Large dinosaurs normally walked with upright torsos and their heavy tails dragging along the ground.

Early dinosaur skeletons were difficult to reconstruct from incomplete fossils, and large bipeds were often compared with kangaroos. Museum mounts helped make the result iconic: a nearly upright torso, hips low behind the body and a heavy tail resting on the ground as a third support.

The pose became culturally self-reinforcing. Illustrations copied museum mounts, and the mounts in turn made the posture seem anatomically established. Yet articulated skeletons did not require such an erect stance, and fossil trackways produced an awkward question: if dinosaurs routinely dragged enormous tails, why were continuous tail grooves so uncommon alongside their footprints?

Biomechanical reconstruction increasingly placed the spine closer to horizontal, with the tail held clear of the ground as a counterbalance to the head and torso. The dinosaur renaissance of the late twentieth century made this more active, balanced posture familiar, and museums remounted major skeletons accordingly.

Occasional fossil tail-drag marks do exist, so no absolute rule says a dinosaur’s tail could never touch the ground. The change concerns the normal walking posture represented by generations of reconstructions.