Sauropod hypothesis: the base of the tail of sauropods was always more less horizontal. That is, the keystoning of the sacrum predicts the angle of the back from the ground. Prove me wrong @mike and @ScottHartman.
@john@mike I agree the tails were not arched into the air, even in the most keystoned of sacra, so it sets a minimum angle of the back. That's actually how I've been doing my new (or reposed) sauropod skeletals for 4 years or so now.
@mike@john We do here. Turns out sauropod tails go horizontal when I made other (IMO improved) assumptions about foot and pectoral posture, so the synergy with the biomechanical improvement makes it a strong argument to me. Also, while hadrosaurs and prosauropods once got the "tails arched in the air" treatment, those are clearly wrong, so with it not being normal to dinosaurs and there being no extant analog I think the "tail-arching up" model is coasting on social inertia at this point.