Stegouros (, meaning "roofed tail") is an extinct genus of ankylosaurian dinosaurs from the Late Cretaceous Dorotea Formation of southern Chile. The genus contains a single species, Stegouros elengassen, known from a semi-articulated, near-complete skeleton. Stegouros is a member of the Parankylosauria, a clade of small Gondwanan armored dinosaurs. It is characterized by its small body size and distinct arrangement of fused osteoderms forming a macuahuitl-like tail weapon.
Discovery
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. In 2021, the type species Stegouros elengassen was named and described by Sergio Soto-Acuña et al. The generic name combines the Greek stegos, meaning "roof" and oura, meaning "tail", referring to the roof-like covering of the tail end. 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. The holotype specimen, CPAP-3165, was found in a layer of the Dorotea Formation dating from the upper Campanian. It consists of a relatively complete skeleton with skull and lower jaws. The rear part, the hindlimbs, sacrum, pelvis and tail, was preserved articulated. The front parts were scattered over a small surface. It lacks the skull roof, the rear lower jaws, the shoulder blades, the right humerus and the pubic bones. Some osteoderms were recovered. It represents an adult individual.
Description
Size and distinguishing traits
Stegouros is a very small ankylosaur. It was estimated at to be about 1.8–2 metres (5.9–6.6 ft) long and stood less than two feet tall. Only the tail was considered to be strictly distinctive. 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. 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.
Cranium
The head is probably proportionally large, though it is difficult to precisely gauge the proportions due to limited preservation. The praemaxillae of the front snout are toothless, short, high and narrow, and completely fused at the midline. The front palate to which they contribute is high. The maxillary bones merge seamlessly into the plate-shaped lacrimal bones that slope backwards. The maxillary bones have internal branches that form a secondary palate. The row of teeth starts slightly in front of the lacrimal bone and continues below the eye socket. The entire top of the eye socket is formed by fused supraoccipitalia that form a continuous thickened canopy. Similar to Kunbarrasaurus the posterior parts of the skull are not fused so that the sutures remain visible. These parts are rough with pits so that raised bone pates or horn plates were probably present. In the lateral braincase, the basisphenoid is short, shorter than the basioccipital of the lower occiput.
In the lower jaws, the central predentary, the bony core of the lower bill, is short and high with thin upper branches that are longer than the lower branches. The dentarium or os dentale is undulating in side view. The row of teeth is curved inwards, so that the rows of teeth of the lower jaws combined show an hourglass shape when viewed from above. The dentarium bears fourteen teeth, some of which the fossil preserves. The dentary teeth are leaf-shaped and high. The teeth are asymmetrical in external view. They have a convex cingulum that extends upwards into fluted ridges ending in serrations. The cingulum is asymmetrical: horizontal when seen from the outside but an arc seen viewed the inside, that is slightly oblique towards the front.
Postcrania
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