Elessaurus is an extinct genus of archosauromorph from the Early Triassic of Brazil. It contains a single species, Elessaurus gondwanoccidens. It possessed a variety of features common to basal archosauromorphs, particularly basal tanystropheids such as Macrocnemus. However, it is uncertain whether Elessaurus was a particularly close relative of tanystropheids, and it might instead be closer to other major archosauromorph clades. The genus name refers to "Elessar", an alternate name of the character Aragorn from J.R.R. Tolkien's Lord of the Rings trilogy.
Discovery
Elessaurus is known from a single holotype specimen, UFSM 11471, which consists of most of a leg connected to parts of the hip and the base of the tail. Despite being fairly complete by the standards of its locale, many of the specimen's bones are flattened or fragmentary. UFSM 11471 was collected from the Bica São Tomé site, an outcrop of the Sanga do Cabral Formation in Rio Grande do Sul, Brazil. Based on the presence of Procolophon, the Sanga do Cabral Formation is likely Early Triassic (Induan-Olenekian) in age.
The generic name of Elessaurus is derived from the word "Elessar". This word originated from the fictional language of Quenya, which was invented by J.R.R. Tolkien for elves in his Middle Earth series. It translated to "elf-stone", and is one of the many names of the character more well known as Aragorn. Another name of Aragorn is "long-shanks", justifying its connection to Elessaurus, which possessed characteristically elongated shin bones. The specific name translates to "Western Gondwana", the region of the Early Triassic world which is now South America.
Description
The sacral rib of the second sacral (hip) vertebra bifurcates towards its contact with the ilium. Most of this sacral rib has a broad connection with the ilium, but at its rear it sends a pointed prong backwards. This trait is similar to Macrocnemus but unlike other tanystropheids, many of which do not have bifurcating sacral ribs. On the other hand, the caudal (tail) vertebrae resemble Tanystrachelos and Jesairosaurus in possessing transverse processes which project backwards and upwards. The ilium has a straight and blade-like upper edge, a pronounced postacetabular process, and a short yet distinct preacetabular process.
The femur is slender and sigmoid, similar to that of Macrocnemus and Augustaburiania. The femoral head is not distinctly wider than the shaft and its caudofemoralis attachment ridge is in the form of an internal trochanter (like lizards and basal archosauromorphs) rather than a fourth trochanter (which is present in archosauriforms). The femur widens slightly near the knee, where it has a pair of distinct condyles. The tibia and fibula are about 20% longer than the femur, a characteristic which is observed in Macrocnemus as well as avemetatarsalian archosaurs (like pterosaurs and early dinosaurs). The tibia may have a groove on its outer surface near the ankle like proterochampsids, but this may instead be an area of crushed bone. The fibula is thin and has a knob for the iliofibularis muscle near the knee.
The foot is asymmetrical, with the fourth metatarsal being the longest bone in the foot as with most non-archosauriform, non-tanystropheid archosauromorphs. Also like most early archosauromorphs, the fifth metatarsal is a short, hooked, L-shaped bone. The tarsus has six bones: an astragalus, calcaneum, distal tarsals I, III, and IV, and a centrale. This situation mirrors that of Macrocnemus bassannii and non-tanystropheid, non-archosauriform archosauromorphs. There is no foramen (hole) on the suture between the astragalus and calcaneum. The outer surface of calcaneum has an expansion known as a calcaneal tuber. A calcaneal tuber is known in archosauriforms and some allokotosaurs, as well as at least one tanystropheid, Tanytrachelos.