Siats /see-ats/ is an extinctgenus of large neovenatoridtheropod dinosaur known from the Late CretaceousCedar Mountain Formation of Utah, US. It contains a single species, Siats meekerorum. S. meekerorum could be the first neovenatorid discovered in North America and the geologically youngest allosauroid yet discovered from the continent. It was initially classified as a megaraptoran, a clade of large theropods with very controversial relationships. This group may be examples of tyrannosauroids, neovenatorid allosauroids, or basal coelurosaurs.
The holotype specimen, FMNH PR 2716, consists of material from a single individual that is considered skeletally immature on the basis of the incomplete fusion of neural arches to the centra of the dorsal vertebrae. Siats is characterized by seven diagnostic, including four autapomorphic, traits. Its autapomorphies include the subtriangular cross section of the distal caudal vertebrae, elongated centrodiapophyseal laminae lacking noticeable infradiapophyseal fossae on the proximal caudals, a transversely concaved acetabular rim of iliac pubic peduncle, and the presence of a notch on the end of the truncated lateral brevis shelf. Other notable traits include the broad neural spines on the dorsal vertebrae. Siats represents one of the largest known theropods from North America. Zanno and Mackovicky, using other megaraptoran taxa as a proxy, estimated the length of the holotype individual at long, scaling upwards from the femur. Additionally, using a femur circumference regression, they estimated its mass at roughly 4 tons. The authors also wrote that the holotype specimen was already comparable in size to Saurophaganax and Acrocanthosaurus despite the visible neurocentral sutures showing absence of ossification, suggesting a skeletally immature individual. If it is a neovenatorid, the discovery of Siats also reveals that allosauroids did not yield dominance in North America to tyrannosauroids until the late Cretaceous. However, there has been and still is substantial disagreement on the classification of megaraptorans, which are usually found to be either neovenatorids or tyrannosauroids, often with both interpretations appearing in the same paper. The placement of Siats within Megaraptora itself has also been a source of controversy, with recent analyses placing it as a non-megaraptoran neovenatorid irrespective of the placement of Megaraptora.
Phylogeny and classification
Siats was initially classified as a megaraptoran neovenatorid closely related to Chilantaisaurus upon its discovery in 2013, based on the presence of pronounced centrodiapophyseal laminae bracketed by deep infradiapophyseal fossa on the caudal neural arches, similar to that of the megaraptoran Aerosteon. In a 2014 description of a juvenile Megaraptor specimen, the referral of Siats to Megaraptora was contested, and megaraptorans were found to more likely be tyrannosauroids rather than neovenatorids such as Siats. The paper noted that, although sharing various features with Neovenator, Siats was very different from megaraptorans in the structure of its dorsal vertebrae, ilium, and fibula. A subsequent analysis conducted by Coria and Currie, which even placed megaraptorans as neovenatorids, still placed Siats and Chilantaisaurus as neovenatorids outside of Megaraptora. On the other hand, the phylogenetic study published by Bell et al. recovered Siats as a member of Coelurosauria of uncertain phylogenetic placement within this group; the analysis indicated that Siats might either be a relative of ornithomimosaurs, a theropod more closely related to maniraptorans than tyrannosauroids, a basal megaraptoran, or a tyrannosauroid more closely related to tyrannosaurids than Xiongguanlong.
The cladogram presented below follows Zanno & Makovicky.
