T. rex takes a walk

Four footprints finally give an adult Tyrannosaurus rex a walking pace.

Footprints offer a glimpse of a living animal, moving and interacting with its environment. Bones are left behind by a dead animal. If we're honest, these tracks don't tell us a whole lot that we didn't already know – we know T. rex could walk on land. The tracks don't record running, or any unusual behaviour – it is just a sequence of four steps. But it's still pretty incredible to know that an adult T. rex walked in exactly that place, and how long its steps were, and that it placed its feet one in front of the other in quite a narrow line.

We infer it's an adult T. rex mostly through a process of elimination: the tracks are huge — among the largest three-toed tracks known. The only animal in the right place at the right time with feet that size is adult T.rex; we can date it to 66.5 million years ago, from magnetic reversals recorded in nearby rocks. T. rex bones are abundant at this locality; several partial skeletons come out of the same Badlands. Hadrosaurs, plant-eating dinosaurs, also made big three-toed tracks, but their toe impressions are broad and blunt, and their feet tend to be wider than long, so we can rule them out.

We can get a rough idea of speed from a combination of foot length and stride length, and in this case it's about 4 mph, a brisk walk for a human. That doesn't mean T. rex couldn't move faster, just that in this case, it was taking a leisurely stroll. A seven-tonne animal ambling along at walking pace is what physics predicts, but prediction is not observation; four footprints are the observation.

Ongoing thread. More from Peter L. Falkingham to follow.