Dinosaurs first evolved 230 million years ago, and died out suddenly 65 million years ago. For 165 million years they were the dominant land-living animals on our planet. In addition to studying their skeletons, we can learn a lot about the biology and behaviour of dinosaurs from their fossilised footprints.
A single footprint is called a track, and a trail of footprints made by a moving animal is called a trackway. They show us whether the animal was two footed (bipedal) or four footed (quadrapedal), walking, running, its size, weight, and age and even whether it was ravelling in a herd or alone. Most bipedal dinosaurs, including Tyrannosaurus, walked on their toes and were built for speed and agility. The quadrapedal dinosaurs, like Brachiosaurus, although mostly large and heavy also walked mainly on their toes. Even the huge sauropods tip-toed on their front feet and were mostly only slightly flat-footed on their back feet.
Most dinosaurs walked either on two legs or on four, although some used both forms of locomotion as chimpanzees do today. The gait (width) of the trackway of bipedal dinosaurs was narrow due to their upright posture, whereas that of quadrupedal dinosaurs was wider to give it stability. However, as both types of animals increased their speed their gaits became narrower. From the stride length of a trackway it is possible to calculate how fast the animal was moving, and an increase in stride length indicates that it changed speed.
Most bipedal dinosaurs had a leg length or hip height approximately four times the length of their footprint. The same calculations applies for quadrupedal dinosaurs, but in this case it is the length of the back feet only that are measured. In these four-footed animals the length of the body can be obtained by measuring the distance between the mid point of the back feet and that of the front feet, which is the same as that between the hip and the shoulder. In both cases, once these sizes have been determined they are compared with known skeletons to estimate the overall length of the animal.
Dinosaur tracks are known from all over the world. Many recent discoveries have been made in Africa, North and South America, Asia, Europe and Australia. In Britain, most tracks have been found in Dorset, Yorkshire, the Isle of Wight and South Wales. The oldest known tracks are 225 million years old from middle Triassic rocks of France. They are three toed, bird-like tracks made by a fairly small bipedal dinosaur. Many other reptile tracks were found at the same site, showing that reptiles dominated the world at the time of the early dinosaurs. However, by early Jurassic times, 190 million years ago, dinosaurs had become the dominant land animals and at many track sites only dinosaur tracks are known. One reason for their success was that they evolved in agility and intelligence. Changes to the structure of their hip, knee and ankle joints allowed them to stand erect rather than sprawl like earlier reptiles and amphibians.
Tracks also tell us a number of other things about the life habits of dinosaurs. Drag marks made by animals' tails are known from trackways of lizard-like reptiles and short-legged amphibians but none are known from dinosaur trackways, showing that dinosaurs must have held their tails off the ground. Some dinosaur skeletons show damaged bones and healed injuries and tracks can also show evidence of injuries. A trackway in Utah, USA shows a dinosaur with a damaged toe and the trackway has alternating long and sort steps, indicating that the animal was limping.
Were dinosaurs solitary or did they move in groups? Trackway evidence suggests both. Many multiple animal tracksites are known from around the world, suggesting that some dinosaurs were social. Good examples from the Purgatoire River in the USA preserve five parallel brontosaur trackways made by a group moving along an old shoreline, and in South Korea a trackway shows eighteen three-toed ornithopods travelling together.
Not all trackways made by animals travelling in groups were made by animals of the same age. In rocks of Cretaceous age from the Davenport Ranch, Texas, twenty-three parallel brontosaur trackways have been found containing those of both old and young animals. Scientists have been able to reconstruct the position and movement of the whole herd, sowing that they were strung out four or five animals abreast, with the larger animals leading the way and the younger ones following behind.
Some of the trackways are huge and show evidence of hundreds of dinosaurs. A single limestone ridge in Colorado, containing over 1300 tracks, is the largest continuous tracksite in North America. The tracks occur in at least four layers of limestone and shale, deposited in a shallow lake during the Jurassic period, which shows that the lake was visited by dinosaurs over a long period of time. However, even this is minor by comparison with some of the 'megatracksites' that are known. A single 'megatracksite' in Utah, USA, covering 1000 square miles may contain over a billion tracks, while several are up to 100,000 square miles in extent and may contain a hundred billion tracks.
Some footprint sites give some indication of dinosaur distribution. Two tracksites of the same age in the western United States contain different communities in different regions, with iguanodons in Colorado and sauropods in Texas.
The trackways of other animals have been found with those of dinosaurs, including bird tracks. Although similar in shape these are generally smaller and have a much wider spread to the toes. Interestingly, when dinosaur tracks were first found, they were thought to be those of birds because of the close similarities. But why should bird tracks be found with those of dinosaurs? Perhaps the birds followed the dinosaurs along the lake shores feeding on insects and worms disturbed by their larger relatives. Relatives? Yes, because scientists believe that birds evolved from dinosaurs. The oldest known fossil bird, Archaeopteryx, is of late Jurassic age and by the Cretaceous Period bird tracks became increasingly abundant.