To give a brief summary (Ortega et al,2010) in their description of Concavenator noted series of small bony bumbs running in a single line on the ulna and proposed that these are homologous to quill knobs. Darren Naish and Mickey Mortimer came independently to the conclusion that instead of quill knobs the structure represents intermuscular line. And indeed looking at the Alligator forelimb musculature the line of knobs fits rather nicely between flexor ulnaris and the extensor carpi radialis brevis:
However let us not forget our feathered winged friends. If we are going to make conclusions based on muscular reconstruction, then we must also take into account the birdies
And here's a nice comparison modified from an excellent chart done by John Conway.
As you can see Birds simply do not have the large extensor carpi radialis brevis complex of crocodilians and the flexor ulnaris attachment area is above the mid line of the ulna.
So how should we reconstruct the musculature? It's problematic. Especially when we consider the locomotary difference between the two extant animals. Crocodilian with it's terrestrial/aquatic lifestyle, quadrupedality and postural diversity and in the other hand flying bipedal birds with heavily limited wrist motion. I tend to lean on birds due to closer relationship, locked radii&ulnae which would pose limitations to musculature and bipedal locomotion which provides similar pressures for forelimb musculature.
So it seems that the whole intermuscular line hypothesis isn't all that straightforward. In phylogenetic bracketing terms it's level II inference. It might or it might not be intermuscular line but that's depending on muscular reconstruction.
4 kommenttia:
A very well argued point. Concavenator is going to court controversy for sometime to come. Claims for a feathered Allosauroidea as opposed to a feathered member of the Coelurosauria clade are certainly newsworthy, but whether this indicates that the Neotetanurae, basal to the Coelurosauria were feathered is open to debate, this could be a case of convergent evolution.
Well, this gets complicated. Alligator has two muscles which extend over most of the lateral ulnar shaft- the extensor carpi radialis pars ulnaris and the flexor ulnaris. Both were undistinguished as the extensor carpi ulnaris by Gishlick (2002) since their origin shares fibers, but his ectepicondyloulnaris seems homologous to the flexor ulnaris. Gishlick says it's only in birds and not crocs, but it has the same association with the extensor carpi and inserts on the same area as the flexor ulnaris does in Alligator. This is what Conway identifies as the flexor ulnaris in the bird in his illustration. This leaves the extensor carpi origin on the ulna as a small circular area (the proximal pink spot in Conway's drawing). So it's true that birds don't have the long intermuscular division that crocs do, but what about carnosaurs?
Luckily, Gishlick examined the muscle scars on a wide range of theropod forelimbs. Plateosaurus, Allosaurus, Coelurus, Compsognathus and Ornitholestes have the primitive condition, while maniraptorans have a bird-like reduced flexor ulnaris. As Concavenator is firmly within the croc-like grade, my interpretation of the line seems justified as a level 1 inference.
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