Argentinosaurus was probably a primitive member of the group. It's broad vertebrae had small, peg-and-socket articulations that were located just above the spinal cord opening.peg-and-socket structures were common in saurischians, and probably kept the animals backbones rigid. These spinal feathers were absent in later titanosaurus(the lithostrotions), but it is unknown why they evolved more flexible backbones. Argentinosaurus is immense ribs were hollow, cylindrical tubes.