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New sauropod paper in May issue of Palaeontology
New sauropod paper in the May issue of Palaeontology
JOSÉ L. CARBALLIDO, JEAN S. MARPMANN, DANIELA SCHWARZ-WINGS & BEN PABST
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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