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Siats
Temporal range: Cenomanian, 99–94.5 Ma
↓
Skeletal reconstruction of known elements
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
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 an extinct genus of large theropod dinosaurs known from the Late Cretaceous Cedar Mountain Formation (Mussentuchit Member) of Utah, United States. The genus contains a single species, Siats meekerorum. It was initially classified as a megaraptoran, a clade of large theropods with controversial relationships. Alternative positions within the Neovenatoridae, Allosauroidea, and Tyrannosauroidea have also been proposed.

Discovery and naming

[edit]
Dorsal vertebra

Siats was described by Lindsay E. Zanno and Peter J. Makovicky in 2013. The generic name references a man-eating monster in Ute mythology. The specific name meekerorum honors the geologist John Caldwell Meeker and his family for their support of paleontological research.[1][2]

Siats is known from the holotype specimen, FMNH PR 2716, a partial postcranial skeleton housed at the Field Museum of Natural History, Chicago. The specimen consists of five dorsal vertebrae, eight caudal vertebrae, a chevron, a partial right ilium, ischium, fibula, tibia, and several right and left pedal phalanges. 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 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.[1][2][3]

Description

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Size of Siats compared to a human

The Siats 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 four autapomorphies. These 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]

Speculative life restoration as an Allosauroid

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 to Allosaurus anax and Acrocanthosaurus despite its immaturity.[1]

Classification

[edit]
Caudal vertebral centrum
Dorsal vertebral centrum

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 megaraptoran Aerosteon. These results, following Zanno & Makovicky (2013), are displayed in the cladogram below:[1]

Allosauroidea

In the 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 could be distinguished from megaraptorans in the structure of its dorsal vertebrae, ilium, and fibula.[4] 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.[5] However, Bell et al. (2016) recovered Siats as a member of Coelurosauria of uncertain phylogenetic placement within this group; their analyses found variable positions for Siats, as 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.[6]

Naish and Cau (2022) recovered the taxon as the basalmost megaraptoran, with this clade diverging after Xiongguanlong, and supported Siats and Chilantaisaurus as representing a wave of gigantism in tyrannosauroids preceding the Tyrannosauridae.[7] In their 2025 analysis of allosauroid phylogenetics, Kellermann, Cuesta & Rauhut consistently recovered Siats 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 found Siats within Megaraptora as part of Tyrannosauroidea:[7]

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References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Siats

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from Grokipedia
Siats is an extinct genus of large theropod dinosaur from the Late Cretaceous period, approximately 99 million years ago, in what is now Utah, United States. The type species, Siats meekerorum, was described in 2013 based on a partial postcranial skeleton, including elements of the axial column, pelvic girdle, and hind limb, representing an immature individual; no cranial material is known. The holotype specimen (FMNH PR 2716) and a referred specimen (FMNH PR 3059) were discovered in the Mussentuchit Member of the Cedar Mountain Formation, Emery County, Utah, about 10 meters below the base of the Dakota Formation. Estimated from the holotype's femur length of approximately 1,166 mm, S. meekerorum was one of the largest known megaraptorans, with a body mass of about 4 tons for the holotype and likely reaching lengths of 9–12 meters and masses over 4 tons in adulthood. Initially classified within , Allosauroidea, and Neovenatoridae, and tentatively within , Siats represents the first evidence of megaraptorans in , indicating a for this during the stage of the Upper ; however, recent phylogenetic analyses as of 2024 have suggested alternative placements, such as within . As an , it dominated the ecosystem, competitively excluding early tyrannosauroids from large-bodied niches and delaying the evolutionary rise of tyrannosaurids as top predators in until after the apparent extinction of neovenatorids like Siats. The genus name derives from a predatory, man-eating monster in Ute Native American folklore, while the specific honors the Meeker family, who own the land where the fossils were found.

Discovery and naming

Discovery

The specimen of Siats meekerorum, cataloged as FMNH PR 2716, was discovered in 2008 by paleontologists Lindsay E. Zanno and Peter J. Makovicky during a expedition in , USA. The site was located in the Mussentuchit Member of the Cedar Mountain Formation, a mid-Cretaceous ( stage, approximately 98 million years ago) stratigraphic unit known for its rich theropod fauna. Excavation efforts spanned from 2008 to 2010, recovering a disarticulated yet associated partial postcranial skeleton from an immature individual preserved in a paucispecific bonebed alongside small theropod and abundant microvertebrate remains. The preserved elements include portions of the axial column—such as parts of five dorsal vertebrae (a cranial dorsal centrum, two fragmentary dorsal neural arches, a probable fifth cranial dorsal vertebra, and a caudal dorsal centrum) and up to 13 caudal vertebrae (three rostral caudal neural arches and several distal caudals)—as well as elements of the pelvic girdle and hind limb: a partial right ilium, proximal portions of the right and left ischia, a long section (~460 mm) of the fibular shaft, the distal end of the third metatarsal, a damaged distal articulation of metatarsal II or IV, and pedal phalanges II-1, III-2, and IV-3. Immaturity of the specimen is indicated by the incomplete neurocentral fusion in most vertebrae, except for the distal caudals. Preparation of the fossil material was conducted at the Field Museum of Natural History in Chicago, where Zanno and Makovicky performed detailed mechanical removal of the enclosing sediment to isolate and stabilize the bones for study. Their initial analyses focused on anatomical comparisons and phylogenetic placement, leading to the formal description of Siats as a neovenatorid allosauroid, published in November 2013 in Nature Communications. This publication highlighted the specimen's significance in understanding theropod diversity and apex predator dynamics in Late Cretaceous North America.

Etymology

The genus name Siats is derived from the name of a predatory, man-eating monster in the legends of the Ute Native American tribe from Utah, occasionally spelled as "See-atch" and pronounced "see-ats." This choice reflects the dinosaur's inferred role as an apex predator in its Late Cretaceous ecosystem, aligning with paleontological naming conventions that draw from mythology to evoke the animal's formidable nature. The species epithet meekerorum honors the late John Caldwell Meeker and his family, who established an endowment fund supporting research and early-career scientists at the Field Museum in . This recognition underscores a tradition in of commemorating key contributors to geological and fossil studies, particularly those advancing knowledge of formations like the Cedar Mountain where Siats was found. Overall, the binomial Siats meekerorum exemplifies how nomenclature blends cultural mythology with tributes to scientific patrons, a practice common in the field to highlight both ecological dominance and human legacies in discovery.

Description

Overall size and build

Siats meekerorum was a large-bodied theropod , with the specimen (FMNH PR 2716) representing a skeletally immature individual based on unfused neurocentral sutures in the preserved vertebrae. This subadult is estimated to have measured about 9 meters (30 feet) in length, with a body mass of approximately 3.9 metric tons (4.3 short tons), derived from circumference regressions. Adult individuals likely attained lengths of 9–12 meters (30–39 feet), potentially approaching the dimensions of the North American theropod atokensis. The overall build of Siats was robust, characteristic of large allosauroid theropods, featuring a bipedal posture supported by elongated hindlimbs that indicate adaptations for agile predation over considerable distances. Proportions included a relatively long axial column with elongated dorsal centra (more than twice as long as tall) and compact neural spines, contributing to a powerful, streamlined frame suited for apex predation. In comparisons to other mid-Cretaceous North American theropods, such as those from the Cedar Mountain Formation, Siats stands out as one of the largest known carnivores, exceeding 1 metric ton in mass and dominating the size class of contemporary predators.

Specific anatomical features

The specimen of Siats meekerorum (FMNH PR 2716) preserves a disarticulated partial from a skeletally immature individual, including portions of five dorsal vertebrae, up to 13 caudal vertebrae, a partial right ilium, proximal portions of the right and left , a nearly complete fibular shaft, and several pedal phalanges, but lacks the , forelimbs, and most of the , which limits comprehensive reconstruction of the overall morphology. The preserved dorsal vertebrae exhibit camellate internal pneumaticity, with slit-like pneumatopores on the centra and pneumatic foramina present on all presacral elements, alongside platycoelous centra that are elongated cranially (more than twice their height) and low neural spines approximately 50% of centrum height. Caudal vertebrae display amphicoelous to platycoelous with an axially concave ventral surface lacking a midline groove, and the neural arches feature fan-shaped, backswept transverse processes along with deep infrapre- and infrapostzygapophyseal fossae. Among the seven diagnostic traits identified for the , key features include pneumatic prezygapophyses on the presacral vertebrae, hypertrophied caudal centrodiapophyseal laminae, and a peg-and-socket articulation between the ilium and . The partial ilium indicates a deep pelvic girdle, with a transversely concave acetabular rim on the pubic peduncle and a truncated lateral brevis shelf that has a notched caudal end and subparallel margins defining a deep brevis fossa. The elements, including the shaft and pedal phalanges, suggest robust construction, with the showing a mediolaterally compressed proximal end and slight craniocaudal expansion along its transversely compressed shaft. Four autapomorphies distinguish S. meekerorum: craniocaudally expanded centrodiapophyseal laminae on proximal caudal vertebrae lacking infradiapophyseal fossae; a subtriangular cross-section in distal caudal vertebrae; a transversely concave acetabular rim of the iliac pubic peduncle; and a truncated lateral brevis shelf with a notched caudal end. The preserved pedal phalanges include elements such as II-1, III-2, and IV-3, along with the distal end of metatarsal III, featuring specific articulations that contribute to the diagnostic profile, though details on phalangeal articulations are limited by the fragmentary of the material.

Classification

Initial classification

Siats meekerorum was formally described and classified by paleontologists Lindsay E. Zanno and Peter J. Makovicky in 2013 as a member of Neovenatoridae, a subclade within the larger group Allosauroidea. This placement positioned Siats as a carcharodontosaurian allosauroid, emphasizing its role as an in the Late Cretaceous ecosystems of . The classification was based on a comprehensive phylogenetic analysis incorporating 355 morphological characters scored across 61 theropod taxa, with the specimen (FMNH PR 2716) providing sufficient diagnostic features to support this affiliation. The initial assessment highlighted similarities in pelvic girdle and morphology to other neovenatorids, particularly the pronounced centrodiapophyseal laminae bracketed by deep infradiapophyseal fossae observed in and , which were interpreted as homologous traits. Additional synapomorphies for Neovenatoridae, such as tab-like dorsal postzygapophyseal extensions and a preacetabular cuppedicus shelf on the ilium, further reinforced this grouping, with Bremer support values exceeding 5 indicating robust . Within this framework, Siats was recovered as a basal member of , positioned sister to and outside , distinguishing it from the emerging tyrannosaur dominance in later North American faunas. At the time of description, Siats was noted as one of the largest known allosauroids from during the mid-Cretaceous, with an estimated adult body size rivaling based on a partial measuring approximately 1,166 mm in length—surpassing comparable elements from contemporary taxa like . This size underscores its ecological significance as a top predator in the Cedar Mountain Formation, representing the youngest and among the most substantial allosauroid records from the continent.

Recent phylogenetic analyses

Following the initial description of Siats meekerorum as a neovenatorid carcharodontosaur in 2013, subsequent analyses from 2014 to 2016 began exploring alternative affinities, particularly suggesting closer ties to tyrannosauroids and challenging its placement among megaraptorans or other allosauroids. A 2014 study on a juvenile Megaraptor specimen highlighted shared cranial features, such as premaxillary foramina and a straight manual phalanx I-1, that aligned megaraptorans more closely with basal tyrannosauroids than traditional allosauroids, prompting reevaluation of Siats' position due to its large size and North American occurrence. A 2016 analysis of manual anatomy in Australian megaraptorans like Australovenator suggested retention of allosauroid-like traits and argued against close ties to tyrannosauroids, contributing to the ongoing debate over the group's affinities. These early shifts set the stage for more explicit inclusion of Siats in broader matrices. A 2022 phylogenetic study incorporating Eotyrannus and other mid-Cretaceous theropods recovered Siats as the basalmost megaraptoran within a monophyletic Tyrannosauroidea. This placement emphasized Siats' role in bridging North American and Gondwanan tyrannosauroid diversity during the Cenomanian. The analysis used a modified matrix from prior works, yielding 24 most parsimonious trees with a consistency index of 0.42. A 2025 re-evaluation of allosauroid relationships, focusing on carcharodontosaurid material from but incorporating a comprehensive theropod , recovered megaraptorans—including Siats—within as part of , positioned stemward to more derived forms. This study highlighted Siats' contribution to understanding gigantism trends in the , with its estimated 30-foot length (approximately 9 meters) paralleling relatives in body size evolution during the Early-Late transition. The analysis produced multiple topologies but consistently supported 's tyrannosauroid affinities, aligning with recent consensus as of 2025. Ongoing debates in these analyses center on character scoring ambiguities that affect tree topologies, particularly the shape of the ilium—debated as either horizontally oriented (favoring tyrannosauroid placement) or vertically aligned (supporting allosauroid retention)—and the extent of vertebral pneumaticity, where Siats' anterior caudals exhibit large foramina akin to megaraptorans but differing from typical carcharodontosaurids in chamber complexity. Such scoring variations, often rescored in sensitivity analyses, have led to polytomies in strict consensuses but generally stabilize Siats as a basal megaraptoran tyrannosauroid in recent high-impact matrices.

Paleoecology

Geological setting

Siats meekerorum was discovered in the Mussentuchit Member of the Cedar Mountain Formation, located in central within the Western Interior Basin of . This member represents the uppermost unit of the formation and is dated to the early stage of the , with high-precision U-Pb zircon dating from interbedded ash layers constraining deposition between approximately 99.5 and 98.9 million years ago. Biostratigraphic evidence from the vertebrate fossils further supports this mid-Cretaceous age, marking a transitional period between Early and Late Cretaceous faunas in western . The Cedar Mountain Formation, including the Mussentuchit Member, accumulated in a setting associated with the , where tectonic loading from westward-advancing thrust sheets led to and infilling. ologically, the member consists primarily of fluvial channel deposits—characterized by coarse-grained, laterally accreting sandstones—interbedded with fines such as smectitic mudstones derived from altered . These strata reflect a distributive fluvial system with low- to moderate-sinuosity channels flowing northeastward across distal foredeep to forebulge depozones, indicating a dynamic environment influenced by proximal mountain sources. Paleoclimate during Mussentuchit deposition transitioned toward more humid conditions compared to underlying members, with estimated annual precipitation of 883–910 mm, facilitated by shifting summer winds that delivered moisture from the encroaching Western Interior Seaway. This supported seasonal river systems and a vegetated floodplain, as evidenced by carbonaceous materials and the absence of arid indicators like caliche soils. The associated fauna underscores the diverse ecosystem, including the ornithopod Tenontosaurus, smaller theropods such as Moros intrepidus, and other vertebrates like the hadrosauroid Eolambia, reflecting a rich continental assemblage preserved in these fluvial-lacustrine settings.

Ecological role

Siats meekerorum occupied the role of within the Mussentuchit Member ecosystem of the Cedar Mountain Formation, dominating the top carnivore niche during the stage of the mid-Cretaceous. As the largest known theropod in this fauna, it likely preyed primarily on large herbivores, including hadrosauroids such as Eolambia caroljonesi, which were abundant in the local assemblage. This genus coexisted with a diverse of smaller theropods, including dromaeosaurids and diminutive early tyrannosauroids, indicating niche partitioning to reduce competition. While Siats exhibited no strong habitat preference across and channel environments, smaller predators like certain dromaeosaurid morphotypes favored specific settings, such as , suggesting spatial segregation in behaviors. Its low abundance in the fossil record (approximately 14% of theropod teeth) further supports opportunistic strategies that targeted large prey while avoiding direct overlap with mid-sized carnivores. The presence of Siats as a gigantic neovenatorid theropod contributed to a mid-Cretaceous phase of large-bodied predator diversity in North America, bridging the ecological gap between the decline of carcharodontosaurians in the Early Cretaceous and the later dominance of tyrannosaurids. By competitively excluding tyrannosauroids from apex roles, Siats likely constrained their body size evolution until the extinction of such large allosauroids opened the niche for tyrannosaurid gigantism in subsequent stages. This dynamic highlights Siats' pivotal position in the transitional theropod faunas of the period.
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