The prespinal complex in sauropods: A zipper model.

SEBASTIÁN APESTEGUÍA; PABLO A. GALLINA

San Juan

Congreso; IV Congreso Latinoamericano de Paleontología de Vertebrados; 2011

Universidad Nacional de San Juan

During vertebrate ontogeny, the neural spines of dorsal vertebrae originate from two equal halves that fuse differentially from bottom to top (Romer, 1956, p. 226).This occurs along a ‘wave’ that follows a cranial progression along the vertebral series, from sacrum to neck (Salgado, 1999, p. 212). Embryonic hemispines complete its ontogenetic closure in most sauropods and the suture that evidences the fusion remains evident in the middle axis of the spine (Romer 1956, p. 226). In basal sauropods the plane of fusion is included in a wide rugose zone (‘prespinal bump’) whereas in Macronaria it is reduced to a vertical rugose rod: the ‘prespinal rugosity’. Built over its solid base develops a ‘prespinal lamina’, a commonly tall and sharp ridge of bone over the median axis of the anterior side of the neural spine in different sectors of the axial skeleton of Neosauropoda. Although present in cervical and anterior dorsal vertebrae, it is strong and well-developed in posterior dorsal, sacral and caudal vertebrae (where there are no evidences of bifurcation, even in diplodocoids). The study of the laminar development along the axial series is a useful tool in order to understand their constituent components and homologies. In the anterior dorsal vertebrae of basal eusauropods, spinoprezygapophyseal laminae (SPRL) add to the ‘prespinal rugosity’ to form a profusely striated and vertical ‘prespinal bump’. This widens toward the summits of the neural spine. In the bifid-spined taxa (e.g., some diplodocoids), the ‘prespinal rugosity’ is present between the paired SPRL at mid dorsal vertebrae, when the median cleft (=bifurcation) in the neural spine is reduced (Wilson, 1999). In both bifid and single-spined diplodocoids as well as in the single-spined basal titanosauriforms, the incorporation of the SPRL to the ‘prespinal rugosity’ occurs slowly along the entire dorsal series, being only complete toward the posterior-most dorsal vertebrae. On the other hand, in advanced titanosaurs the process is summarized to only the first two dorsal vertebrae. This way, all the additional changes along the dorsal series of a derived titanosaur include aspects that never occurred in a basal titanosauriform; they are new and not comparable to other sauropod lineages. As portrayed, the prespinal lamina bears a complex evolutionary history that affects the homologies and phylogenetic resolution of the involved taxa. Along phylogeny, the primary ‘prespinal rugosity’ is successively strengthened by the addition of at least the two recognized generations of SPRL (Bonaparte et al., 2006): the MSPRL at first (Prespinal Complex 1) and the (LSPRL) later (Prespinal Complex 2). This way, the ‘prespinal lamina’ of a derived titanosaur has a three-component ‘prespinal lamina’ on its neural spine, thus being non-homologous, for example, to the wide and rough ‘prespinal bump’ of a basal eusauropod. The pattern briefly summarized here, although more complex than that of previous proposals, is an oversimplification of an evolutionary process that we are just beginning to interpret, and most probably implied several additional steps.