The Fossils That Are Only Footsteps
In most natural history museums, the skeletons get the crowds. Towering, toothy, impossible to walk past. Somewhere off to the side, there's often a slab of rock with a few dents in it that most people stroll straight by.
Look closer, though. The dents are three-toed. They come in a line, left, right, left, right, crossing what used to be a stretch of soft mud. There's no animal anywhere. Not a scrap of bone.
And yet that slab may be the most lifelike thing in the building. A skeleton tells you what an animal was. A line of footprints tells you what it did, on one particular day, for a few seconds, a very long time ago.
A fossil of an action
Palaeontology, the study of ancient life, has two quite different kinds of evidence. The familiar kind is body fossils: bones, teeth, shells, the remains of the animal itself.
The other kind is trace fossils. These are evidence of what an animal did rather than what it was made of: footprints, burrows, even fossilised dung, which goes by the wonderfully dignified name of coprolites. Their study has its own name, ichnology, and a fossil footprint is called an ichnite.
A trace fossil is a record of behaviour. It's the difference between finding someone's car and finding the skid marks it left on the road.
How mud remembers a step
Think about walking along a beach. In dry sand your footprints are shapeless scuffs. Right at the water's edge they fill with water and vanish. But somewhere in between, on firm, damp sand, you leave a crisp print with every toe showing.
Ancient mud worked the same way. To get a good fossil footprint you need ground that's soft enough to take an impression but firm enough to hold it. Then it has to harden, baked by the sun over days or even months, before a fresh layer of mud or ash covers it over. Over a very long time, the layers turn to rock.
Which means a footprint isn't a neat cast of the foot. Its shape depends on three things at once: the ground, the foot and the movement. The same animal could leave a deep, slurred print in sloppy mud and a shallow, sharp one on firmer ground a few metres on. A footprint shows how a foot met the mud on that day.
Reading a trackway
One print is interesting. A line of them, a trackway, is where it gets really useful.
Researchers measure a few simple things. The step is the distance from one footprint to the next, left to right. The stride is the distance from one footprint to the next print of the same foot, left to left. And the trackway width shows how far apart the feet landed from side to side.
A narrow trackway suggests legs tucked underneath the body, upright, like ours. A wide one suggests limbs set further out to the side. Some trackways show both the front feet and the back feet, which go by the Latin terms manus (the "hand", the front foot) and pes (the hind foot), and that tells you the animal was walking on all fours.
The spacing gives you a rough idea of speed, too. Long strides from a small foot suggest an animal that was moving quickly. Short strides suggest a stroll. It's only ever an estimate, but it's an estimate of a living animal in motion, which no skeleton can give you.
Good at "what", bad at "who"
Now for the counter-intuitive part. Footprints are excellent at showing behaviour and surprisingly poor at identifying the animal.
Many meat-eating dinosaurs, the two-legged group called theropods, had feet that looked much alike. Their footprints can be almost impossible to tell apart. So scientists usually give tracks their own names, separate from the names of species known from bones, because they often can't say for sure which animal made them.
And here's the twist that catches people out. Finding bones near a trackway doesn't solve the problem. Bones found nearby are probably not from the animal that made the tracks. Bones and footprints usually fossilised under quite different conditions, and tracks that end at the animal's own body are exceptionally rare.
So the bones tell you who. The tracks tell you what happened.
Herds, and a chase that might not be one
Some sites have several trackways running side by side, all heading the same way. That may suggest a group of animals moving together. It's a tantalising hint of social behaviour, preserved in rock.
Some sites go further, with the prints of a meat-eater and a plant-eater crossing the same stretch of ground. It's very tempting to read that as a hunt, frozen mid-pursuit.
It might be. It might also be a later passage: two animals walking the same route at different times.
The timing problem
That's the honest caveat with trackways. A muddy surface can stay open for some time before it's buried, perhaps hours, perhaps longer. Two sets of prints on the same surface could have been made minutes apart or long after one another, and the rock doesn't always tell you which.
So a "chase" may be two separate walks across the same mud. A "herd" may be several animals following the same convenient route on different occasions. Footprints are wonderful evidence of behaviour, but the dramatic stories people read into them come with a large "maybe" attached.
Nothing of the animal, and all of it
There's something rather moving about this. A skeleton is what's left once life has gone. It tells you an animal existed. A footprint was made while the animal was alive, with its weight pressing down, mid-step, going somewhere.
Bones tell you that an animal lived. Footprints tell you it walked here, on this particular day. The most lifelike fossils of all turn out to be the ones with nothing of the animal in them.
Sources
Natural History Museum (London), "Dinosaur footprints: how do they form and what can they tell us?". https://www.nhm.ac.uk/discover/dinosaur-footprints.html (accessed 28 Oct 2026)
UC Museum of Paleontology (Berkeley), "How do we study fossil tracks?". https://ucmp.berkeley.edu/science/trackways/trackways4.php (accessed 28 Oct 2026)
US National Park Service, "Trace Fossils". https://www.nps.gov/subjects/fossils/trace-fossils.htm (accessed 28 Oct 2026)
UC Museum of Paleontology (Berkeley), "Trace fossils". https://ucmp.berkeley.edu/paleo/fossils/trace.html (accessed 28 Oct 2026)
