Trace Fossils

A trace fossil is the preserved sign of an animal's behaviour, rather than a body part. Trace fossils include footprints, burrows, nests, tooth marks, fossilised eggs and even fossilised droppings (called coprolites). Although they do not include any of the creature itself, trace fossils can be amazingly informative: they tell us things about how an ancient animal lived that bones alone cannot reveal. Did it walk on two legs or four? Did it travel in herds? How fast could it run? What did it eat? Some of the best answers come from trace fossils, not body fossils.

• Common types5+Tracks, burrows, nests, coprolites, gastroliths
• Oldest trace fossilsApprox. 550 million yearsWorm-like burrows
• Biggest dinosaur footprintApprox. 1.7 m longFound in Western Australia
• Famous trackwayLaetoli, TanzaniaHuman ancestors walking upright, 3.6 mya
• Largest coproliteApprox. 65 cm longProbably from a T. rex
• Special name for pooCoproliteFrom Greek for "dung stone"

What trace fossils tell us

Body fossils (bones, teeth, shells) tell us what an animal looked like. Trace fossils tell us how it behaved. Some of the things they reveal:

• How an animal walked (two or four legs, in a line or sprawled).
• How fast it moved (worked out from how far apart its footprints are).
• Whether it lived alone or in groups (multiple parallel trackways suggest herd behaviour).
• What it ate (from preserved stomach contents and coprolites).
• Whether it cared for its young (from nesting sites with eggs and babies).
• Where it lived (burrows often preserve the position of the burrow in relation to other things in the local environment).

The main types

• Footprints and trackways: probably the most famous trace fossils. Made when soft mud or sand was stepped on, then quickly covered by more sediment that preserved the print.
• Burrows and tunnels: dug by worms, crabs, small mammals and many other creatures. Often filled with different-coloured sediment so they stand out in the rock.
• Nests and eggs: dinosaur nests have been found with eggs, embryos and even babies inside. Some shoreline sites preserve entire colonies of nesting birds or reptiles.
• Coprolites: fossilised droppings. The contents reveal what an animal ate.
• Gastroliths: smooth pebbles found in fossilised stomach contents. Some dinosaurs (and many modern birds) swallowed stones to help grind up food.
• Tooth marks: bite marks on bones reveal who was eating whom, even when the eater itself is not fossilised.

Famous trackways

Some of the most exciting trace fossils are large groups of footprints preserved together.

• Laetoli, Tanzania: in 1976, anthropologist Mary Leakey discovered a set of footprints preserved in volcanic ash 3.6 million years ago. The prints were made by early human ancestors walking on two legs. The Laetoli prints showed that our ancestors had been walking upright long before they evolved big brains. One of the most important fossil finds of the 20th century.
• Davenport Ranch, Texas: a herd of large dinosaur footprints, all going the same direction, evidence that some dinosaurs travelled in groups.
• Lark Quarry, Australia: hundreds of small dinosaur footprints fleeing from a single large predator, captured in stone 95 million years ago. One of the most dramatic trace fossil sites in the world.
• Dinosaur State Park, Connecticut: a single layer of mud preserves over 2,000 dinosaur footprints from 200 million years ago.

Fact Scientists can work out how fast a dinosaur was running from its footprints. By measuring the length of each footprint and the distance between strides, plus making assumptions about leg length, they calculate the running speed. Most fossil trackways show dinosaurs walking at a leisurely 4 to 7 km/h. A few show smaller theropods running at over 40 km/h: about as fast as a galloping horse.

Coprolites: fossil poo

A coprolite is a fossilised dropping. They might sound silly but they are surprisingly useful. By looking at what is preserved inside, palaeontologists can work out exactly what an ancient animal ate. Coprolites have been found containing:

• Fish scales and bones (proving certain dinosaurs ate fish).
• Crushed dinosaur bones (showing T. rex really did chew up other dinosaurs).
• Seeds and pollen (showing what plants were around at the time).
• Insect parts (showing what small carnivores hunted).

The largest known coprolite is approximately 65 cm long and is thought to have been produced by a T. rex. It is on display at the Smithsonian.

Did you know? Many British beaches contain hidden trace fossils. The cliffs of Compton Bay on the Isle of Wight occasionally reveal large dinosaur footprints from approximately 125 million years ago, made by Iguanodon and others walking across what was then a warm river floodplain. Finding one is exciting but also a race against time, because the soft sandstone wears away quickly once exposed to waves and weather.

Deeper dive: what we have learned from dinosaur nests

One of the biggest revolutions in dinosaur research came from the discovery of dinosaur nests in the 1920s and again in dramatic ways in the 1970s and beyond. Before then, dinosaurs were imagined as cold-blooded, slow-witted creatures that laid eggs and abandoned them.

The Yale palaeontologist John Ostrom rocked the science world in 1969 when he described Deinonychus, a fast, intelligent-looking predator that he argued might be warm-blooded. Then in 1978, American palaeontologist Jack Horner discovered the first known dinosaur nesting site, in Montana. He named the species Maiasaura ("good mother lizard") after the discovery, which showed:

• Dinosaurs returned to the same nesting sites year after year (the site held many nests).
• They laid eggs in carefully arranged rings.
• The babies stayed in the nest after hatching (their teeth showed wear, meaning the parents brought them food).
• The babies grew rapidly, suggesting active warm-blooded metabolism.

Later finds in Mongolia revealed dinosaurs preserved in the act of brooding their eggs, just like birds. One famous fossil from China shows a small dinosaur called Oviraptor sitting protectively on a nest, killed by a sandstorm before it could move. Almost every modern image of dinosaurs as active, social, partly bird-like creatures comes from trace fossils discovered in the last 50 years.

For more, see dinosaurs and how fossils form.