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Leonerasaurus, new Argentine Jurassic sauropodomorph

From: Ben Creisler

In case this PLoS One article has not been mentioned yet:

Pol, D., Garrido A. & Cerda, I.A. (2011)
A New Sauropodomorph Dinosaur from the Early Jurassic of 
Patagonia and the Origin and Evolution of the Sauropod-
type Sacrum. 
PLoS ONE 6(1): e14572. 

The 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 Findings
A 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.

The 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.

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