A trace fossil, also called an ichnofossil (pronounced "ee-knoh-fos-uhl"), is a type of fossil that shows evidence of an organism's activity, such as movement or behavior, but does not preserve the actual body parts of the organism. Trace fossils are different from body fossils, which are the preserved remains of an organism’s body parts, often changed by chemical processes or minerals over time. The study of trace fossils is called ichnology, and the scientists who study them are called ichnologists.
Trace fossils can include physical marks left on or in the ground by an organism. Examples include burrows, holes made by waste from an organism, footprints, feeding marks, and spaces left by plant roots. These features are created when an organism interacts with its environment.
In a broader sense, the term trace fossil can also include other materials made by an organism, such as coprolites (fossilized droppings) or structures formed by biological activity, like stromatolites. However, not all structures found in rock layers are trace fossils. For example, marks made by empty shells rolling on the ocean floor are not considered trace fossils because they are not created by the behavior of an organism.
The study of traces, called ichnology, is divided into two areas: paleoichnology, which focuses on ancient trace fossils, and neoichnology, which studies modern traces. Ichnology is challenging because trace fossils often show how an organism behaved, not what kind of organism it was. Scientists classify trace fossils into groups based on their shape and the behavior that likely created them.
Occurrence
Traces are more commonly found as fossils than in modern sediments. This makes it hard to understand some fossils by comparing them to modern traces, even if these traces are still living or common today. The main challenges in studying living burrows include finding them in hardened rock and reaching those formed in deeper water.
Trace fossils are best preserved in sandstone because the size of the sand particles and the way the sediment was deposited help protect them. They can also be found in shale and limestone.
Classification
Trace fossils are often hard to link to a specific animal or plant. This happens because the creature that made the fossil is rarely found near its tracks. Also, different animals can leave the same kind of track. Because of this, traditional classification methods are not used. Instead, a new classification system has been created. At the top level of this system, five different types of behaviors are recognized.
Trace fossils are also grouped into categories called form genera. Some of these categories are divided further into groups that resemble species. These groups are based on the shape, structure, and the type of behavior that might have created the fossil.
To keep the names of body fossils and trace fossils separate, special names called ichnospecies are used for trace fossils. These names are organized differently in the rules for naming animals compared to names used for body fossils. For more details, see the classification of trace fossils. Examples include:
Information provided by ichnofossils
Trace fossils are important clues about ancient ecosystems and environments because they are usually found in the place where the organism that made them lived. These fossils can only reliably tell us two things: the type of sediment present when it was formed and the energy level of the environment where it was created. Some scientists have tried to use trace fossils to determine if a deposit was from the ocean or land, but these methods have not been proven to be reliable.
Trace fossils show indirect evidence of ancient life, such as footprints, tracks, burrows, borings, and feces left by animals, rather than the preserved bodies of the animals themselves. Unlike most other fossils, which form after an organism dies, trace fossils record the actions of an organism while it was alive. Because body fossils can be moved far from where an animal lived, trace fossils are better at showing the actual environment an animal lived in, making them more useful for studying ancient ecosystems.
Trace fossils are created when animals do activities like walking, crawling, burrowing, or feeding. Examples include tetrapod footprints, worm trails, and burrows made by clams and arthropods.
Some of the most famous trace fossils are the large, three-toed footprints left by dinosaurs and related animals. These footprints help scientists learn how these animals lived. While dinosaur skeletons can be reconstructed, only their footprints can show exactly how they stood and walked. These tracks can reveal details about how the animals moved, their stride length, and whether their front legs touched the ground.
Most trace fossils are less obvious, such as the trails made by segmented worms. These soft-bodied animals rarely leave body fossils because they lack hard parts.
Ichnofossils can sometimes show signs of illness or injury in ancient animals, especially when many examples of the same type of track are found and differences in the tracks can be clearly seen.
Fossil footprints made by four-legged vertebrates are hard to match to a specific species, but they can still provide useful information, such as how fast the animal moved, its weight, and its behavior. These fossils form when amphibians, reptiles, mammals, or birds walked across soft (likely wet) mud or sand that later hardened enough to keep the impressions before more sediment covered them. Some fossils even show how wet the sand was, which can help scientists estimate ancient wind directions.
Groups of trace fossils are often found at certain water depths and can also indicate the saltiness and clarity of the water at the time.
Some trace fossils are used as local markers to date the rocks they are found in. For example, the burrow Arenicolites franconicus appears only in a 4 cm (1 1/2 in) layer of the Triassic Muschelkalk epoch in southern Germany.
The beginning of the Cambrian period is marked by the first appearance of the trace fossil Treptichnus pedum.
Trace fossils are also helpful because some appear before the organisms believed to have made them, which extends the time range these organisms are known to have existed.
Ichnofacies
Ichnofacies are groups of trace fossils that appear again and again in different places and times. A scientist named Adolf Seilacher first introduced the idea of ichnofacies. He showed that geologists can learn about the environment when rocks were formed by looking at how different fossils are grouped together. The main ichnofacies found in scientific studies include Skolithos, Cruziana, Zoophycos, Nereites, Glossifungites, Scoyenia, Trypanites, Teredolites, and Psilonichus. These groups of fossils are not random. Instead, the types of fossils found depend mostly on the environment where the animals that made them lived. Factors like water depth, saltiness, hardness of the ground, oxygen levels, and other conditions determine which animals can live in certain areas. By studying how ichnofacies change over time, scientists can learn about changes in the environment. For example, scientists have used ichnological studies near the time of major extinction events, like the Cretaceous–Paleogene extinction, to understand what environmental changes caused these events.
Inherent bias
Most trace fossils are found in sediment from the ocean. There are two main types of traces: those made on the surface of sediment, like footprints, and those made inside layers of sediment, like burrows.
Traces on the surface of sediment in shallow ocean areas are less likely to become fossils because waves and water movement can easily destroy them. In deeper, calmer ocean areas, conditions are better for preserving small, detailed traces.
Evolution
The earliest complex trace fossils, excluding those made by microbes like stromatolites, appeared between 2,000 and 1,800 million years ago. These are too old to have been made by animals, and scientists believe they were created by amoebae. Possible "burrows" as old as 1,100 million years may have been made by animals that fed on the bottom of microbial mats, which helped protect them from a harsh ocean. However, these burrows have uneven widths and tapering ends, making it hard to prove they were made by living things. The person who first described them no longer believes they are real.
The first widely accepted evidence of burrowing dates to the Ediacaran (Vendian) period, about 560 million years ago. During this time, most traces and burrows were horizontal, lying on or just below the seafloor. These traces were likely made by moving organisms with heads, probably bilaterally symmetrical animals. The traces suggest simple behaviors, such as feeding above the seafloor and burrowing to avoid predators. Some vertical burrows, like Skolithos, are also known from the Ediacaran period. The makers of these burrows, such as Skolithos declinatus in Russia (555.3 million years old), are unknown, but they may have been filter feeders that ate nutrients from the water. These burrows can be as dense as 245 per square decimeter. Some Ediacaran trace fossils are found alongside body fossils. For example, Yorgia and Dickinsonia are often found at the ends of long pathways of trace fossils that match their shapes. These organisms may have used their cilia-covered undersides to feed mechanically. The potential mollusk-related Kimberella is linked to scratch marks, possibly made by a radula, which suggests active crawling or burrowing around 555 million years ago.
As the Cambrian period began, new types of trace fossils appeared, including vertical burrows like Diplocraterion and traces usually linked to arthropods. These changes show a greater variety and complexity of behaviors.
Trace fossils are especially important during the Cambrian period because they provide information about organisms that did not have hard parts, which are rare in the fossil record. While it is difficult to match trace fossils to the organisms that made them, the record suggests that large, bottom-dwelling, bilaterally symmetrical animals were quickly diversifying during the early Cambrian.
Later, diversification happened more slowly, and many trace fossils were made independently by unrelated groups.
Trace fossils also give the earliest evidence of animals living on land. The first known fully terrestrial animals appear in the Cambro-Ordovician period, with trackways found in the Ordovician Tumblagooda sandstone. These trackways help scientists understand the behavior of other land-dwelling organisms. The trace Protichnites, from the Cambrian period, shows the movement of an amphibious or terrestrial arthropod.
Other notable trace fossils
Traces left by invertebrates, such as Hibbertopterus, a large "sea scorpion" or eurypterid from the early Paleozoic era, are clearer than the traces mentioned earlier. This marine arthropod left a well-preserved track found in Scotland.
Over time, bioerosion has created a detailed record of marks, such as borings, scratches, and scrapings, on hard surfaces. These trace fossils are generally grouped into two categories: macroborings and microborings. The intensity and variety of bioerosion are marked by two key events. One is the Ordovician Bioerosion Revolution (see Wilson & Palmer, 2006), and the other occurred during the Jurassic period. For a complete list of references about bioerosion, see the External links below.
The oldest known tetrapod tail-and-footprints date to the later part of the Devonian period. These vertebrate impressions have been discovered in Ireland, Scotland, Pennsylvania, and Australia. A sandstone slab containing a tetrapod track, dated to 400 million years ago, is among the earliest evidence of a vertebrate walking on land.
Important human trace fossils include the Laetoli footprints in Tanzania, imprinted in volcanic ash 3.7 million years ago—likely made by an early Australopithecus.
Confusion with other types of fossils
Trace fossils are not the same as body casts. For example, the Ediacara biota mainly includes copies of organisms left in sediment. A footprint is not just a copy of the bottom of a foot, and the resting mark of a seastar looks different from an impression of a seastar.
Early scientists thought many structures found in layers of sedimentary rocks were fucoids, which are a type of brown algae. However, even in the early days of studying trace fossils, some were identified as animal footprints and burrows. In the 1880s, A. G. Nathorst and Joseph F. James compared these "fucoids" to modern traces, showing that most were actually animal trails and burrows. Real fossil fucoids are very rare.
Pseudofossils, which are not real fossils, should not be mixed up with ichnofossils, which are real signs of ancient life.
History
Charles Darwin's book The Formation of Vegetable Mould through the Action of Worms is an early example of a scientific study called ichnology. This work explains how animals, such as earthworms, mix soil through their burrowing activities, a process known as bioturbation.