What Are Trace Fossils?
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Trace fossils are fossils of activity rather than anatomy: footprints, burrows, nests, bite marks and droppings. A skeleton tells you what an animal was. A trace fossil tells you what it was doing, and that information is almost impossible to get any other way.
Body fossils versus trace fossils
A body fossil preserves part of an organism, such as a bone, shell or tooth. A trace fossil preserves evidence of behaviour left in sediment which then hardened into rock.
The formal name is ichnofossil, and the study of them is ichnology. Because a trace fossil records a moment of activity, it captures a living animal in a way a skeleton cannot. A footprint was made by an animal that was alive and moving on that exact spot.
The main types
Trace fossils come in several recognisable forms:
- •Footprints and trackways, showing gait, speed and direction
- •Burrows and borings, revealing how animals lived in sediment or wood
- •Nests and eggs, showing reproductive behaviour
- •Coprolites, or fossil droppings, preserving diet
- •Bite and gnaw marks on bone, showing feeding or predation
What footprints tell us
A trackway is more informative than a single print. The distance between steps, combined with an estimate of leg length, allows researchers to calculate how fast the animal was moving. Some dinosaur trackways record a clear switch from walking to running.
Trackways have also settled arguments. Sauropod tracks show no tail drag marks, confirming that these animals held their tails clear of the ground. Parallel trackways of many individuals moving the same way are good evidence for herding behaviour.
Nests, eggs and droppings
Fossil nests have transformed how dinosaurs are understood. Sites in Montana revealed nesting colonies with young that appear to have stayed in the nest, implying parental care. Fossils of small theropods have been found crouched over egg clutches in a brooding posture identical to modern birds.
Coprolites sound like a joke but are seriously useful. One large specimen attributed to a tyrannosaur contains crushed bone fragments, direct evidence that the animal bit through the skeletons of its prey rather than stripping flesh neatly.
The catch with trace fossils
Trace fossils have one persistent problem: it is usually impossible to say which species made them. A footprint records foot shape, not identity, and the animal that left it is rarely found nearby.
For that reason tracks are given their own names separate from any body fossil species. It is a slightly awkward system, but it keeps the evidence honest by not claiming more than the fossil actually shows.
Reading a dinosaur chase
One trackway site in Texas records what appears to be a large theropod following a sauropod, with the predator's prints overlapping the plant eater's in a way suggesting it came afterwards.
Interpretations of this site have been debated for decades, and it may record a chase or simply two animals crossing the same ground at different times. The disagreement is instructive. Trace fossils invite narrative reconstruction, and researchers have to work hard to distinguish what the sediment actually shows from what a good story would require.
The takeaway
Trace fossils are footprints, burrows, nests and droppings that record behaviour rather than anatomy, and they are the main source of evidence for how extinct animals moved, fed and raised their young.