Dinosaur footprints: how do they form and what can they tell us?
Fossilised bones are some of the most tangible evidence of a dinosaur, but they aren't the only way to study these prehistoric animals.
Preserved footprints, also known as ichnites, are a type of trace fossil and a window into the lives of dinosaurs.
They formed in the same way our footprints do when walking on soft ground like mud. But rather than being washed away, evidence of some of these reptiles' movements has survived for millions of years.
How do fossil footprints form?
The impression or print left behind when an animal's hand or foot pushes into the ground is called a track. Where they directly impact the ground is referred to as a true track.
As an animal takes a step, the ground below the hand or foot is compressed. This displacement forms features below the true track. These are known as undertracks, underprints, ghost prints or transmitted tracks.
Undertracks can extend anywhere from a few centimetres up to a metre below where the animal's hand or foot pressed into the ground. In prints that are preserved - for millions of years in the case of dinosaurs - sometimes you'll find both tracks and associated underprints, and in other places only one or the other will survive.
Tracks can also survive as natural casts. These are made by the material that filled the original track.
A trackway is more than one consecutive footfall from the same animal.
Why do some footprints fossilise?
A dinosaur could leave innumerable footprints, but only one skeleton. However, for tracks to form and preserve, conditions must be just right.
The consistency of the ground influences the shape, size and depth of the track and any associated underprints. For a perfect print, the ground can't be too hard or too soft.
If the ground is too hard, the resulting print would be very shallow, show very limited detail or not form at all.
If the ground is too soft, the track could collapse in on itself. If these prints survive, they would look distorted. For example, digit marks could turn into slits instead of distinct fingers or toes. Once prints form, they could easily degrade and be filled or washed away.
The soft ground of ancient shorelines or mudflats are common locations to find fossilised dinosaur tracks. For example, those found at the Red Gulch dinosaur tracksite in Wyoming, USA, were made during the Jurassic Period when this area was the shoreline of the Sundance Sea.
Unlike bones, which needed to be covered quickly once a dinosaur died to preserve as much of the animal as possible, tracks first needed to be baked hard by the Sun. This would have taken anywhere from days to months depending on the conditions.
Only then would a layer of mud, ash or similar help to preserve the tracks.
In some places, fossilised tracks make it look as though dinosaurs would have been walking up impossibly steep inclines, such as the near vertical Cal Orcko tracksite in Bolivia. But this is where the geology of the ground has changed dramatically over millions of years. The dinosaurs would have been walking over much flatter ground - the Cal Orcko site was a riverbed around 200 million years ago.
What can dinosaur footprints tell us?
Dinosaur tracks are a type of trace fossil. These are evidence of an animal's activity when it was alive, but are not part of the animal itself. Scientists that study this type of fossil are known as ichnologists.
It is almost impossible to tell exactly which species of dinosaur made a track - for example, many theropods have similar-looking three-toed feet. Additionally, bones and tracks don't line up exactly - the bones lack the soft tissue that was part of the foot or hand that made the print
Bones found close to a tracksite are unlikely to belong to the dinosaur that made the tracks, as they would have fossilised under different conditions. Termination trackways, where a dinosaur fossil is associated with its final steps, are exceptionally rare.
Instead, ichnologists are generally able to identify which dinosaur group made a track using clues such as the size and shape of a print. Geographic location and the age of the rocks may help narrow down the potential species.
Experts can also determine whether a trackway was made by a bipedal or quadrupedal dinosaur - one that moved on two or four legs. Bipedal footprints were made by either theropods or ornithopods - although some of the latter, such as Mantellisaurus, are thought to have spent time on all-fours too.
Theropods, such as Tyrannosaurus, Baryonyx or Velociraptor, had narrower and longer footprints than ornithopods. Theropod footprints typically have long, slender toes and a V-shaped outline. Ornithopod tracks lack distinctive claw marks and generally have a more rounded appearance with wider digits.
Thyreophorans (armoured dinosaurs) including stegosaurs and ankylosaurs, ceratopsians such as Triceratops, and sauropods like Diplodocus, were quadrupeds.
The differences between ceratopsian, stegosaur and ankylosaur tracks are subtle. Each had five fingers, but ceratopsians had four toes, stegosaurs had three and ankylosaurs had three or four. Stegosaurs and ankylosaurs overlapped in time and area, so telling their tracks apart can be tricky. Ceratopsians lived much later than stegosaurs and the number of toes helps to distinguish them.
Ankylosaurs generally had longer toes than ceratopsians. Additionally, ceratopsians may have walked on the tips of their fingers so wouldn't leave a palm print, whereas ankylosaurs walked with their palms flat on the floor.
Sauropods produced the largest tracks of all dinosaurs. Their footprints were wide and circular with five toes. Sauropods' handprints were smaller in comparison and had a crescent-like outline. Most sauropods had claws on their hands, although often only on the thumb, but evidence of these aren't always seen in tracks. The feet usually had three claws.
In some places only sauropod handprints are found. This may be due to the type of the ground they were walking on and how they distributed their weight. Some scientists have suggested this is evidence of sauropods swimming, using their hands to pull themselves along rivers.
In prints produced in perfect conditions, scientists may be able to spot skin impressions or other evidence of the animals' soft anatomy, as well as claw marks and indications of how flexible the digits were.
Tracks are a record of how a dinosaur moved. Trackways show how long a dinosaur's stride was. This can be interpreted from the spacing of the prints. It is sometimes also possible to estimate how fast the dinosaur was moving.
A series of parallel tracks may suggest that animals were moving in a group and could indicate possible herd behaviour. Some experts propose that some trackways with prints made by different types of dinosaurs are evidence of prehistoric chase scenes. However, predator and prey prints in the same place may have been made hours or even weeks apart.
A direct link with the past
Dinosaur tracks provide a snapshot of when these animals roamed across our planet. They are direct evidence of how an individual was behaving at a specific moment in time.
Fossilised bones aren't necessarily found where the animal lived, they could have been washed to a new location. But tracks were made by a dinosaur moving about its environment - so they are an important link between these prehistoric animals and the habitats they lived in.