PHYLOGENETIC RELATIONSHIPS OF MUSSAURUS PATAGONICUS: TESTING THE EFFECT OF ONTOGENETIC VARIABLE CHARACTERS IN TREE TOPOLOGY

ALEJANDRO OTERO; DIEGO POL; JAIME POWELL

Raleigh

Congreso; 73 Annual Meeting Society of Vertebrate Paleontology; 2012

Society of Vertebrate Paleontology

Mussaurus patagonicus is a basal sauropodomorph dinosaur from the Late Triassic Laguna Colorada Formation, Patagonia, Argentina. This taxon is known from multiple individuals of different ontogenetic stages, including hatchlings, juveniles, and adult specimens, providing a study case of ontogenetic changes in a basal sauropodomorph. Skull morphology is known from hatchlings and juveniles whereas postcranial remains are known from all ontogenetic stages. Here we test the effect of ontogenetic changes in the inferences on the phylogenetic position of M. patagonicus, scoring specimens from different ontogenetic stages as separate operational taxonomic units: only adult specimens (lacking cranial characters), only non-adult specimens (including cranial characters), and a mixture of non-adult (cranial) and adult (postcranial) characters in the same data matrix in order to test the impact of ontogenetic variations in the retrieved tree topology. The results depicted M. patagonicus as a sister group of massospondylids when scored only juvenile specimens on one hand, and the same resolution when scored only juvenile cranial characters or only juvenile postcranial characters. On the other hand, when we scored characters solely from adult individuals, all MPTs depict M. patagonicus as a basal member of Anchisauria, being the sister group of the clade composed by Aardonyx + more derived sauropodomorphs. M. patagonicus is therefore showing a more derived position forming part of the basal sauropodomorph-sauropod transition. The same results are retrieved when we scored a mixture of juvenile cranial and adult postcranial characters. Certain characters related to limb proportions in the juvenile specimens, such as the skull/femora and humerus/femur ratios show derived states, like those present in the quadrupedal clade (i.e., Eusauropoda) suggesting putative quadrupedal locomotion. Those characters, however, present plesiomorphic states in the adult specimens, suggesting, at least, facultative bipedal locomotion. On the contrary, most character states related to the manus in the juvenile specimens show no changes when compared to adult specimens, depicting this structure as a conservative module through ontogeny. The results shows the character states that vary through ontogeny and such variation does not always are present plesiomorphic states in early ontogenetic stages, but instead a mixture of apomorphic and plesiomorphic states reflecting in different tree topology. These changes are shown to be relevant for determining the phylogenetic position of M. patagonicus, as its affinities changes if adult morphology is ignored, suggesting that a careful evaluation of ontogenetic stages is needed for testing the phylogenetic relationships of basal sauropodomorphs.