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New sauropod paper in May issue of Palaeontology

New sauropod paper in the May issue of Palaeontology


New information on a juvenile sauropod specimen from the Morrison Formation and 
the reassessment of its systematic position

Keywords: Sauropoda; Neosauropoda; Titanosauriformes; Brachiosauridae;    
Brachiosaurus; ontogeny; early juvenile sauropod

Abstract:  Morphological changes in the ontogeny of sauropods are poorly known, 
making difficult to establish the systematic affinities of very young 
individuals. New information on an almost complete juvenile sauropod (SMA 0009) 
with an estimated total length of about 2 m is here presented. The specimen was 
described as a diplodocid owing to the presence of some putative synapomorphies 
of this group. However, recent further preparation revealed the absence of 
diplodocid characters and the presence of macronarian derived characters. To 
test the affinities of this specimen, a phylogenetic analysis was conducted. 
The strict consensus tree recovers the specimen as a basal titanosauriform, in 
an unresolved relation with Brachiosaurus and Giraffatitan. Nevertheless, a 
brachiosaurid assignment is here suggested in base of the widely accepted 
monophyly of this group (only recovered when SMA 0009 is placed within this 
group). Although the existence of a new taxon cannot be complet!
ely ruled out, the combination of derived and plesiomorphic characters in the 
specimen suggests its assignment to Brachiosaurus. Sixteen extra steps are 
needed to place this specimen within Diplodocidae. The high cost to place this 
specimen within this group is owing to the fact that several diplodocid 
characters are absent in SMA 0009, such as the absence of divided 
centroprezygapophyseal lamina in cervical vertebrae, procoelous anterior caudal 
centra, composed lateral lamina in anterior caudal vertebrae, elongated middle 
caudal vertebrae, short cervical ribs and caudolateral projection of distal 
condyle of metatarsal I. Finally, the systematic position reveals few major 
ontogenetic transformations. These affect the pneumatic structures (e.g. change 
from simple pleurocoels in the cervical vertebrae to complex pleurocoels and 
the development of lateral excavations in the dorsal vertebrae) but also 
include unrecorded transformations of the neural spine (e.g. the development o!
f the spinodiapophyseal lamina, the widening of the neural spines in the dorsal 
vertebrae) and allometric growth in some limb bones.

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