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a new "prosauropod" indicates lotsa convergence in early sauropodomorphs



what a surprise! :)

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0014572

Diego Pol, Alberto Garrido and Ignacio Cerda report on a new early
sauropodomorph from Patagonia. A partial and partially articulated
individual is describes from what is probably early Jurassic
sediments, but may also be late Triassic. The critter has an odd
mixture of characters, including four sacral vertebrae (an extra
caudosacral), but some also significantly less derived, hinting at a
good amount of homoplasy going on. Exactly what I have been talking
about for years, based on what I have seen from the Late Triassic of
Europe. The biedpal-lightweight-prosauropod -->
quadrupedal-heavy-sauropod story was just that: a "just-so" story, and
this animal hammers another nail into the coffin.


Diego Pol, Alberto Garrido, Ignacio A. Cerda

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. doi:10.1371/journal.pone.0014572

ABSTRACT

Background
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.

Conclusions/Significance
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.