3D GEOMETRIC MORPHOMETRICS OF THE PROXIMAL ULNAR ARTICULAR SURFACE OF SMALL MAMMALS AND ITS APPLICATION IN PALEOBIOLOGICAL INFERENCES OF TWO TYPOTHERES (NOTOUNGULATA) FROM THE SANTA CRUZ FORMATION

MUÑOZ, VIZCAÍNO, CANDELA

Mendoza

Congreso; 4th international palaeontological Congress; 2014

Articular surfaces of limb bones are very informative in morphofunctional studies, providing information about the relative movements between adjacent bones. The elbow joint is a particularly complex hinge involved in flexo-extension movements between humerus and ulna as well as rotational movements between these two and the radius. The proximal articular surface of the ulna was examined in living and extinct small mammals in order to assess its usefulness as a proxy for this articulation in functional studies. The extant sample was composed with 11 species of eutherians: 5 histricomorph rodents, 6 carnivorans and one primate. The extinct species studied were the Early Miocene (Santacrucian) typotheres (Notoungulata) Hegetotherium mirabile (Hegetotheriidae) and Interatherium robustum (Interatheriidae). These typotheres have been described as rodent-like in overall form of estimated body masses of less than 10 kg. The proximal articular surfaces of the ulnas were examined through geometric morphometrics in three dimensions after digitizing with a NextEngine Desktop 3D Scanner. Forty five landmarks were taken with the Landmark Editor software and Principal Component Analyses (PCA) was used to explore the morphospace structure. The PCA of the whole surface was inconclusive; therefore successive analyses were made subdividing it. The PCA for the posterior part of the facet is the most informative allowing recognizing morphospaces with functional value. In PC1, shape changes from narrower to a wider facet. In PC2, shape varies from laterally flattened and less anteroposteriorly concave trochlea, medial width sub-equal to posterior width, and non-projected proximal lip to a more laterally convex and more anteroposteriorly concave trochlea, medial width narrower than posterior width, and projected proximal lip. Diggers and generalized species lie at high values of PC1 and more cursorial species at mid-low values of PC1 and high of PC2; the rest at mid-low values of PC1 and PC2, with climbers lowest values of PC2. The typotheres lie with the cursorial species. These results differ from those obtained in a previous work with the distal humeral facet, which placed I. robustum among climbers and H. mirabile in its own morphospace. This may be due to differential optimization of the facets of a joint, and must be considered in before making paleobiological inferences. The study of a portion of a facet permits a better focus on the details of the chosen area, and allows detecting patterns produced by differential optimization on certain parts of a joint.