Is Kayentavenator a young "Megapnosaurus" kayentakatae?

For the Kayentavenator conclusion, we'll see how it compares to other coelophysoids, and "Megapnosaurus" kayentakatae in particular.

Gay's diagnosis

Gay lists several characters in the diagnosis which are meant to distinguish it from other theropods, including kayentakatae. One of these is the supposed medial accessory femoral condyle discussed in the last post. Another is the acetabular shape, which is described as arching "upward under the acetabular rim, making it taller than it is wide." However, it is clear from the illustration and Tykoski's photo that the supracetabular crest is broken. If it was pendent as in other coelophysoids, the apparent acetabular depth would decrease. If the above speculation about the pubic peduncle being broken in Gay's illustration is correct, this would also add to the acetabulum's length once corrected for.

Left- "Megapnosaurus" kayentakatae partial pelvis in lateral view (after Tykoski, 1998). Above right- Kayentavenator partial ilium in lateral view (after Gay, 2010).  Hypothesized complete ilial peduncle and supracetabular crest dotted in, along with ischium. Below right- Kayentavenator partial ilium in ventral view (after Tykoski, 2005).  Note large ventral surface of pubic peduncle, differing from Gay's drawing.

The greater trochanter and femoral head are stated to be fused, which seems to be an unorthodox way of saying there is no concavity between them. Gay notes in the description Coelophysis shares this morphology, and this is true for all other coelophysoids as well. Gay describes a mediodistal crest on the femur extending from the medial condyle for at least half the length of the element. While this might be assumed to be the medial epicondyle on the anteromedial edge common in non-tetanurines (but always less than a third of femoral length), kayentakatae also has a longer (40% of femoral length) crest extending from the medial condyle on the posterior shaft. Of course if the distal femora have been switched as proposed above, the crest would become lateral instead. A posterior lateral crest is present in kayentakatae's holotype, but this is intermediate in length between the medial epicondyle and postromedial crest. These posterior crests are undeveloped or poorly developed in Dilophosaurus, Liliensternus, Coelophysis and Segisaurus, though seem present in at least some Megapnosaurus. The distal femur is photographed in anterior view and shows no evidence of a medial epicondyle (though the dark coloring in this area could indicate the bone surface is broken off), while any posterior crest is of course unobservable. The transverse groove on the proximal femoral head surface is said to be unique among theropods, but is polymorphic in Coelophysis (e.g. NMMNH P-29046 and P-54620, UCMP 129618), so could be expected in some kayentakatae individuals as well.

The caudal centra are said to be highly constricted, with the description further specifying "minimum width of approximately 4mm, with an articular surface diameter of 17mm (Figure1)." Based on the scale in that figure, the articular surface is indeed close to 17 mm, but the minimum central height is 11 mm instead of 4 mm. While kayentakatae caudals have not been illustrated in lateral view, these proportions are similar to other coelophysoids like Megapnosaurus and Liliensternus.

A few additional characters are listed as being specifically distinct from kayentakatae. The anterior trochanter is said to be placed more medially, but this is not true. It should be noted that only robust femora are otherwise known for kayentakatae, which makes comparison of minor details questionable. The trochanter of Kayentavenator is actually more laterally placed than gracile individuals of Dilophosaurus and Megapnosaurus, but comparable to Liliensternus. The "groove in ventral surface of femoral head" is presumedly a typo for the groove in the proximal surface, which is dealt with above. The "spike on medial surface of tibia" is a typo for the lateral fibular crest, as indicated by Gay using the same character with 'lateral' substituted for 'medial' to distinguish Kayentavenator from Coelophysis, Megapnosaurus and Dilophosaurus. The crest of course is not a spike, and as previously noted is stated to be large in kayentakatae as well.

Thus the supposed diagnostic characters are all problematic. The accessory femoral condyle and acetabular shape may be misinterpreted, the distal femoral crest is unique as described but impossible to homologize, the caudal proportions and "fused" greater trochanter are normal for coelophysoids, and the transverse femoral head groove is prone to individual variation.

Is Kayentavenator distinct from other coelophysoids?

Comparing Kayentavenator to other coelophysoids is made difficult not only by the poor preservation and juvenile status of the former, but also the wanting description and figures, as well as the general lack of postcranial characters in coelophysid diagnoses. The one complete caudal centrum of Kayentavenator lacks a ventral median groove, unlike at least some of kayantakatae's centra. Yet this varies within the tail of many theropods like Eustreptospondylus, so is probably unimportant. The pubic peduncle being longer than wide is more similar to Megapnosaurus than Dilophosaurus and Liliensternus, as is the narrow and ventrally pointed ischial peduncle. The femoral head is more elongate than other coelophysoids (including Halticosaurus- contra Gay), but this shows individual variation that could be accommodated by kayentakatae as noted in the last post. If Gay's correct about the anterodistal femoral fossa, this is like Megapnosaurus, Segisaurus and kayentakatae but unlike Coelophysis, Liliensternus and Dilophosaurus. The fibular crest is larger in Segisaurus than in Coelophysis, which is in turn larger than in Dilophosaurus. Based on Gay's description, Kayentavenator is more similar to coelophysids in this respect, but without figures it's difficult to determine. Similarly, any comparisons of vertebral fossae or distal femoral ridges are hindered by their unknown homology due to a lack of description and figures.

Based on published evidence, Kayentavenator seems to be a coelophysid. Past that, it's difficult to tell. There's nothing verified that is more similar to kayentakatae than to other coelophysids, so I don't think it should be referred to that species. There are already at least two Kayenta coelophysids after all (kayentakatae and the Shake-n-Bake taxon which is too small and fused to belong to Kayentavenator). There are a few supposed diagnostic characters that have escaped definite rejection (short anteriorly projected pubic peduncle on the ilium; accessory medial femoral condyle; ambiguous mediodistal femoral crest longer than half of shaft length), but I'm hesitant to believe these are real based on the lack of appropriate femur illustrations, differences from the ilium's photo in Tykoski's thesis, and generally large amount of errors present in the paper. Further analysis may vindicate Gay or may identify features shared only with kayentakatae. At the moment, whether one makes Kayentavenator a nomen dubium depends on how much one trusts Gay's description.

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

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.