Systematic reevaluation of Toni , the juvenil sauropod from the Morrison Formation

CARBALLIDO, JOSÉ. L.; SCHWARZ-WINGS, DANIELA; MARPMANN, SEBASTIAN; SANDER, P. MARTIN; PABST, BENN

Pittsburgh

Congreso; 70th Anniversary Meeting, Society of Vertebrate Paleontology; 2010

Morphological changes in the ontogeny of sauropods are poorly known, making it difficult to establish the systematic affinities of very young individuals. A headless but almost complete juvenile sauropod nicknamed “Toni” with an estimated total length 2 m was recently recovered from Morrison Formation (Upper Jurassic, western USA). The specimen was described as a diplodocid due to the presence of some putative synapomorphies of this group. However, recent further preparation revealed a number of non-diplodocid characters. To detect the affinities of “Toni”, a phylogenetic analysis was conducted. In the strict consensus tree the specimen was recovered among titanosauriforms as a basal brachiosaurid. Fifteen extra steps are needed to place this specimen among Diplodocidae. Characters supporting titanosauriform affinities include: low sacral neural spines, middle caudal vertebrae with anteriorly positioned neural spine, and undivided distal condyle on metacarpal I. The absence of medially divided centroprezygapophyseal lamina in the posterior dorsals and the anteroposteriorly compressed femur support its inclusion in Brachiosauridae. Several diplodocid characters are absent in “Toni” such as the bifid cervical neural spines, procoelous first caudal centrum, weakly procoelous anterior caudals, long middle caudal vertebrae, short cervical ribs, and caudolateral projection of distal condyle of metatarsal I. The autapomorphic characters recovered in the analysis can reflect major ontogenetic transformations. These mainly affect pneumaticity (e.g., change from simple pleurocoels in the cervicals to complex pleurocoels and the development of lateral excavations in the dorsal vertebrae) but also include unexpected transformations (e.g., development of the spinodiapophyseal lamina, widening of the neural spines in the dorsal vertebrae). We thus identify the specimen as a juvenile of Brachiosaurus or a new titanosauriform taxon. The study of “Toni” allows a comparison of character acquistion during ontogeny and phylogeny.