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Juvenile brachiosaurid from Morrison Formation

From: Ben Creisler

A new online paper:

New information on a juvenile sauropod specimen from the Morrison
Formation and the reassessment of its systematic position.
Palaeontology (advance online publication)
doi: 10.1111/j.1475-4983.2012.01139.x

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 completely 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 of the spinodiapophyseal lamina,
the widening of the neural spines in the dorsal vertebrae) and
allometric growth in some limb bones.