Concavenator Part II - Becklespinax comparison, ulnar muscle details and a last lunch

It seems Naish had the same ideas I did regarding the similarity to Becklespinax and the supposed ulnar quill knobs being part of an intermuscular line.  That we both came up with these conclusions so quickly leads me to wonder why none of the article's reviewers did, or if they did, why they weren't taken into account by the authors. In fact, Becklespinax's description was cited (as reference 26 to an "undetermined tetanuran") though Ortega et al. incorrectly say "none of these [high-spined taxa] has elongated neurapophyses restricted to two presacral dorsal vertebrae."  Naish noted he proposed the same kind of short posterior dorsal sail for Becklespinax in 2007, and indeed the idea goes back to the Crystal Palace Megalosaurus which were given an anterior hump based on the Becklespinax material.  In regard to the ulnar morphology, Jura/Jason noted on the DML that the authors didn't examine any other explanations for the ulnar ridge and knobs.  He's correct to state that we shouldn't be so quick to accept new hypotheses without testing, and I have a feeling we'll see Concavenator inappropriately used as evidence for carnosaurian feathers without question for years to come.

In any case, what does a comparison of Concavenator with Becklespinax reveal?  Becklespinax has "tall neurapophyses of the eleventh and twelfth dorsal vertebrae (five times the height of the centra)", which is listed in Concavenator's diagnosis.  It also has "accessory centrodiapophyseal lamina on the transverse processes of the posterior dorsal vertebrae" on the last two dorsals, which is listed as another diagnostic feature that is also present in baryonychines.  Naish noted both lack pleurocoels, but this is normal for theropod posterior dorsals and is only untrue in certain groups like carcharodontosaurids, tyrannosaurids and caenagnathoids.  In regard to Becklespinax, Olshevsky (1991) stated "The firm contact between the apexes of the neural spines of vertebrae #9 and 10 (and, presumably, later vertebrae in the series) is a diagnostic feature of the specimen that occurs in no other known theropod genus."  This is also found in Concavenator, in which the anterior  neural spine actually seems to overlap the posterior one at their tips.  He also stated that "It is possible
that the neural spine of vertebra #8 is not broken off at the top but is naturally shorter than die spine of #9. This would constitute another very strong diagnostic feature of the genus, but it requires physical examination of the specimen before it can be confirmed."  This is found in Concavenator as well, to an even greater extreme.  Naish (online, 2007) noted Becklespinax has a spine table on its posterior two dorsals, but the condition in Concavenator has not been reported yet.

Dorsal vertebrae 10-12 of Concavenator (left, after Ortega et al., 2010) and Becklespinax (right, after Owen, 1856).  Sediment around neural spines removed to show shape better.  Red dotted line in Concavenator indicates possible real outline of twelfth neural spine, red dotted line in Becklespinax indicates possible dorsal limit to tenth neural spine.

So simply based on the published diagnoses for each genus, Becklespinax and Concavenator are indistinguishable.  Naish correctly stated Becklespinax is Valanginian (not Barremian like I wrote), which does make it a bit earlier stratigraphically than Concavenator.  Still, stratigraphy is frowned upon as a diagnostic character, or else Triceratops would be Agathaumas and such.  So we'll have to look in greater detail at the two genera, which isn't easy because Concavenator was described in Nature (so has an extremely brief description, though Ortega et al. do provide several excellent close up photos in the supplementary information) and Becklespinax hasn't been redescribed in a modern context (though Naish has done so in his unpublished thesis).  Also hindering matters is the fact Concavenator's neural arches are largely hidden by the dorsal ribs.  The exposed centra and diapophyseal areas are identical from the photos (which isn't saying much), but the neural spines do differ.  In Becklespinax, the tenth spine is taller and there is much more space between the eleventh and twelfth spines.  Also, in Becklespinax the neural spines are distally expanded while they are distally tapered in Concavenator.  I do wonder how much of this is real in Concavenator, especially in the twelfth spine.  That one has a strongly notched posterior edge that isn't matched at all by the life restoration's smooth triangular fin, leading me to wonder if its spine was distally expanded like Becklespinax's in life.  In all three characters, Becklespinax is closer to the condition in basal carnosaurs such as Sinraptor, which matches stratigraphy in a simplistic way.

Based on these comparisons, for the moment I consider both taxa valid, though closely related.  Whether they are congeneric or not is a subjective decision of course, but I don't think sinking Concavenator is a better idea than doing so to Effigia or any of the numerous genera Paul synonymized in his new book.  It's possible the distinguishing characters are individual or sexual variation, especially if the hump/sail was for display, but it's hard to know with a stratigraphically separated sample size of two.

Based on the ulnar musculature of modern alligators (Meers, 2003), the intermuscular line in Concavenator seems to be between the flexor ulnaris and the extensor carpi radialis brevis (Haro, DML).  This is shown in the figure below where you can see how anteriorly placed the line and knobs are in Concavenator compared to the quill knobs of Velociraptor and birds.  Note too that Concavenator's ulna is shown in anterolateral view based on the position of the radial articular surface (outlined in all three in green).  Thus the line would appear to be even further anteriorly positioned if the bone were in the same perspective as the Alligator and Velociraptor ulnae.  I've sketched in a rough guide to where the flexor ulnaris and the extensor carpi radialis brevis would be in Concavenator.  Based on the topographic similarity, I think we can be fairly certain the structure is an intermuscular line and not quill knobs.

Ulnae in lateral view of Alligator (top, after Meers, 2003), Concavenator (middle, after Ortega et al., 2010) and Velociraptor (bottom, after Turner et al., 2007). Radial articular surface outlined in green, relevent muscle placement added to Concavenator in colors matching Alligator, quill knob positions added in red for Velociraptor.

A final thought- has anyone else noticed the series of twenty or so small vertebrae illustrated in the gut area of Concavenator?  They seem too short for distal caudals, especially with their high neural spines and transverse processes.  I can only assume they are stomach contents, but it's odd the authors didn't mention it.

References- Owen, 1856. Monograph on the fossil Reptilia of the Wealden Formation. Part IV. Palaeontological Society Monographs. 10, 1-26.

Olshevsky, 1991. A Revision of the Parainfraclass Archosauria Cope, 1869, Excluding the Advanced Crocodylia. Mesozoic Meanderings. 2, 196 pp. 

Meers, 2003. Crocodylian forelimb musculature and its relevance to Archosauria. The Anatomical Record. Part A, 274A, 891-916.

Naish, online 2007. http://scienceblogs.com/tetrapodzoology/2007/10/_becklespinax_and_valdoraptor.php


Turner, Makovicky and Norell, 2007. Feather quill knobs in the dinosaur Velociraptor. Science. 317, 1721.

Ortega, Escaso and Sanz, 2010. A bizarre, humped Carcharodontosauria (Theropoda) from the Lower Cretaceous of Spain. Nature. 467, 203-206.