Siats

*Siats meekerorum*

***Siats* *meekerorum*** was the apex predator of the Cenomanian Stage. It's name is a combination of two factors; the word Siats, comes from a man eating monster in Ute Mythology, whereas Meekerorum honors the late geologist John Caldwell Meeker, who bequeathed a fund for paleontological research. The holotype specimen is found in Utah, Cedar Mountain formation (Mussentuchit Member), which also gave us the well-known *Deinonychus*. The holotype is also the only known specimen from this genus, described by paleontologist Lindsay E. Zanno in year 2009. It is one of the largest theropod dinosaurs known from North America. Skeleton includes traces of wide, crocodilian - shaped teeth. Dr. Zanno thinks that is a clue that giant crocodiles. Today, Siats is generally known for being the fourth largest terrestrial carnivore North America has ever known. It was initially classified as a megaraptoran, a group with controversial relations, but is now mostly associated with allosauria.

## Discovery

First when Zanno and Makovicky discovered the skeleton, they thought it belongs to carcharodontosaurid type of dinosaur, possibly close related to *Acrocanthosaurus* or *Shaochilong*. Most phylogenetic analysis suggests that it was a carcharodontosaurid (Neovenatorid?). The paper describing a juvenile *Megaraptor* suggested that *Siats* is not a megaraptoran, and instead is likely a neovenatorid, as it shares several traits with *Neovenator*, but differs 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. The discovery of Siats also reveals that allosauroids did not yield dominance in North America to tyrannosauroids until the late Cretaceous

*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. FMNH PR 2716 is the first specimen referable to the Neovenatoridae discovered in North America and represents the geologically youngest allosauroid yet discovered from the continent.

## Description

The holotype came from a single immature individual, based on the incomplete fusion of neural arches to their centra in the vertebral column. *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 the third largest theropod from North America, after *Acrocanthosaurus* and *Tyrannosaurus*. It has been suggested that this individual would have been about nine meters (30 ft) in length and weighing roughly 2.5 metric tons. However, a full grown *Siats* would have grown about as long as the earlier carcharodontosaurid *Acrocanthosaurus*, measuring about 12 meters (40 ft) in length and nearly 4 metric tons in weight. The discovery of this neovenatorid also reveals that allosauroids did not yield dominance in North America to tyrannosauroids until the late Cretaceous.

### Size

Holotype specimen is about 9-11 m long, over 3 m tall and weighs 4-5 tonnes. However, the description paper of Siats states that the holotype is skeletally immature. Hypothetical adults could most likely reach sizes of over 12 m in length and over 5 tonnes in weight, what surely makes this Theropod one of the largest predators in the North America.

## Phylogeny

The cladogram presented below follows Zanno & Makovicky (2013). Updated by Porfiri et al., 2014.

The cladogram below follows Naish and Cau (2022), who found *Siats* within Megaraptora as part of Tyrannosauroidea

## Environment

Siats meekerorum was found in the Mussentuchit Member of the Cedar Mountain Formation. It lived alongside ankylosaurs, the giant Sauropod Abydosaurus, Deinonychus, the small tyrannosaur Moros and ornithischians such as Tenontosaurus and Eolambia. Due being recovered as carcharodontosaurid, Siats very likely mainly gone after Eolambia, Tenontosaurus and Abydosaurus.

## Appearance in other media

### *Jurassic Park*

* Siats appeared in the Jurassic World toys.

## References

1. https://en.wikipedia.org/wiki/Siats

2. https://www.nature.com/articles/ncomms3827

3. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0157973&from=article\_link

4. https://www.deviantart.com/bluethedakotaraptor/art/Siats-meekerorum-820886118

1. ↑ Cite error: Invalid `<ref>` tag; no text was provided for refs named `Naish e12727`