Los bumps de Concavenator ¿quill knobs o cicatriz muscular? en el 75th Annual Meeting de la SVP

Concavenator también tuvo su lugar en la sección de póster de la SVP que tuvo lugar en Dallas. Una de las características de este terópodo es la presencia de una serie de protuberancias o "bumps" en la superficie posterolateral de la ulna. Estos bumps, tanto por morfología como por situación, parecen ser homólogos a los quill knobs de las aves actuales. Los quill knobs representan la zona de inserción de las plumas remeras de las alas de las aves al hueso de la ulna mediante pequeños ligamentos. Por tanto, la presencia de una estructura similar en Concavenator supondría que estaría provisto de estructuras tegumentarias, más primitivas que las plumas actuales. Un estudio reciente plantea una hipótesis alternativa sobre el origen de dichas estructuras, asemejándolo a las crestas intermusculares que presentan las aves en la parte anterior de la ulna. Además, aún sin ser propuesta, también podría formularse la hipótesis de que se tratase de la cicatriz de la inserción de algún músculo ulnar. Sin embargo, la reconstrucción propuesta en el trabajo presentado en el congreso de la musculatura del brazo de Concavenator, muestra que dichas hipótesis son refutadas y que la hipótesis inicial es la más parsimoniosa hasta la fecha.

Para más información, aquí está el resumen (en inglés) del libro de abstract del congreso:

The Concavenator corcovatus holotype (MCCM-LH 6666) is a skeleton of a carcharodontosaurid from the Las Hoyas fossil locality (Lower Cretaceous, Spain). This specimen shows unusual features, such as elongated neurapophyses of the dorsal vertebrae and a series of small bumps on the ulna.
The right ulna is completely preserved and all sides are visible, except for the medial side, which is hidden. The bumps are present on the posterolateral surface of the ulna. Three of them, located more laterally, are the most marked, and their separation is 6 mm. The two most distal bumps are posteriorly directed and their separation is from 10.6 to 16.7 mm.
These bumps are topologically homologous to the quill knobs of birds. Although quill knobs are generally situated on the posterior surface of the ulna, some taxa, such as Gallinula, develop these knobs on the lateral surface. Altenative hypotheses to explain these bumps consist of their association to an intermuscular crest or an attachment scar for the insertion of an ulnar muscle.
A myological reconstruction of the forelimb of Concavenator is performed in order to test these hypotheses. The reconstruction is carried out based on the osteological description, anatomical comparison with extant taxa, and using the Extant Phylogenetic Bracket (EPB).
The insertions of M. triceps brachii (TB) and M. anconeus (AN), and the origin of the ulnar head of M. abductor pollicis longus (APL) are reconstructed on the ulna of Concavenator. The first is located on the posterior surface of the olecranon. The insertion site of AN is situated on the anteroposterior surface of the ulna based on EPB. The origin of APL is located on the anterior surface of the ulna.
The bumps are not located between these muscles. Thus, the hypothesis that the bumps could be an intermuscular crest is refuted. As for the second hypothesis, the bumps could be a muscle scar of AN. However, the insertion of this muscle is a fleshly attachment that produces a featureless surface, without osteological correlates.
In conclusion, the myological reconstruction does not provide any evidence supporting the hypothesis that the series of bumps can be interpreted as an intermuscular line or attachment scar. However, the existence of birds with quill knobs in the same position as the bumps of Concavenator inclines us to consider that this is, so far, the most parsimonious interpretation. This interpretation indicates the presence of skin appendages in Concavenator, preceding the wing feathers present in Maniraptora.