Kayentavenator is not a tetanurine

Only five posts this month?!  Time to end September with a Kayentavenator series.

You may remember Kayentavenator from the discussions it ignited on the DML over whether Gay published it validly under ICZN rules.  I think the name is nomenclaturally valid, but that's not what I want to write about today.  The author rightly complained that the ICZN debate had completely overshadowed any attempt to discuss the paper's contents.  The only discussion so far has been that of Cau, who noted some anatomical identification issues and found it to be a basal theropod in his Megamatrix.  Unfortunately for Gay, the paper's contents hold up to scrutiny worse than the naming issue does.

Supposed tetanurine characters

UCMP V128659 was discovered in 1982 and referred to Syntarsus kayentakatae by Rowe (1989), as a subadult gracile individual. Tykoski (1998) did not examine it for his redescription of the species, but later (2005) examined it for his PhD thesis and considered it to be "probably referrable to "Syntarsus" kayentakatae" without discussion. Gay (2010) described the specimen as the new taxon Kayentavenator elysiae. Based on a small phylogenetic analysis, Gay placed Kayentavenator in Tetanurae but outside Avetheropoda. This was based on several characters which are all flawed.

The pubic articulation of the ilium is also larger than the ischial articulation in kayentakatae, and by a larger amount than Kayentavenator. The pubic peduncle's distal surface is also longer than wide in ceratosaurs and coelophysids like Megapnosaurus. The cnemial process arises from the lateral surface of the tibia in almost all theropods including kayentakatae. The trochanteric shelf is absent in all gracile and juvenile ceratosaurs and coelophysids, so cannot be used to place the juvenile Kayentavenator holotype in Tetanurae. Finally, the anterodistal femoral fossa is said to be non-elliptical in shape, which refers to a character originally used by Perez-Moreno et al. (1993). Ironically, in Perez-Moreno et al.'s analysis, the avetheropods were coded as having an elliptical fossa unlike Gay's analysis. In truth, avetheropods do not have fossae that are more or less oval than that of more basal theropods. While the fossa is poorly developed in Coelophysis, it is illustrated by Rowe in kayentakatae (as being non-elliptical due to its flat medial edge, for what it's worth) and is stated to be distinct in Segisaurus as well.

Gay also lists a feature in the description that is supposedly diagnostic of tetanurines- a pronounced sheet of bone projecting from the medial surface of the tibia, referring to Naish's (1999) description of BMNH R9385. Yet this must be a mistake as the feature Naish describes is the fibular crest on the lateral surface. However, Segisaurus has a prominent fibular crest comparable to tetanurines', while kayantakatae's is also described as large. There are therefore no characters placing Kayentavenator in Tetanurae.

Supposed differences from coelophysids

Gay states Kayentavenator "lacks a crista tibiofibularis and its associated groove, which are present in all coelophysoids and Dilophosaurus." Yet coelophysoids do not have a tibiofibular crest, the structure labeled as such by Rowe in kayantakatae being the ectocondylar tuber present in almost all theropods. Gay's description of Kayentavenator's femoral condyles is confusing as the "accessory condyle" is said to project from the medial condyle, yet the only accessory condyle in theropods including birds (which Gay states the accessory condyle of Kayentavenator resembles) is the ectocondylar tuber which is associated with the lateral condyle. Unfortunately, this supposedly unique morphology is not illustrated, with the femur only photographed in anterior view. Since the distal femur is separated from the proximal end in at least one element (and presumably the other, as the total length of both is said to be difficult to determine), it seems at least possible Gay confused the right and left distal femora. This would give them standard theropod ectocondylar tubers instead of apomorphically lacking the tuber and having a unique medial accessory condyle. In any case, young kayentakatae and Dilophosaurus specimens lack the deep groove lateral to the ectocondylar tuber, so its absence in the juvenile Kayentavenator specimen (confirmed by Tykoski, 2005) is expected.  

Kayentavenator is coded differently than "Coelophysisidae" [sic] in Gay's matrix for several additional characters. The caudal vertebrae are coded as having pleurocoels in the neural arch, which is not possible since pleurocoels are by definition a feature of vertebral centra. Furthermore, Gay states the position of the two partial preserved neural arches is impossible to ascertain. The pneumatic fossae are stated to face anteriorly on each side of the neural arch, indicating they may be anterior peduncular fossae as in Coelophysis cervicals, or even anterior infradiapophyseal fossae which all theropod presacrals possess. Since the neural arches are so fragmentary they cannot even be placed in the vertebral column (they are assumed by Gay to be from the posterior region only because the other remains are from the pelvis and hindlimb), they could even be backwards and merely exhibit posterior peduncular fossae as in all coelophysid cervicals including those of kayentakatae. Again, this supposedly unique feature is not illustrated, making evaluation difficult.

The brevis fossa is coded as being deep unlike coelophysoids, but coelophysoids including Megapnosaurus have deep brevis fossae. Coelophysids are oddly coded as lacking a supracetabular crest, which is untrue. They are also incorrectly coded as having an acetabular height only a third or less of the acetabular length, which is not true of any theropod (e.g. the ratio in kayentakatae is 88%). For the character "Pubic peduncle of ilium depth: 0, extends ventrally to the same level as ischiadic peduncle; 1, extends more ventrally than ischiadic peduncle.", coelophysids are coded as having nonexistant state 2 unlike Kayentavenator's state 0. In actuality coelophysids including kayentakatae have pubic peduncles extending ventral to their ischial peduncle. This is also true in Kayentavenator based on the stereophotograph in Tykoski's (2005) thesis, which does not agree with Gay's drawing. The photo also shows a complete articular surface on the pubic peduncle, while no obvious anteroventral corner to the process exists in Gay's illustration. Perhaps the peduncle was broken off during Gay's examination?

The obturator foramen is coded as open in Kayentavenator, despite Gay illustrating the ventral edge as closed but broken and stating the ventral margin was missing. Oddly, Kayentavenator is coded as having a pubic fenestra while coelophysids are not, despite the fact the latter are the theropods best known for having pubic fenestrae. Gay codes Kayentavenator as having a more propubic pelvis (~30 degrees from horizontal) than coelophysids (~45 degrees). This would be based off the angle of the pubic peduncle's articular surface, but as noted above, the preservation of this surface in Gay's illustration is in doubt.

The femoral head is coded as being subequally long and deep (in anterior/posterior view) while coelophysids' are coded as proximodistally elongate. However, the transverse width (from medial edge of femoral head to medial edge of shaft) is only 68% of the proximodistal height of the head, which is close to that in the kayentakatae holotype (63%). Since Coelophysis varies between 43% and 62%, a difference of 5% seems within plausible individual variation in kayentakatae. The anterior trochanter is aliform (as confirmed by Tykoski, 2005) while coelophysids' were incorrectly coded as absent. In actuality, many gracile coelophysoids (e.g. Dilophosaurus, Megapnosaurus) have aliform anterior trochanters as well. Gay codes coelophysids as having an anterior trochanter (contra the previous character) which does not extend proximally past the femoral head's ventral margin unlike Kayentavenator, but coelophysids' anterior trochanters do in fact extend past the femoral head's margin (e.g. kayentakatae- Rowe, 1989). Finally, the proximomedial fibular sulcus is coded as absent in coelophysids unlike Kayentavenator, but this feature is present in all adult coelophysids and was even made famous by kayentakatae.

In all, the characters which supposedly differ from coelophysids are miscodings or based on questionable morphologies (perhaps switched distal femora, possibly broken pubic peduncle, uncertain neural arch position). Supporting the placement of Kayentavenator in the Coelophysoidea is the presence of a divided articular facet on the pubic peduncle of the ilium, as illustrated by Tykoski.

Next up, is the Kayentavenator specimen a valid taxon of coelophysoid or a juvenile kayentakatae?

References- Rowe, 1989. A new species of the theropod dinosaur Syntarsus from the Early Jurassic Kayenta Formation of Arizona. Journal of Vertebrate Paleontology. 9, 125-136.

Perez-Moreno, Sanz, Sudre and Sige, 1993. A theropod dinosaur from the Lower Cretaceous of Southern France. Revue de Paleobiologie. 7, 173-188.

Tykoski, 1998. The osteology of Syntarsus kayentakatae and its implications for ceratosaurid phylogeny. Unpublished Masters Thesis, University of Texas at Austin. 217 pp.

Naish, 1999. Theropod dinosaur diversity and palaeobiology in the Wealden Group (Early Cretaceous) of England: Evidence from a previously undescribed tibia. Geologie en Mijnbouw. 78, 367-373.

Tykoski, 2005. Anatomy, ontogeny and phylogeny of coelophysoid theropods. PhD Dissertation. University of Texas at Austin. 553 pp.

Gay, 2010. Notes on Early Mesozoic theropods. Lulu Press. 44 pp.