Siats

''Siats'' (/see-ats/) is an extinct genus of large theropod dinosaur known from the Late Cretaceous Cedar Mountain Formation of Utah, United States. It contains a single species, ''Siats meekerorum''. It was initially classified as a megaraptoran, a clade of large theropods with very controversial relationships. ''Siats'' may be a neovenatorid allosauroid, a coelurosaur of uncertain phylogenetic position, or a tyrannosauroid.

Discovery

''Siats'' was first described in 2012 and named by Lindsay E. Zanno and Peter J. Makovicky in 2013 and the type species is ''Siats meekerorum''. The generic name is derived from the name of ''Siats'', a man-eating monster in the Ute mythology. The specific name ''meekerorum'' honors the late geologist John Caldwell Meeker who bequeathed a fund for the support of paleontological research, his widow Withrow Meeker and their daughter Lis Meeker, one of the volunteers in the research project.

''Siats'' is known from the holotype FMNH PR 2716, a partial postcranial skeleton housed at the Field Museum of Natural History, Chicago. FMNH PR 2716 consists of five dorsal and eight caudal vertebrae, a chevron, partial right ilium, ischium and fibula, a partial left tibia, and several right and left pedal phalanges. FMNH PR 2716 was discovered by Lindsay Zanno, as a part of a 2008 expedition of the Field Museum led by Peter Makovicky. The bones were first seen protruding out of a hillside, prompting the excavation. It was collected between 2008 and 2010 from the Mussentuchit Member of the Cedar Mountain Formation, in Emery County of Utah, dating to the early Cenomanian stage of the Late Cretaceous, approximately 94.5 million years ago.

Description

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 (i.e. unique), 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 (2013), using a femur circumference regression, estimated its mass at roughly . 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. In 2019, its maximum adult size was estimated up to in length and in body mass.

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 this animal as well as with 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 (2016), which even placed megaraptorans as neovenatorids, still placed ''Siats'' and ''Chilantaisaurus'' as neovenatorids outside of Megaraptora. However, Bell ''et al.'' (2016) 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 (created prior to these more recent studies) follows Zanno & Makovicky (2013).

References

{{Taxonbar, from=Q15221367

Prehistoric tetanurans
Late Cretaceous dinosaurs of North America

Fossil taxa described in 2013
Taxa named by Lindsay Zanno
Paleontology in Utah