How Fast Could It Run? Speed From the Spacing of Footprints

A sequence of footprints preserves several measurements directly. Stride length is the distance between successive prints of the same foot, pace length is the distance between consecutive prints of alternating feet, and trackway width indicates how far apart the feet were placed laterally. Print length gives an estimate of foot size, from which hip height is estimated using a ratio derived from measured animals, conventionally around four times foot length for a bipedal dinosaur. Those quantities together are what the speed estimate uses. The trackway also records direction, whether the animal was turning, whether it changed pace, and in some cases whether it was limping, which no skeleton can show.

The method rests on a relationship established from living animals:

• •Animals of different sizes moving in dynamically similar ways relate speed, stride and hip height in a consistent way
• •The relationship was published by Robert Alexander in 1976 and is the standard tool
• •Relative stride length, meaning stride divided by hip height, indicates whether an animal walked or ran
• •Values below about two indicate walking, and above about two and three quarters indicate running
• •Speed follows from stride length, hip height and gravity
• •Hip height must be estimated from the print, which is the largest source of error

Applied across many trackways, the method gives a consistent picture that is more modest than popular depictions. The overwhelming majority of dinosaur trackways record walking, at speeds of a few kilometres per hour, which is unsurprising since animals spend most of their time not running and since a running animal is less likely to leave good prints. A small number record faster movement, with the highest estimates from theropod trackways reaching speeds comparable to a human sprinter, and one Australian site preserving what has been interpreted as many animals moving quickly together. Large sauropods appear consistently to have walked, with no trackway indicating anything faster, which is consistent with biomechanical arguments that animals of that size could not run.

Speed is one of several things a trackway records and the others are frequently more interesting. Groups of parallel trackways heading the same direction at the same time indicate animals moving together, which is among the better arguments for social behaviour. Trackways of different sizes together have been read as adults with young. Sites where many trackways cross at various angles record a place animals repeatedly visited, typically a water margin. Predator tracks following herbivore tracks have been reported and are difficult to establish, since the two may have passed hours apart. Changes in depth along a trackway record the substrate and the animal's weight distribution. And a trackway where prints on one side are consistently shallower has been interpreted as an injured animal favouring a limb.

The assumptions deserve stating because the numbers are quoted with more confidence than they carry. Hip height estimated from foot length uses a ratio that varies between groups and between growth stages, and an error there propagates directly into the speed. The substrate matters enormously, since an animal moving across soft mud places its feet differently from one on firm ground, and prints deform as the sediment settles, changing apparent size. The formula was derived from living animals whose proportions differ from many dinosaurs. A trackway captures a few seconds, which may not be typical of the animal. And the identity of the trackmaker is generally inferred rather than known, since a print rarely comes with a skeleton attached.

Stride length and hip height estimated from print size feed a relationship derived from living animals, giving speed, with relative stride length indicating whether the animal walked or ran. Most trackways record walking at a few kilometres per hour, and the fastest theropod estimates approach a human sprinter. Hip height estimated from foot length is the largest source of error.