stegosauria.

However, the ends are strongly transversely broadened, especially in the epiphyses, and there is a well-developed deltopectoral crest projecting anteriorly.

On the outer posterior margin of the humerus, a conspicuous ridge runs downwards with a small bump at the upper end, in the same position in which Stegosauria have a tubercule for the attachment of the musculus triceps brachii.

The radius is slender but the ulna is robust and broadened at the top with a well-developed olecranon, an upper projection to bend the elbow.

The fourth trochanter, the protrusion on the posterior femoral shaft that served to attach the tail retractor muscle, is small and shaped like a vertical ridge.

These eight scutes were not in apparent association with any skeletal element except one that was directly adjacent to a neural arch of a vertebra.

Near the left hand was a cluster of four smaller osteoderms, fifteen to twenty millimeters in diameter, with more pointed keels.

On the upper right ulna is a small round bone plate, keeled with a concave inner surface, next to a flat triangular osteoderm.

[1] Between the ilium and the neural spines of the sacrum is a continuous thin layer of ossified skin armor covered with vein grooves and pits.

[1] Two pairs of small cone-shaped osteoderms with pointed keels and concave lower surfaces are present at the base of the tail.

Ankylosaurids typically have a tail club but the Stegouros weapon has a very distinct build, representing a type not yet known from Ankylosauria.

The weapon consists of seven pairs of flattened osteoderms that form an elongated structure that covers the tip of the tail.

[1] Numerous ossicles in the form of circular disks, four to five millimeters in diameter, were found around all skeletal elements.

Flattened vertebrae have also been found in Antarctopelta, suggesting that a similar tail weapon was present in that species.

[1] Stegouros was found by Soto-Acuña et al. to belong to a distinct lineage of small ankylosaurs known from the Cretaceous of southern Gondwana, also including Kunbarrasaurus from Australia and Antarctopelta from the Antarctic Peninsula.

The results of this phylogenetic analysis are shown below:[1] Lesothosaurus Scutellosaurus Emausaurus Scelidosaurus Huayangosaurus Stegosauridae Kunbarrasaurus Antarctopelta Stegouros Nodosauridae Liaoningosaurus Gobisaurus Shamosaurus Ankylosaurinae The discovery of Stegouros has an important implication for the early history of ankylosaurians; it revealed that a novelly discovered, distinct lineage of ankylosaurs, the Parankylosauria, exist, diverging from Euankylosauria (the combined clade of Ankylosauridae and Nodosauridae) very early in the Cretaceous and surviving until the Maastrichtian, distinguished by a unique tail weapon, dubbed 'macuahuitl', evolved convergently to ankylosaurid tail clubs.

It also suggests, through phylogenetic bracketing, that two genera found to be related by analyses by Soto-Acuña et al. (Antarctopelta and Kunbarrasaurus) possessed the same tail weapon.