Ontogenetic changes in the postcranial skeleton of Mussaurus patagonicus (Dinosauria, Sauropodomorpha) and their impact on the phylogenetic relationships of early sauropodomorphs
ALEJANDRO OTERO; DIEGO POL
Journal of Systematic Palaeontology
Taylor and Francis
Early sauropodomorphs were diverse in Gondwana, being particularly well represented in the South American continent. Mussaurus patagonicus is one of the best known non-sauropod sauropodomorph that inhabited the Southern hemisphere. Its importance relies on its phylogenetic position close to Sauropoda and also because it is known from a well represented ontogenetic series, including embryos, neonate and late immature skeletons, which are particularly scarce among Sauropodomorpha, and mature. In this regard, Mussaurus represents an excellent opportunity to explore anatomical and paleobiological constraints during the ontogeny of early stages of the evolution of the group. We present the osteology of the postcranial skeleton of immature specimens of Mussaurus, highlighting main anatomical changes that occured during its ontogeny. The phylogenetic position of this taxon based on mature specimens is evaluated through a parsimony analysis, corroborating its position closer to Sauropoda than most other early sauropodomorphs. Immature stages of this taxon were also evaluated in a phylogenetic framework, showing an overall phylogenetic signal that positioned them closer to the root of Sauropodomorpha than the mature specimens. However, the cranial and some postcranial anatomical partitions of neonates and late immature specimens have a different phylogenetic signal, showing derived traits present in Sauropoda and related taxa (supporting the hypothesis of paedomorphic evolution in certain regions of the skeleton). Our analysis show that most appendicular apomorphies in Mussaurus appear late in the ontogeny, whereas axial characters (in particular for OS 1), including those of the skull and the presacral vertebrae, show derived character states early in the ontogeny that is congruent with the phylogenetic position of mature specimens. Ontogenetic series of other sauropodomorph species, however, are required to test if this pattern is extensive to the entire group.