What Is the Definition of Trace Fossils?
All living creatures leave behind traces of their activities as they build dwellings, rest, hide, forage and hunt for food, or as they travel about. Sometimes this evidence is preserved by geologic processes over long periods of time, and those traces can give us insight into the lives of creatures from the very distant past.
What are Trace Fossils?
Trace fossils are a type of impression left in sedimentary rock formations, made by the activities of ancient animals. Trace fossils are marks of the creature's passing, and can be as varied as the footprints of dinosaurs, burrows of marine worms, paths made by clams and mollusks across the sea floor or even the droppings or feces of animals. Trace fossils differ from the preserved bodily remains of animals such as skeletons or mineralized forms that preserve the shape of body tissues.
How are trace fossils created?
Most trace fossils are made when a creature has been active in wet mud or sandy soil. The mud or soil dries and hardens, preserving the impressions or droppings, which are later covered by more soil or sediment. Over many years, the sediment is transformed into sedimentary rock.
What can be learned from trace fossils?
Trace fossils are valuable because they can provide us with information about how the animals lived, such as how they moved about, foraging and hunting, how they constructed nest or burrows and even how they fled from predators or fought with them. Trace fossils can reveal details of an animal's posture and walking gait, such as whether they stood erect or moved on two or more limbs. By comparing evidence of an animal's activities to known patterns of modern creatures, information can be deduced about the organism's anatomical structure and interaction with their environment, even if no body-form fossils are present. In some cases, an animal's diet can be determined by the fossilized droppings or feces.
Some trace fossils, made by soft bodied creatures such as worms, nematodes and snails, are the only evidence we have of these type of creatures in the ancient world. The traces they left behind can also help us understand the nature of the environment, climate and ecology of their eras. Trace fossils are often useful in helping to establish dates for sedimentary layers in which other types of fossils are found.
Why are trace fossils valuable for scientific studies?
The scientific study of ancient trace fossils is called paleoichnology. Paleontologists keep records of trace fossils from different rock strata from locations throughout the world. Trace fossils have been discovered in every geologic era starting with the Late Precambrian to the Ice Age. They are found in sedimentary rocks such as limestone, siltstone, sandstone, coal and shale. Trace fossils have been formed in underwater environments as well as on dry land. The presence of trace fossils in a rock strata can also help determine the type of environment and local conditions that existed in the place and time that the impressions were made. Trace fossils are considered important to scientists who study geology, as they provide information about the formation of of the surface features of the Earth's upper crust.
Can trace fossils lead to other discoveries?
Often, trace fossils can indicate areas where more rare fossils that preserve the bodily forms of animals might be found, with further investigation or excavation. For example, areas that have dinosaur tracks visible on ancient river banks may be near fossilized bones that were buried in the silt, where the river took a sharp bend. By tracing the course of the ancient river, such bones are often discovered partially exposed on eroded slopes.
An Example of Evidence from Trace Fossil Footprints
One very interesting example of the type of story that trace fossils can reveal is the case of a set of tracks that showed a struggle between a Tyrannosaurus Rex and a Triceratops. The three-toed tracks of the Tyrannosaur were widely spaced and deeply impressed as it ran swiftly in pursuit of the Triceratops. The trackway showed where the Triceratops swung around to confront its foe and defend itself with its three sharp horns. The footprints dispersed about indicated where the huge beasts fought in deadly combat. Excavation in the same area found the skeletized fossil remains of both, who died still locked in an embrace as they struggled.
Have trace fossils ever been found of humans or their ancestors?
Trace fossils that show the passing of the distant ancestors of humans have been found in sedimentary rocks formed from volcanic ash deposits. Footprints from a small family group including adults and children walking barefoot through the ash were preserved from over three million years ago.
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