Remember the (subadult) Alamosaurus

Actually named after the Ojo Alamo Formation, not the tourist trap in Texas. Otherwise a fairly normal Late Cretaceous titanosaur, and one you can actually draw interacting with T. rex.

New edit: The new specimen that was mounted in Dallas has made it clear that there are important changes in the neck between the younger specimens and a large adult. You can see the adult version here: [link]

As near as I can tell the proportions shown here are those of a half-grown individual.