Scolotosuchus is an extinct genus of rauisuchid from the Early Triassic of Russia. The only known species, Scolotosuchus basileus, has been discovered at the Donskaya Luka Locality. It differs from all other rauisuchids in the morphology of its spine, which allowed it to both resist great stress and gave it a great range of mobility. Additionally, it may have lacked osteoderms, with its spine instead being supported in a similar manner as in dinosaurs. At 3 m (9.8 ft) it was likely the apex predator of its ecosystem.
History and naming In 1999 several isolated fossil remains of a large rauisuchid were described from the Donskaya Luka Locality within the Lipovskaya Formation, located in the Central Russian Upland. The taxon this material was attributed to was named Scythosuchus basileus by paleontologist Andrey Gerasimovich Sennikov, who at the time thought it was a large but short-necked rauisuchid. Later research conducted by Sennikov however called the validity of Scythosuchus into question, as the holotype specimen, a poorly preserved neck vertebrae, was found to be similar to the fossils of the poposauroid Tsylmosuchus. This resulted in Scythosuchus being considered a junior synonym of Tsylmosuchus. However while the holotype itself was found to lack distinguishing traits, the type series of Scythosuchus also included a series of thoracic vertebrae notable for their short spinous processes and rugose dorsal ends. These vertebrae had previously been used to diagnose Scythosuchus and clearly belonged to a distinct type of early pseudosuchian unrelated to Tsylmosuchus. The discovery of additional vertebrae of similar morphology supported the idea that a large rauisuchid was present at Donskaya Luka, eventually leading Sennikov to establish the new genus Scolotosuchus, this time with the thoracic vertebrae serving as the holotype specimen. In addition to vertebrae, a partial pelvis, hindlimbs and a calcaneum are also assigned to this new genus. The name Scolotosuchus is derived from the "Skolotoi", the native name of the Scythians that once inhabited the region the fossils were found in. This mirrors the name of Scythosuchus, the taxon that the type material was once assigned to. The suffix "suchus" meanwhile derives from the Greek soukhos meaning crocodile, a common element in the names of prehistoric pseudosuchians. The species name "basileus" also derives from Greek and means king. It was chosen in part due to the fact that it had previously been the species name of Scythosuchus, but also do reference the royal Scythians that lived north of the Sea of Azov.
Description Scolotosuchus is known for the peculiar morphology of its vertebrae which is highly variable along the spinal column. The neck and early thoracic vertebrae are characterized by their short and low spinous processes, with the neural spine being of approximately equal height and length. Another feature present in both the later neck vertebrae as well as the early thoracic vertebrae are the well developed rugosities on the ends of their spinous processes. These are considered to be hypertrophied apophyseal ossifications by Sennikov, rather than the result of osteoderms fusing with the vertebrae. Sennikov also notes that the frequent discovery of isolated neural arches may indicate that the suture between the neural arch and the centra of the vertebrae are only weakly fused or not fused at all even in adult individuals. This would cause the vertebrae to effectively fall apart once the muscles begin to rot and the body decomposes or gets buried. The anterior articular processes (prezygapophysis) are notably wider than the posterior articular processes (postzygapophysis) in the neck and first thoracic vertebrae, but more similar size in the following trunk vertebrae. The later neck vertebrae, around the 7th or 8th, have high and narrow neural canals formed by the fact that the neural arches are composed of long legs and relatively short spinous processes. Both the hyposphene and the hypantrum, located at the back and front of each vertebra respectively, are well developed and the hypantrum in particular is accompanied by accessory laminae and depressions in the bone. The middle vertebrae of the neck show a neural spine that is bifurcated, giving it a Y-shape. This well defined bifurcation is another trait that is unique to Scolotosuchus among rauisuchids, as any other members of this family with bifurcated neural spines have this feature only weakly developed. This applies to both Batrachotomus and Stagonosuchus (which are considered rauisuchids by Sennikov), in which the bifurcation is additionally restricted to only the front of the neural spine. Sennikov also mentions Mandasuchus as the only rauisuchid with a fully bifurcated neural spine besides Scolotosuchus, however other authors such as Sterling J. Nesbitt do not regard these taxa to be rauisuchids at all. The preacetabular process of the illium is short and projects slightly cranially, the fact that this process is only poorly developed is seen in no other rauisuchid other than Ticinosuchus from the Middle Triassic of Europe. The subvertical crest is only sloping slightly, which sets Scolotosuchus apart from the American Postosuchus. The dorsal blade is low and the upper margin almost straight, both features typical for rauisuchids and clearly aligning Scolotosuchus with this group rather than with the contemporary ctenosauriscid Bystrowisuchus, which possessed a saddle-shaped dorsal blade. The ischium likewise falls within the typical rauisuchid anatomy, with a wide proximal end that connects to the illium and a narrow distal end. The femur is sigmoid in shape and elongated with a slightly compressed shaft and moderately wide ends, again generally resembling what is expected of a rauisuchid. The tibia and a fragmentary fibula are both described as being massive, with the former also being elongated and having a rounded proximal and oval distal ending to it. The calcaneum matches the crocodiloid-type ankle joint with a short tuber. The region where the calcaneum articulares with the astragalus is however convex, bearing slight resemblance to members of the Euparkeriidae, but otherwise shows no depressions or projections. Sennikov estimates that Scolotosuchus may have reached a length of approximately 3 m (9.8 ft).
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