3D Geometric Morphometrics of the proximal ulnar articular surface of small mammals and its application in paleobiological inferences fo two typotheres (Notoungulata) from the Santa Cruz Formation

MUÑOZ, N. A.; VIZCAÍNO S. F.; CANDELA, A. M.

Mendoza

Congreso; 4th International Palaeontological Congress; 2014

Articularsurfaces of limb bones are very informative in morphofunctional studies,providing information about the relative movements between adjacent bones. Theelbow joint is a particularly complex hinge involved in flexo-extensionmovements between humerus and ulna as well as rotational movements betweenthese two and the radius. The proximal articular surface of the ulna wasexamined in living and extinct small mammals in order to assess its usefulnessas a proxy for this articulation in functional studies. The extant sample wascomposed with 11 species of eutherians: 5 histricomorph rodents, 6 carnivorans and one primate. The extinctspecies studied were the Early Miocene (Santacrucian) typotheres (Notoungulata)Hegetotherium mirabile(Hegetotheriidae) and Interatheriumrobustum (Interatheriidae). These typotheres have been described as rodent-likein overall form of estimated body masses of less than 10 kg. The proximalarticular surfaces of the ulnas were examined through geometric morphometricsin three dimensions after digitizing with a NextEngine Desktop 3D Scanner.Forty five landmarks were taken with the Landmark Editor software and PrincipalComponent Analyses (PCA) was used to explore the morphospace structure. The PCAof the whole surface was inconclusive; therefore successive analyses were madesubdividing it. The PCA for the posterior part of the facet is the mostinformative allowing recognizing morphospaces with functional value. In PC1,shape changes from narrower to a wider facet. In PC2, shape varies fromlaterally flattened and less anteroposteriorly concave trochlea, medial widthsub-equal to posterior width, and non-projected proximal lip to a morelaterally convex and more anteroposteriorly concave trochlea, medial widthnarrower than posterior width, and projected proximal lip. Diggers andgeneralized species lie at high values of PC1 and more cursorial species atmid-low values of PC1 and high of PC2; the rest at mid-low values of PC1 andPC2, with climbers lowest values of PC2. The typotheres lie with the cursorialspecies. These results differ from those obtained in a previous work with thedistal humeral facet, which placed I.robustum among climbers and H.mirabile in its own morphospace. This may be due to differentialoptimization of the facets of a joint, and must be considered in before makingpaleobiological inferences. The study of a portion of a facet permits a betterfocus on the details of the chosen area, and allows detecting patterns producedby differential optimization on certain parts of a joint.