What Is a Dinosaur Trackway? Footprints Record Behaviour That Bones Cannot
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A skeleton tells you what an animal was built like. A line of footprints tells you what it was doing on one particular afternoon, how fast it was moving, whether it was alone, and how it held its body while walking, and none of that can be read from bones.
What a footprint preserves
Tracks form when an animal walks on sediment of exactly the right consistency, firm enough to hold an impression and soft enough to take one, and then the impression must be buried gently and quickly before rain, wind or the next animal destroys it. That narrow window means tracks are preserved in particular environments, especially lake margins, tidal flats and river floodplains, which biases the record towards animals that frequented those places. What survives is not simply a foot shape, since the sediment deforms below the surface as well, producing undertracks at lower layers that can be mistaken for separate prints, and the true track surface must be identified carefully. A print records the shape and size of the foot, the number and arrangement of toes, whether claws touched the ground, and sometimes skin texture. A trackway, meaning a connected sequence, records far more: stride length, the width of the gait, whether the animal was walking or running, and whether the limbs were held directly beneath the body rather than sprawling to the sides.
What can be calculated from them
Several quantities can be derived from measurements of a trackway with reasonable confidence:
- •Hip height, estimated from footprint length by a multiplier that varies between groups and is roughly four times for many bipedal dinosaurs
- •Speed, calculated from stride length and estimated hip height using a relationship derived from living animals, which gives walking speeds for most trackways and occasional running speeds
- •Gait, since the ratio of stride to hip height distinguishes walking from trotting and running
- •Posture, because narrow trackways with prints close to the midline demonstrate limbs held under the body, which was important evidence in overturning the older view of dinosaurs as sprawling
- •Whether the animal was bipedal or quadrupedal, and in some sauropod trackways whether the forefeet touched at all
- •Group behaviour, when multiple individuals of the same type travelled in the same direction at the same time on the same surface, which is the strongest evidence available for herding
The naming problem
Tracks are named separately from body fossils, using their own system of ichnogenera and ichnospecies, and that is not a quirk but a necessity. A footprint cannot usually be matched to a species, because many related animals had similar feet, because the same animal makes different-looking prints on different substrates, and because the track-maker is almost never found lying at the end of its own trail. Naming the track independently allows tracks to be classified and compared without claiming to identify the animal, and a track name therefore describes a shape rather than a creature. Occasionally the association is confident, when the geography, the age and the foot anatomy all match a known animal, but the default position is caution. The same discipline covers burrows, borings, nests, coprolites and feeding traces, collectively called trace fossils, and it is a distinct field with its own literature. The practical consequence is that a headline naming the species that made a set of prints is usually going beyond what the evidence supports.
What trackways have settled
Several long-running questions have been answered by tracks rather than by skeletons. Tail dragging was assumed for decades and is contradicted almost universally by trackways, which show no tail drag marks, establishing that large bipedal dinosaurs held their tails clear of the ground as a counterbalance. Herding in sauropods and in some ornithopods is supported by parallel trackways of multiple individuals, including sites where smaller individuals travelled in the centre of a group. Speed estimates from tracks generally give modest walking paces, with genuinely fast trackways being rare, which tempers reconstructions built from limb mechanics alone. Swimming behaviour has been proposed from trackways showing only claw scrapes, as if an animal were punting along the bottom, and remains debated. Some famous sites, including extensive surfaces in Texas, Bolivia and Lark Quarry in Australia, have supported detailed behavioural interpretations that were subsequently revised, which is itself instructive about how much interpretation a surface of footprints can bear.
The takeaway
Footprints need sediment of exactly the right firmness and rapid gentle burial, which biases the record towards lake margins and tidal flats. A connected sequence gives hip height, walking speed, gait and whether limbs were held under the body. Tracks carry their own names because a print rarely identifies a species. The absence of drag marks in trackways is what established that large bipeds carried their tails clear.