Fossa tricipitalis in penguins evolution

ACOSTA HOSPITALECHE, C.; HAIDR, N.

Diamante

Congreso; 9th International Meeting of the Society of Avian Paleontology and Evolution; 2016

Centro de Investigaciones Científicas y Transferencia de Tecnología a la Producción

FOSSA TRICIPITALIS IN PENGUIN EVOLUTIONHAIDR, NADIA1 and ACOSTA HOSPITALECHE, C2 1IBIOMAR, CONICET, Bvd. Brown 2915, U9120ACD, Puerto Madryn, Argentina. nadiahaidr@gmail.com2 División Paleontología Vertebrados, Museo de La Plata, Paseo del Bosque s/n, B1900FWA, La Plata, Argentina. CONICET. acostacaro@fcnym.unlp.edu.arPenguins present a hyper-specialized wing morphology. Regarding the musculature, the m. triceps humeralis (m.t.h.) and m. triceps scapularis extend the antebrachii in flying birds but in penguins this function is very limited and is carried out by the m. triceps scapularis. The m. triceps humeralis is composed by the m.t.h. dorsalis and the m.t.h. ventralis. The latter originates in the fossa tricipitalis, its belly occupies the whole fossa and extends distally into a strong and long tendon towards a sesamoid bone (ossified from the same tendon) in the elbow, to finally insert on the olecranon. Morphology of the fossa tricipitalis, constitutes a diagnostic character in penguins. All members of the crown-group Spheniscidae present a bipartite fossa (feature developed later in ontogeny), whereas all paleogene species have a single fossa. This partition appears during the trend of specialization toward the main efficiency in diving, and might have a functional significance. Muscle dissection in Pygoscelis adeliae and Spheniscus magellanicus were performed in order to understand the relationship between the structure and function of the m.t.h. and the fossa tricipitalis. The m.t.h. ventralis appears divided in two functional heads that originates in a different compartment of the fossa tricipitalis. Fibers run obliquely toward the tendon (two tendons for Spheniscus), where they converge with those of the other head. As opposed to one headed muscles, action of these confluent paired heads would cause a contraction without significant shortening of the muscle, and therefore, no elongation of the antibrachii. During contraction the sesamoids are firmly attached to the articulation making it more rigid and avoiding rotation of the elements of the zeugopodio. In Paleogene species, the fossa tricipitalis is invariably single, implying a different disposition of fiber packages. Without the arrangement in two heads, the elbow articulation might be not so stable. Bipartition of the fossa would confer modern penguins an advantage in diving, turning the wing into a more stable surface for pushing the water.