A new sauropodomorph dinosaur from the Early Jurassic of Patagonia and the origin and evolution of the sauropod-type sacrum.

Diego Pol,Ignacio A Cerda,Alberto Garrido,Andrew Allen Farke

doi:10.1371/journal.pone.0014572

Abstract

BackgroundThe origin of sauropod dinosaurs is one of the major landmarks of dinosaur evolution but is still poorly understood. This drastic transformation involved major skeletal modifications, including a shift from the small and gracile condition of primitive sauropodomorphs to the gigantic and quadrupedal condition of sauropods. Recent findings in the Late Triassic–Early Jurassic of Gondwana provide critical evidence to understand the origin and early evolution of sauropods.Methodology/Principal FindingsA new sauropodomorph dinosaur, Leonerasaurus taquetrensis gen. et sp. nov., is described from the Las Leoneras Formation of Central Patagonia (Argentina). The new taxon is diagnosed by the presence of anterior unserrated teeth with a low spoon-shaped crown, amphicoelous and acamerate vertebral centra, four sacral vertebrae, and humeral deltopectoral crest low and medially deflected along its distal half. The phylogenetic analysis depicts Leonerasaurus as one of the closest outgroups of Sauropoda, being the sister taxon of a clade of large bodied taxa composed of Melanorosaurus and Sauropoda.Conclusions/SignificanceThe dental and postcranial anatomy of Leonerasaurus supports its close affinities with basal sauropods. Despite the small size and plesiomorphic skeletal anatomy of Leonerasaurus, the four vertebrae that compose its sacrum resemble that of the large-bodied primitive sauropods. This shows that the appearance of the sauropod-type of sacrum predated the marked increase in body size that characterizes the origins of sauropods, rejecting a causal explanation and evolutionary linkage between this sacral configuration and body size. Alternative phylogenetic placements of Leonerasaurus as a basal anchisaurian imply a convergent acquisition of the sauropod-type sacrum in the new small-bodied taxon, also rejecting an evolutionary dependence of sacral configuration and body size in sauropodomorphs. This and other recent discoveries are showing that the characteristic sauropod body plan evolved gradually, with a step-wise pattern of character appearance.

Highlights

Sauropods are one of the most recognizable groups of dinosaurs, characterized by their gigantic size, quadrupedal stance, and extremely long cervical and caudal regions of the vertebral column
It has long been recognized that sauropods evolved from the much smaller, gracile, and bipedal primitive sauropodomorphs, a paraphyletic assemblage of taxa previously known as ‘prosauropods’ [1,4,5,6,7,8,9]
Geological Setting The dinosaur remains were recovered from the uppermost part of the Las Leoneras Formation (Figure 1), a sequence of continental deposits of presumed Lower Jurassic age briefly described by Nakayama [28]

Summary

Introduction

Sauropods are one of the most recognizable groups of dinosaurs, characterized by their gigantic size, quadrupedal stance, and extremely long cervical and caudal regions of the vertebral column. These are among the most noticeable features of the sauropod body plan, which was maintained relatively unchanged during their success as the dominant herbivores of the Jurassic and Cretaceous [1,2,3]. A series of recently described forms from the Late Triassic– Early Jurassic of Gondwana have been interpreted either as sauropod outgroups or basal sauropods [10,11,12,13,14] These have partially filled this gap and contributed to understanding the evolutionary origins of sauropods. Recent findings in the Late Triassic–Early Jurassic of Gondwana provide critical evidence to understand the origin and early evolution of sauropods

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Conclusion