| Siats | |
|---|---|
 | |
| Skeletal reconstruction of known elements | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Clade: | Coelurosauria (?) |
| Clade: | †Megaraptora |
| Family: | †Megaraptoridae (?) |
| Genus: | †Siats Zanno &Makovicky,2013 |
| Type species | |
| †Siats meekerorum Zanno & Makovicky, 2013 | |
Siats (/see-ats/) is anextinctgenus of largetheropoddinosaurs known from theLate CretaceousCedar Mountain Formation (Mussentuchit Member) ofUtah, United States. The genus contains asingle species,Siats meekerorum. It was initially classified as amegaraptoran, a clade of large theropods with controversial relationships. Alternative positions within theNeovenatoridae,Allosauroidea, andTyrannosauroidea have also been proposed.
Siats wasdescribed byLindsay E. Zanno and Peter J. Makovicky in2013. Thegeneric name references a man-eatingmonster inUte mythology. Thespecific namemeekerorum honors thegeologist John Caldwell Meeker and his family for their support ofpaleontological research.[1][2]
Siats is known from theholotype specimen, FMNH PR 2716, a partialpostcranialskeleton housed at theField Museum of Natural History,Chicago. The specimen consists of fivedorsal vertebrae, eightcaudal vertebrae, achevron, a partial rightilium,ischium,fibula,tibia, and several right and left pedalphalanges. The specimen was discovered by Lindsay Zanno as a part of a 2008 Field Museum expedition with 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 theCedar Mountain Formation inEmery County of Utah, dating to the earlyCenomanian stage of theLate Cretaceous, approximately 94.5million years ago.[1][2][3]
TheSiats holotype specimen consists of material from a single individual that is considered skeletally immature based on the incomplete fusion of neural arches to the centra of the dorsal vertebrae.Siats is characterized by seven traits, including fourautapomorphies. These include the subtriangularcross section of thedistal caudal vertebrae, elongated centrodiapophyseal laminae lacking noticeable infradiapophysealfossae on theproximal 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 broadneural spines on the dorsal vertebrae.[1]
Siats represents one of the largest known theropods from the 'mid'-Cretaceous of North America. Using a femur circumference regression, Zanno and Mackovicky (2013) estimated its mass at roughly 3.9 metric tons (4.3 short tons). They further wrote that the holotype was already comparable in size toSaurophaganax andAcrocanthosaurus despite its immaturity.[1]
In its 2013 description,Siats was initially classified as a megaraptoran within the Neovenatoridae, based on the presence of pronounced centrodiapophyseal laminae bracketed by deep infradiapophyseal fossa on the caudal neural arches, similar to that of the megaraptoranAerosteon. These results, following Zanno & Makovicky (2013), are displayed in thecladogram below:[1]
In the 2014 description of a juvenileMegaraptor specimen, the referral ofSiats to Megaraptora was contested, and megaraptorans were found to more likely be tyrannosauroids rather than neovenatorids such asSiats. The paper noted that, although sharing various features withNeovenator,Siats could be distinguished from megaraptorans in the structure of its dorsal vertebrae, ilium, and fibula.[4] A subsequent analysis conducted byCoria andCurrie (2016), which even placed megaraptorans as neovenatorids, still placedSiats andChilantaisaurus as neovenatorids outside of Megaraptora.[5] However, Bellet al. (2016) recoveredSiats as a member ofCoelurosauria of uncertain phylogenetic placement within this group; their analyses found variable positions forSiats, as a relative ofornithomimosaurs, a theropod more closely related tomaniraptorans thantyrannosauroids, abasal megaraptoran, or a tyrannosauroid more closely related totyrannosaurids thanXiongguanlong.[6]
Naish andCau (2022) recovered the taxon as the basalmost megaraptoran, with this clade diverging afterXiongguanlong, and supportedSiats andChilantaisaurus as representing a wave of gigantism in tyrannosauroids preceding the Tyrannosauridae.[7] In their 2025 analysis of allosauroid phylogenetics, Kellermann, Cuesta & Rauhut consistently recoveredSiats as a megaraptoran, noting that it shares a specific position of pneumatic fossa with this clade.[8] The cladogram below follows Naish and Cau (2022), who foundSiats within Megaraptora as part of Tyrannosauroidea:[7]
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