1363 Int. J. Morphol., 30(4):1363-1368, 2012. Shape Variation and Allometry in the Precloacal Vertebral Series of the Snake Daboia russelli (Viperidae) Variación Morfológica y Alometría de las Vértebras Precloacales en el Ofidio Daboia russelli (Viperidae) Ioannis Sarris; Jesús Marugán-Lobón; Beatriz Chamero & Ángela D. Buscalioni SARRIS, I.; MARUGÁN-LOBÓN, J.; CHAMERO, B. & BUSCALIONI, A. D. Shape variation and allometry in the precloacal vertebral series of the snake Daboia russelli (Viperidae). Int. J. Morphol., 30(4):1363-1368, 2012. SUMMARY: Understanding the variation of the ophidian vertebral morphology is an essential tool in snake paleobiology, but so far this field remains hardly investigated. A major problematic is the still scarce knowledge about the basis of homogeneity of intracolumnar shape variation along the vertebrae of the precloacal region in these animals. For instance, this variation can be overwhelmingly low in cases such as in vipers, for which it seems almost impossible to describe a concrete regionalization of the precloacal region without ambiguity. This study has applied geometric morphometrics to analyze if the shape variation of the vertebrae of the precloacal vertebrae of an adult specimen of Daboia russelli allows differentiating any sort of parcellation within the column of this organism. We have also explored if size is associated with the organization of vertebral shape along the axial skeleton. The multivariate analyses showed that the main pattern of vertebral shape variation in D. russelli concerns the neural spine and the hypapophysis, whereas the shape of the vertebral centrum appears to be nearly invariant along the series. Our analysis also showed that the precloacal region can be sudivided into two portions that merge in a transitional boundary of largest vertebrae in the middle of the column. From this middle region towards the distal ends of the column vertebrae become smaller changing their shapes in two antithetical ways. KEY WORDS: Intracolumnar variation; Russells viper; Axial skeleton; Serial homology; Vertebral morphology. INTRODUCTION The features that best characterize snake evolution are the total reduction of limbs and an extreme body elongation, often encompassed by hundreds of vertebrae (Gans, 1975; Romer, 1956). Regardless of the mechanisms involved in this phenomenon, the homogenization of verte- bral shapes blurs the boundaries between axial areas of the adult snake, making it very difficult to assign an isolated vertebra to a putative position in the column (Romer; Hoffstetter & Gasc, 1969). In fact, the recognition of regions in the vertebral column causes disagreement among ophidiologists (Cohn & Tickle, 1999; Caldwell, 2000; Tsuihiji et al., 2006; Woltering et al., 2009). The vertebrae are elements of a seriated structure, and their variation is continuous along the precloacal region of the column. Thus, their variability can be accurately assessed applying morphometrics methods (e.g. Thireau, 1967; Szyndlar, 1984), and in particular with geometric morphometrics (GMM) (Bruner & Bartolino, 2008). Using this tool, Polly & Head (2004) attempted to predict verte- bral shape relative to position along the vertebral column in snakes, concluding that the largest probability of success was concentrated at the distal parts of the column, stressing that shape variability is larger at the middle of the trunk. However, such an approach required an arbitrary subdivision of the precloacal region, neglecting the biological fact that vertebral size and shape may vary orderly and continuously along the column (Sarris & Chamero, 2009). If this order is case-specific, it is necessary to first evaluate its pattern within a single individual, as a means to single out the sources of vertebral variability. This study assesses the pattern of intra-individual changes of the precloacal vertebral series of an adult specimen of Daboia russelli (Viperidae) using GMM. We provide a detailed mapping of the pattern of size and shape variation and the association of the vertebral series of Daboia following this analytical protocol: 1) assess the shape variability of vertebrae using Principal Components Analysis, 2) evaluate the integration among vertebral Unidad de Paleontología, Departamento de Biología, Universidad Autónoma de Madrid, 28049, Cantoblanco, Madrid, Spain.