Trace fossils

Trace fossils, or ichnofossils, are the remains of the activity of an organism, recorded in the fossil record as disturbances in the sediment. Trace fossil formation generally requires that the movement of an organism occurs in an area containing fine-grained sediment in a semiaquatic medium.

Types of Fossils

Paleontologists define a fossil as the remains or impressions of an organism preserved for a minimum of ten thousand years. Fossils generally form in sedimentary rock, composed of hardened layers of clay and silt deposited through millions of years. Fossils do not form in igneous rock, which results when magma from the Earth's core cools at the surface or underwater, forming solid structures. Because magma is superheated at the time of its deposition, it usually destroys any potential fossils before cooling sufficiently to allow for fossil formation.

Paleontologists use the term body fossil to refer to the fossilized remains of all or part of an organism's body. In a process called replacement, the decay of the organic material making up an organism's body is coupled with the slow accumulation of minerals from surrounding sediment, forming a cast that can preserve the three-dimensional structure of the organism within. Most body fossils form from decay-resistant tissues, including bones, teeth, shells, and other hard parts of an organism. In some cases, body fossils can preserve the outlines of soft internal organs and other parts of the organism's anatomy.

Another class of fossils, trace fossils or ichnofossils, comprises the fossilized remains of an organism's activity within its environment. Trace fossils include fossilized footprints, burrows, and other impressions left by the movement of living organisms in soft sediment. These traces of activity become preserved when the surrounding sediment is buried and then hardens over time, producing a stone copy of the original impression.

Fossilization is rare, and most organisms will not leave any lasting fossilized trace of their existence. Despite more than 165 million years of dinosaurs roaming the Earth, approximately 50 percent of dinosaur species are known from only a single specimen, while more than 80 percent are known from fewer than five specimens. Various other factors influence the likelihood that an organism will be fossilized, including its habitat, general lifestyle, position in the food chain, and whether the organism possessed hard, resistant tissues. In 3.4 billion years of life on Earth, hundreds of thousands of fossils have accumulated in the sediment, leaving paleontologists with a detailed record of life on the planet.

Formation of Trace Fossils

Fossilization is much more likely to occur in marine environments, partly because minerals mix with water in aqueous environments. This liquid solution can better infiltrate a potential fossil site. Many trace fossils result from animals that can burrow into solid or partially solid materials. An organism that burrows through rock is far more likely to leave a trace fossil than an animal that lives in a soft substrate because a softer substrate is more likely to deform before hardening to preserve a trace fossil. For this reason, the burrows of marine animals form a large portion of the existing trace fossil record.

Trace fossils form best in fine-grained sediments, such as sandstone and limestone. Sandstone is composed of crystallized mineral fragments and tiny fragments of rock eroded from larger stones. Before it hardens, sandstone begins as fine silt that gathers in moist or aquatic environments. Semisolid sandstone is the best material for the preservation of trace fossils from terrestrial animals. The material is soft enough that a moving organism can leave a distinct trace and firm enough to maintain the shape of the impression after the organism has moved away.

After the depression is formed, heat from the sun may begin to harden the silt in the fossil site. If left exposed to the elements, the fossil erodes and deforms, so it is important that the entire fossil site be covered by water and additional silt before erosion damages the depression. In many cases, trace fossils result from situations in which the initial mold is later filled by a different type of silt (such as mudstone or limestone), which, when solidified, will have different qualities from the stone making up the mold.

For thousands of years, the entire fossil site has been buried under layers of additional sediment. Later, as the environment changes, the terrain surrounding the fossil site erodes, exposing the hardened sediment containing the trace fossil. The stone filling the fossil depression erodes at a different rate from that containing the depression, leaving the fossil exposed.

Trace fossils are often preserved in environments where body fossils are rare because the chemical and physical processes that destroy organic remains do not have the same effect on trace fossils. For instance, potential body fossils are often damaged by a process called diagenesis, which involve chemical and physical changes within a sample of sediment after deposition. Diagenesis does not damage trace fossils and may actually enhance trace fossil formation by increasing differences between the sediment of the mold and that of the material filling the mold. Therefore, areas like deep marine shales and shallow aquatic environments, which resist the formation of body fossils, often contain a variety of trace fossils. In some cases, trace fossils are the only record of life in chemically or physically active environments where body fossil formation is hindered.

Classification of Trace Fossils

It is usually difficult or impossible for paleontologists to determine the species responsible for a specific trace fossil. Trace fossils left by burrowing marine mollusks and worms, for instance, are often indistinguishable from one another and may have been created either by the same species or by two species of similar size, shape, and habit. Paleontologists have created a set of ethological classes to organize trace fossils based on the type of activity that formed the fossil. There are five or six major groups of trace fossils, which are further divided into ethological subcategories.

Cubichnia. Cubichnia (resting traces) usually take the form of shallow depressions and are thought to result from animals temporarily resting in the substrate. In exceptionally well-preserved samples, cubichnia may also indicate the morphology of the resting organism, making it occasionally possible for paleontologists to determine the species responsible for the resting trace fossil.

Pascichnia and Fodinichnia. Pascichnia (grazing traces) and fodinichnia (feeding traces) are left by organisms feeding on smaller organisms within the substrate of the marine environment. Pascichnia takes the form of shallow grooves and corresponding burrows formed by an animal that feeds by rubbing its jaws across the surface of the sediment in a characteristic pattern and grazing off small organisms in the soil. Fodinichnia appear as cylindrical depressions in the sediment, left by organisms known as deposit feeders that feed on the detritus that accumulates at the bottom of the ocean.

Fugichnia. Fugichnia (escape traces) generally appear as burrows in the sediment, occasionally with angles indicating rapid lateral movement. Paleontologists believe that fugichnia result when an organism has been rapidly covered by sediment and thereafter moves rapidly upward in an effort to escape the sediment. Fugichnia generally involve upward movement, whereas most trace fossils result from organisms burrowing deeper into the sediment.

Domichnia. Domichnia (dwelling traces) show up as burrows or small chambers excavated by ancient marine animals seeking temporary or permanent shelter. In some cases, domichnia may be surrounded by nodules resulting from repeated digging against the sides of the chamber. In other cases, domichnia may include a branching system of interconnected tunnels created when the domicile created by one animal is used again and again by other animals in repeated excavations.

Repichnia. Repichnia (crawling traces) are traces left by the movement of an animal across the surface of the sediment. These include the most famous type of trace fossil, the dinosaur track-ways, or imprints of dinosaur footprints, found in Mesozoic excavation sites around the world. Other types of repichnia include depressions made by slithering or crawling animals observed in marine environments. Repichnia are especially interesting in that they provide information about the locomotive patterns of the animals making the tracks.

Another method for classifying trace fossils is to group them into assemblages called ichnofacies, which contain multiple fossil varieties from a single area, representing different species and ethological categories. Ichnofacies provide paleoecologists with a snapshot of ancient marine communities, showing how different species divided a given territory according to their varying needs.

Ichnofacies are divided according to whether the environment was terrestrial, marine, or freshwater and according to the type of sediment represented in the sample. For example, terrestrial and freshwater habitats with firm sediment are part of the Scoyenia ichnofacies, within which paleontologists find tracks and trails of dinosaurs and other terrestrial or semiaquatic animals that lived in ancient environments. Scoyenia ichnofacies also contain remnants of plant life from ancient environments, sometimes in the form of fossilized seeds and pollen grains, which can be used to further specify the type of climate that existed when the ichnofacies began to form.

Marine ecosystems are represented by various ichnofacies divided according to the type of sediment and the depth of the environment in which they were formed. The Nereites ichnofacies, for instance, contain feeding burrows, locomotor pathways, and various other trace fossils from soft-sediment, deep-water marine environments. The Cruziana ichnofacies, by contrast, represent assemblages of organisms living in mid-depth marine environments where the soil bears evidence of occasional environmental disturbances. The Skolithos ichnofacies are characterized by various feeding and burrowing structures left by organisms living in intertidal marine environments marked by frequent, cyclical changes in water depth.

Importance to

Paleoecology is the study of the interrelationships between extinct animals and their environments. Trace fossils are important to paleoecology because they provide independent information that is not given by other types of fossils. A trace fossil can provide clues about the types of organisms that may be presumed to have lived in an environment where body fossils are rare. In addition, trace fossils can be created by organisms lacking the stiff anatomical structures most conducive to body fossil formation. Trace fossils may, therefore, provide the only evidence of species that would otherwise be absent from the fossil record. These findings can be used to compose colonization patterns.

Additionally, body fossils do not always provide evidence about the types of organisms that lived in specific environments because the bones and other body parts that become fossils can move from one location to another after the animal's death and before the fossilization process begins. By contrast, trace fossils are necessarily tied to the environment in which they were created. For this reason, when paleontologists find a specific kind of trace fossil in a certain environment, they can reason that the organism responsible for the fossil appeared in that exact area at the time of deposition.

Because trace fossils are generally more widely available than body fossils, paleoecologic researchers can use trace fossil findings to fill in research gaps and answer questions about historical conditions that body fossils fail to answer. For example, the fossil record was missing evidence of deep-sea fish older than 50 million years until 2023, when researchers discovered trace fossils in Northwest Italy dating back 130 million years to the Early Cretaceous period that evidence of fish activity thousands of meters deep. This discovery provides evidence of the first known deep-sea vertebrates and is a significant milestone in understanding the evolution of deep-sea vertebrates and biodiversity.

Principal Terms

ichnofauna: the collective remains of trace fossil records of animal activity

ichnofossil: a trace fossil created by the disturbances or activity caused by living animals that leave impressions in the sediment of the Earth

ichnology: a branch of paleontology concerned with tracks, burrows, and other fossilized remains representing the activity of living animals

igneous rock: a rock formed by the solidification of volcanic magma

paleoecology: the study and use of fossils and other geological evidence to reconstruct ecosystems from the past

replacement: a process in which the organic materials of an organism are replaced by mineral deposits after the animal's death, leading to the formation of a body fossil

sedimentary rock: a rock formed from the consolidation of clay minerals

sedimentology: a branch of science dealing with the formation, classification, and study of sedimentary rock

stratigraphy: the study of geological strata, including the age, deposition, and composition of sedimentary rock

taphonomy: the study of the ways organisms and traces of organisms become fossilized

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