Kayenta coelophysoid

Cranium MNA V2623.

The Kayenta coelophysoid is an unnamed genus of coeolophysoid dinosaur that lived in the United States during the Early Jurassic.

## History

UCMP V128659 was assigned "Syntarsus" *kayentakatae* by Rowe (1989). It was discovered in 1982, and is a gracile subadult. Tykoski (2005) agreed. Gay (2010) described the same taxon as a new tetanurine, *Kayentavenator*. Mortimer (2010), however, found there is no published evidence of their synonymy. It has also been called *"Coelophysis" kayentakatae* and *"Megapnosaurus" kayentakatae*, both of which are preferred temporary terms for the taxon until a replacement name is given. *"Syntarsus" kayentakatae* (although now considered invalid by entomologists) is preoccupied by a beetle. The cranium MNA V2623 is the holotype, known from the Kayenta Formation and collected from Sinemurian-Pliensbachian carbonaceous sandstone in 1977. It was discovered by Dr. Kathleen "Kayenta Kay" Smith and named after her in honour of her research of the Kayenta Formation.

## Description

It bears two, small parallel crests that were theorized to be ancestral to later, larger neotheropods such as *Dilophosaurus*. They both have a weak premaxillary/maxillary joint, which creates a hooked premaxillary. UCMP V128659 was discovered from 1982 and referred by Rowe (1989) as a gracile subadult, with Tykoski (2005) agreeing. Tykoski and Rowe (2004) differentiate it by nasal crests and the frontals separated by a midline anterior extension of the parietals. They also note a transverse groove on the anterior astragalar surface as an autapomorphy, but this is also seen in *Coelophysis* and the Shake-N-Bake theropod.

## Classification

Marsh and Rowe (2020) retained "*Syntarsus"* for QC 1 and MNA V2623 instead of using *Coelophysis* or Megapnosaurus since systematics in Coelophysoidea are in constant flux. Congenericity for *Megapnosaurus* would not be supported if *Coelophysis bauri*, *"S." rhodesiensis* and *"S." kayentakatae* do not form clades, further suggested by phylogenetic analyses. Ezcurra et al. (2021) found *M. rhodesiensis* to have been distant from *C. bauri* and *"S." kayentakatae*, who found *Camposaurus*, *Segisaurus* and *Lucianovenator* in their analysis. Ezcurra et al. (2020) find:

During the Description of Pendraig, Spiekman *et al.*, 2021 recover the Kayenta coelophysoid as the sister clade to Coelophysidae.

*"S." kayentakatae* was named by Raath (1969), and suggested to be called *Megapnosaurus*. However, phylogenetic analyses show that inclusion in this genus would make it polyphyletic. McDavid and Bugos (2022) note that this taxon requires a new genus name, suggesting *'Megapnosaurus' kayentakatae* with quotations until this is reaffirmed. McDavid later confirmed that a new genus name was going to be proposed, but that she would not be the one naming it.

## Paleoecology

The Kayenta Formation is from the Glen Canyon Group, and was primarily deposited by rivers and having silty faces made by a sluggish section of the river system. Dating the Kayenta has proven difficult. It had rainy summers and dry winters, with resilient animals. Other theropods include *Dilophosaurus*, *Kayentavenator* and the Shake-N-Bake theropod. Other fauna includes: *Sarahsaurus*, heterodontosaurids, thyreophorans, hybodonts, bony fish, lungfish, salamenders, *Prosailrus*, *Eocaecilia*, *Kayentachelys*, sphenodonts, lizards, Rhamphinian, *Dinnebitodon*, *Kayentatherium*, *Oligokyphus*, morganucodontans, haramiyids, crocodylomorphs, therapsids, bivalves, freshwater mussels, snails and ostracods. Plants include trees and algae and ichnites and coprolites are known. It presents the most diverse ceratosaurid fauna and has a diverse coelophysoid assemblage. The holotype was fossilized on its right side and partially buried, which stabilized the left side of the skull. This exposed the right, where bones likely drifted out during decomposition due to water currents loosening them. The weight of the sediment then distorted it.

## References

* McDavid SN & Bugos JE (2022) Taxonomic notes on *Megapnosaurus* and '*Syntarsus*' (Theropoda: Coelophysidae) *The Mosasaur* 12: 1-5
* https://en.wikipedia.org/wiki/Megapnosaurus
* https://en.wikipedia.org/wiki/Coelophysis%3F\_kayentakatae
* https://twitter.com/SkyeMcDavid/status/1554891381696602113?s=20&t=rxg-Nwz0NzAR\_Gbqb64nGg
* Spiekman SN, Ezcurra MD, Butler RJ, Fraser NC, Maidment SC (2021). "Pendraig milnerae, a new small-sized coelophysoid theropod from the Late Triassic of Wales". Royal Society Open Science. 8 (10): Article ID 210915. doi:10.1098/rsos.210915.