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

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 2m 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.