Abstract

Ontogenetic information is crucial to understand life histories and represents a true challenge in dinosaurs due to the scarcity of growth series available. Mussaurus patagonicus was a sauropodomorph dinosaur close to the origin of Sauropoda known from hatchling, juvenile and mature specimens, providing a sufficiently complete ontogenetic series to reconstruct general patterns of ontogeny. Here, in order to quantify how body shape and its relationship with locomotor stance (quadruped/biped) changed in ontogeny, hatchling, juvenile (~1 year old) and adult (8+ years old) individuals were studied using digital models. Our results show that Mussaurus rapidly grew from about 60 g at hatching to ~7 kg at one year old, reaching >1000 kg at adulthood. During this time, the body’s centre of mass moved from a position in the mid-thorax to a more caudal position nearer to the pelvis. We infer that these changes of body shape and centre of mass reflect a shift from quadrupedalism to bipedalism occurred early in ontogeny in Mussaurus. Our study indicates that relative development of the tail and neck was more influential in determining the locomotor stance in Sauropodomorpha during ontogeny, challenging previous studies, which have emphasized the influence of hindlimb vs. forelimb lengths on sauropodomorph stance.

Highlights

  • IntroductionOne of the most dramatic evolutionary transformations recorded in the history of terrestrial vertebrates is the acquisition of gigantic body size in sauropods, through the transition from small to mid-sized early sauropodomorphs (dinosaurs that were mostly bipedal, omnivorous, and weighed less than 3 tons) to giant eusauropods (large, graviportal and quadrupedal herbivores of more than 10 tons)

  • One of the most dramatic evolutionary transformations recorded in the history of terrestrial vertebrates is the acquisition of gigantic body size in sauropods, through the transition from small to mid-sized early sauropodomorphs to giant eusauropods

  • Changes in the cross-sectional geometry of femora may be indicative of a shift from bipedal to quadrupedal stance within a few months of hatching, as suggested for Dryosaurus[17]; or just the opposite, with femora and tibiae becoming relatively weaker at resisting bending loads with increasing size, concurrent with relative strengthening of humeri, indicating a shift from bipedalism to quadrupedalism as in the hadrosaurids Maiasaura and Iguanodon[18,19]

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Summary

Introduction

One of the most dramatic evolutionary transformations recorded in the history of terrestrial vertebrates is the acquisition of gigantic body size in sauropods, through the transition from small to mid-sized early sauropodomorphs (dinosaurs that were mostly bipedal, omnivorous, and weighed less than 3 tons) to giant eusauropods (large, graviportal and quadrupedal herbivores of more than 10 tons). The ontogenetic shift from quadrupedalism, typical of sauropods, to the facultative or obligate bipedalism of Massospondylus (the ancestral condition) led to inferences that quadrupedalism in sauropods could have evolved through heterochrony (i.e., paedomorphosis), by the retention of early ontogenetic features in adults[28,30] (see[31]) It has not been tested whether the pattern estimated for Massospondylus is applicable to other early sauropodomorphs (largely because of the lack of ontogenetic series) or if it was an ontogenetic trajectory exclusive to its own lineage (or a restricted group to which it belonged, such as massospondylids). Evidence supporting previous shifting of stance in dinosaurs during ontogeny is based on limb proportions and allometric scaling[28], but still there is a lack of unambiguous, more direct osteological or biomechanical evidence supporting quadrupedalism at any age in sauropodomorphs, as noted for other dinosaurian groups[14]

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