What Are 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.

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

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.

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.

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.

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.

Trace fossils have one advantage over bones and shells: they almost never move after they form. A shell can be washed along a beach or carried into deeper water before burial, but a burrow stays exactly where the animal dug it. Geologists use this to read ancient environments. Deep vertical burrows, such as those called Skolithos, tend to form in shifting sandy shallows where animals shelter below the surface, while intricate horizontal feeding trails are more typical of calm, deeper water. Recurring groups of traces like these are known as ichnofacies.

Traces also mark one of the great turning points in the history of life. The earliest animal traces are simple trails across the sea floor, but around the start of the Cambrian period, roughly 540 million years ago, burrows become more complex and reach deeper into the sediment. The change is so distinctive that geologists formally define the base of the Cambrian using the first appearance of a particular branching burrow, Treptichnus pedum, at a reference site in Newfoundland. A record of behaviour, not a body, marks the boundary.

A footprint is not simply a perfect mould of a foot. Prints form best in damp sediment, such as mud along a lake shore. If the mud is too wet the print slumps, and if it is too dry the foot barely marks it. A print usually survives when it dries a little and is then gently covered by new sediment, which can harden into a natural cast on the underside of the layer above. A heavy animal also presses down into layers below the surface, leaving blurred, wider undertracks that can mislead researchers about the true shape of the foot.

Researchers test these ideas by watching modern animals, such as large birds, walk across mud of different firmness. One of the most famous trackways shows why the effort is worth it. At Laetoli in Tanzania, footprints were preserved in damp volcanic ash about 3.6 million years ago, and in 1978 a team led by Mary Leakey uncovered a trail of early human relatives. The prints show a rounded heel and a big toe in line with the others, strong evidence that these hominins walked upright on two legs long before brains grew large.

Estimating speed from footprints is a neat piece of reasoning borrowed from living animals. In 1976 the zoologist R. McNeill Alexander showed that animals of very different sizes, from people to elephants, move in similar ways when their stride length is compared with their hip height. First, hip height is estimated from footprint length. For many two-legged dinosaurs it is commonly taken as about four times the length of the print. Then the stride, the distance between successive prints of the same foot, is measured along the trackway. The ratio of stride to hip height tells you the gait.

A ratio below about 2 usually means walking, while values above about 3 indicate running. Take a print 50 centimetres long, giving a hip height of about 2 metres, and a stride of 3 metres. The ratio is 1.5, a walk, and Alexander's equation gives a speed of a little over 2 metres per second, close to 8 kilometres per hour. Most dinosaur trackways give walking speeds like this, which makes sense, since animals spend most of their time walking. The answers are approximate, because hip height is itself an estimate, but they are anchored in measurements of animals alive today.

In the 1830s quarry workers near Hildburghausen in Germany found slabs of Triassic sandstone covered in strange tracks. Each print looked like a human hand, with four forward toes and what seemed to be a thumb sticking out to the side. With no bones nearby, the maker was named from the tracks alone: Chirotherium, meaning hand beast. Suggestions about its identity ranged widely, including a giant amphibian and various mammals, and the puzzle lasted for nearly a century. It is a classic illustration of the identification problem that runs through the study of trace fossils.

The breakthrough came from studying the foot itself. In 1925 the German geologist Wolfgang Soergel argued that the apparent thumb was really the outer, fifth toe, angled outwards, and that the tracks matched the feet of early archosaurs on the crocodile side of the family tree, a group that also gave rise to dinosaurs. Later skeletons from that group, with feet that fit the prints well, supported him. His method is still standard: compare the number, length and spread of toes in a print with the foot bones of animals from rocks of the same age, and treat the best match as a strong hypothesis rather than a certainty.

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.