Siats

Siats /see-ats/ is an extinct genus of large neovenatorid theropod dinosaur known from the Late Cretaceous Cedar Mountain Formation of Utah, United States. 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.[1] It was initially classified as a megaraptoran, a clade of large theropods with very controversial relationships. This group may be examples of tyrannosauroids,[2] neovenatorid allosauroids,[1] or basal coelurosaurs.[3]

Siats Temporal range: Cenomanian, 94.6 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Skeletal reconstruction of known elements
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Clade:	†Allosauria
Clade:	†Carcharodontosauria
Family:	†Neovenatoridae
Genus:	†Siats Zanno & Makovicky, 2013
Type species
†Siats meekerorum Zanno & Makovicky, 2013

Discovery

Dorsal vertebra

Siats was first described 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.[1] 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.[1]

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.[1] FMNH PR 2716 was discovered by Lindsay Zanno, as a part of a 2008 expedition of the Field Museum led by Peter Makovicky.[4] 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.[5]

Description

Size compared to a human

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.[1]

Life restoration

Siats represents one of the largest known theropods from North America. Zanno and Mackovicky (2013), using other megaraptoran taxa as a proxy, estimated the length of the holotype individual at 11.9 meters (39 ft) long, scaling upwards from the femur. Additionally, using a femur circumference regression, they estimated its mass at roughly 4 metric tons (4.4 short 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.[1]

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.[6] 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.[3] 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.[7]

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.[7] 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.[8] On the other hand, the phylogenetic study published by 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.[9]

Caudal centrum

Dorsal centrum

The cladogram presented below (created prior to these more recent studies) follows Zanno & Makovicky (2013).[1]

Allosauroidea

Metriacanthosauridae     Yangchuanosaurus zigongensis       CV 00214     Yangchuanosaurus shangyouensis           Shidaisaurus jinae       "Sinraptor" hepingensis       Metriacanthosaurus parkeri       Sinraptor dongi     Siamotyrannus isanensis             Allosauria Allosauridae   Allosaurus fragilis     Saurophaganax maximus     Carcharodontosauria Neovenatoridae   Neovenator salerii       Chilantaisaurus tashuikouensis   Megaraptora   Siats meekerorum       Australovenator wintonensis     Fukuiraptor kitadaniensis         Aerosteon riocoloradensis     Megaraptor namunhuaiquii           Carcharodontosauridae     Eocarcharia dinops     Concavenator corcovatus         Acrocanthosaurus atokensis       Shaochilong maortuensis   Carcharodontosaurinae   Carcharodontosaurus saharicus   Giganotosaurini   Tyrannotitan chubutensis       Giganotosaurus carolinii     Mapusaurus roseae

References

1. Zanno, L. E.; Makovicky, P. J. (2013). "Neovenatorid theropods are apex predators in the Late Cretaceous of North America". Nature Communications. 4: 2827. Bibcode:2013NatCo...4.2827Z. doi:10.1038/ncomms3827. PMID 24264527.
2. F. E. Novas; F. L. Agnolín; M. D. Ezcurra; J. I. Canale; J. D. Porfiri (2012). "Megaraptorans as members of an unexpected evolutionary radiation of tyrant-reptiles in Gondwana". Ameghiniana. 49 (Suppl): R33.
3. "Archived copy" (PDF). Archived from the original (PDF) on 2016-08-18. Retrieved 2016-07-24.CS1 maint: archived copy as title (link)
4. Johnson, Steve (22 November 2013). "New dinosaur discovered by Field Museum expedition". Chicago Tribune. Retrieved 22 November 2013.
5. Tucker, R.T.; Zanno, L.E.; Huang, H.-Q.; Makovicky, P.J. (2020). "A refined temporal framework for newly discovered fossil assemblages of the upper Cedar Mountain Formation (Mussentuchit Member), Mussentuchit Wash, Central Utah". Cretaceous Research. doi:10.1016/j.cretres.2020.104384.
6. Jha, Ajit (22 November 2013). "'Man-Eating Monster' Dinosaur Was Top Predator Before T. Rex; Discovery Solves 30 Million Year-Old Puzzle". International Science Times. Retrieved 24 November 2013.
7. Porfiri, Juan D.; Novas, Fernando E.; Calvo, Jorge O.; Agnolín, Federico L.; Ezcurra, Martín D.; Cerda, Ignacio A. (2014-09-01). "Juvenile specimen of Megaraptor (Dinosauria, Theropoda) sheds light about tyrannosauroid radiation". Cretaceous Research. 51 (Supplement C): 35–55. doi:10.1016/j.cretres.2014.04.007.
8. Coria, Rodolfo A.; Currie, Philip J. (2016-07-20). "A New Megaraptoran Dinosaur (Dinosauria, Theropoda, Megaraptoridae) from the Late Cretaceous of Patagonia". PLoS ONE. 11 (7): e0157973. Bibcode:2016PLoSO..1157973C. doi:10.1371/journal.pone.0157973. ISSN 1932-6203. PMC 4954680. PMID 27439002.
9. Phil R. Bell; Andrea Cau; Federico Fanti; Elizabeth T. Smith (2016). "A large-clawed theropod (Dinosauria: Tetanurae) from the Lower Cretaceous of Australia and the Gondwanan origin of megaraptorid theropods". Gondwana Research. 36: 473–487. Bibcode:2016GondR..36..473B. doi:10.1016/j.gr.2015.08.004.