Online exhibits : Special exhibits : Fossil footprints
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Fossil footprints through geologic time
By Allison Vitkus, Karen Chin, and Martin Lockley
Animals have been walking, scurrying, or slithering on this planet for over 540 million years. During the long history of fossil track creation, environments on Earth have gone through many changes. Continents have moved and sea levels have risen and fallen many times click on the graphic at right to see an animation of the movements of the continents over the last 750 million years. As a result, many of the oldest rocks on Earth have been lost through erosion and tectonic plate collisions. Nevertheless, fossil tracks have been found all over the world.
What depositional environments are tracks commonly found in?
Sedimentary rocks preserve records of past habitats. The types of rocks in an area give us clues as to whether the ancient environment was once an ocean shoreline, a shallow lake, a field of sand dunes, or another type of habitat. We tend to find more fossil tracks in sediments deposited in certain environments. As discussed in the How Tracks are Preserved section, burial of tracks improves their chances of fossilization. Thus, tracks made near bodies of water and sand dunes are more likely to be buried before they are destroyed than are footprints made in other environments.
Tracks are commonly found in areas associated with streams or rivers. Sediments deposited by rivers and streams are called fluvial deposits and include mud, silt, and sand. The track below (left) was made in mud that is thought to have been deposited by a flooding river. This particular type of track is called Dromaeopodus shandongensis and was probably made by a theropod dinosaur; theropods are a group of bipedal dinosaurs that include Tyrannosaurus and modern birds.
It is also common to find tracks in sediments that were deposited along the shores, or in the shallow waters of lakes (called lacustrine sediments). Below right are tracks of a web-footed bird found in the Green River Formation of Utah, USA. The Green River Formation is made up of lake sediments deposited during the Eocene Epoch and is known for containing many fossil fish skeletons.
Fossil tracks associated with marine environments were commonly formed along the shores or in shallow waters of the ocean. The trace below left is called Isopodichnus and was probably made by an isopod or another type of crustacean. This trail was made in a shallow sea in North America during the Jurassic Period, when a series of inland seas flooded central western North America.
Tracks can also be preserved in eolian deposits. These are sediments that were moved by wind instead of water. Below right is a type of trackway called Paleohelcura. It is thought to have been made by a scorpion walking across sand dunes during the Permian Period.
Fossil tracks from around the world
The Dinosaur Freeway reveals about 80 tracksites in a single rock formation ranging from northern New Mexico to northern Colorado. These sites preserve tracks made by animals of varying sizes, and many indicate that the dinosaurs that made them were moving parallel to each other. Most of the tracks were made by ornithopod dinosaurs (a group of herbivorous dinosaurs that includes duck-billed and iguanodontid dinosaurs), though some were made by bipedal theropod dinosaurs (including shorebirds) and crocodilians.
One easily accessible fossil site along the Dinosaur Freeway is the Dinosaur Ridge portion of the Morrison-Golden Fossil Area National Natural Landmark near Denver, Colorado. This area contains many ornithopod and theropod tracks. The tracks are found in different layers of sandstone, indicating that the animals walked through the area at different times.
An interesting European dinosaur tracksite can be found near Fátima, Portugal. The tracks were discovered in the bottom of the Pedreiros do Galinha limestone quarry in 1994. They were made in shallow marine sediments during the middle of the Jurassic Period, about 170 million years ago. Large, long-necked, herbivorous dinosaurs, called sauropods, apparently produced the footprints. These Galinha sauropod tracks are notable for being particularly large and well-preserved, and the site was dedicated as a national monument by the Portuguese government in 1996 (Monumento Natural das Pegadas dos Dinossáurios da Serra de Aire). Visitors to Fátima can see the tracks and learn more about them at the visitor center.
Spectacular fossil tracks have been discovered high in the Andes in Bolivia. These tracks were made near the end of the Cretaceous Period around 66 million years ago. Since then, geologic forces have tilted the track beds, forming a steeply angled wall. Workers in a cement quarry discovered the site in 1994, and now over 5,000 dinosaur tracks have been found in what is now Parque Cretácico in Cal Orck'o. The site includes tracks made by ankylosaurs, armored herbivorous dinosaurs that walked around like prehistoric tanks. Ankylosaur tracks were thought to be rare before the discovery at Cal Orck'o. For many years, people doubted that ankylosaurs had lived in South America at all. Theropod and sauropod tracks have also been found at the site, as well as bones from turtles, fish, and crocodilians. Unfortunately a large section of the wall with the best trackways fell down in 2010. Although this section was lost, photographs of the site are among the most spectacular dinosaur track images ever taken.
The Laetoli area fossil beds in northern Tanzania preserve a series of tracksites famous for the footprints of early hominins (a group that includes humans and our close ancestors). Laetoli tracks demonstrate an unusual type of track fossilization because they were preserved in volcanic ash about 3.5 million years ago. These footprints are much younger than most of the fossil tracks described on this website, underscoring the fact that hominins have had a relatively short history on Earth compared to most other animals. At the time they were found, the Laetoli tracks provided the oldest evidence of hominins walking bipedally (walking on two legs). There has been debate about which species actually made the Laetoli tracks, but Australopithecus afarensis (the same species as the ancient skeleton known as "Lucy") appears to be the best track-maker candidate. Fossil tracks of birds, invertebrates, and mammals (including hares, horses, and hyenas) have also been found in the Laetoli sediments.
The Haenam fossil site in South Korea hosts over 500 dinosaur tracks, 400 pterosaur (flying reptiles related to dinosaurs) tracks, and thousands of web-footed bird tracks some of the oldest of their kind known. The fossil tracks are found in the Uhangri Formation, which was deposited during the mid to Late Cretaceous, between about 100 and 72 million years ago. Most of the dinosaur tracks at this site were made by ornithopods, though some tracks are thought to have been made by sauropods. The pterosaur tracks include hind foot impressions as well as hand prints. Studies indicate that pterosaurs almost always walked on all fours, with their wings folded up so their hands touched the ground. The Haenam fossil site was designated as a national monument in 1998, and Uhangri Dinosaur Museum is located near the tracks.
The Lark Quarry "dinosaur stampede" in Australia presents another example of a site with many Cretaceous dinosaur tracks. The tracksite was discovered in the 1960s and is now preserved in the Lark Quarry Conservation Park. Some scientists think that the thousands of fossil footprints represent a stampede of a mixed group of small dinosaurs reacting to the appearance of a large carnivorous theropod dinosaur. Other scientists suggest that Lark Quarry was simply a well-used route for dinosaurs along the edge of a river. The Lark Quarry Dinosaur Trackways site is about 110 km from Winton in central Queensland.
The continent of Antarctica used to be further north and much warmer. Antarctica's current frozen climate makes it difficult to find many fossils there but exciting discoveries have been made. An international team of scientists found fossil vertebrate tracks in the Queen Alexandra Range of Antarctica during an expedition to the continent in 1990-1991. These tracks were made during the Triassic Period and were probably made by early synapsids, a group that includes mammals and their extinct relatives. The trackmaker may have been Lystrosaurus, an ancient relative of modern mammals. Fossil bones of Lystrosaurus have been found in Antarctica, as well as in Africa and India. The distinctive distribution of this terrestrial animal provided important early evidence for the fact that the continents have moved over time.
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1UCM refers to the University of Colorado Museum of Natural History. The collection was originally built by the University of Colorado Denver, and was displayed at the Dinosaur Tracks Museum on the Denver campus. The fossil tracks are now housed at the University of Colorado Museum of Natural History in Boulder, Colorado, and the images of these specimens are courtesy of the University of Colorado.