A trace fossil, also known as an ichnofossil, is the preserved record of an ancient organism’s activity rather than the remains of the organism itself. These remnants offer unique insights into the behavior, movement, and dwelling habits of extinct life forms. Unlike body fossils, trace fossils capture a moment of interaction between the creature and its environment. Examples include fossilized footprints, burrows, trails, and preserved feces, known as coprolites. The study of these structures, called ichnology, helps researchers reconstruct ancient ecosystems where body fossils might be scarce or absent.
Geologic Requirements for Preservation
The location of trace fossils is dictated by the geological processes required for their preservation. The initial activity must occur on a substrate, such as soft sand or mud, cohesive enough to record the impression. Preservation requires extremely rapid burial by a subsequent layer of sediment before the trace is destroyed by erosion or biological activity. The overlying sediment must be of a contrasting lithology, often sand over mud, which effectively seals the impression.
Trace fossils are often found not within the rock layers themselves, but directly on the bedding planes separating different strata. These planes represent the interface where the ancient activity occurred before the next layer of sediment was deposited. Fine-grained sedimentary rocks, such as mudstone, shale, and some sandstones, are the most common hosts because they retain fine detail. The absence of significant sediment mixing by organisms, known as bioturbation, is also necessary, as this mixing can destroy shallow traces.
Discovering Trace Fossils in Marine and Coastal Environments
The vast majority of trace fossils are discovered in rocks originally deposited in marine and coastal settings. These environments feature the continuous, rapid influx of sediment needed for preservation. Shallow marine shelves and intertidal zones are prolific sources due to their high energy and shifting substrates. High-energy settings like sandy shorefaces often yield the Skolithos ichnofacies, characterized by simple, vertical dwelling burrows created by suspension feeders seeking refuge from wave action.
Deep Marine Ichnofacies
Slightly deeper, lower-energy environments, such as those below the fair-weather wave base, are typically defined by the Cruziana ichnofacies. This assemblage shows a greater diversity of traces, dominated by horizontal traces, including complex crawling (Cruziana) and feeding structures made by deposit-feeding invertebrates like trilobites.
Deep-Sea Ichnofacies
Deep-sea environments, characterized by extremely slow deposition and very low energy, host the Nereites ichnofacies. This group is distinguished by highly complex, patterned, and looped grazing trails made by organisms methodically searching for food on the seafloor.
Coastal transitional zones, such as tidal flats and estuaries, also preserve unique traces reflecting fluctuating salinity and energy levels. These areas often exhibit a mix of vertical and horizontal burrows, including the Psilonichnus ichnofacies found in supratidal sandy settings. The constant ebb and flow of tides and storm events create alternating layers of sediment that seal these traces, recording community response to environmental stress.
Trace Fossils in Terrestrial and Continental Environments
Trace fossils are less common in continental environments compared to marine settings, but they provide unique evidence of land-based life. Preservation often depends on rare, high-volume sedimentation events, such as flash floods or volcanic ash falls, which rapidly seal the ground surface. Ancient river floodplains and lake beds are particularly important continental locations for discoveries.
Vertebrate tracks, particularly dinosaur footprints, are the most recognizable continental trace fossils, often preserved in ancient mud flats or lake margins. A significant example is the preservation of hominin footprints, like those at Laetoli, sealed by volcanic ash more than three million years ago. Other continental traces include rhizoliths, which are fossilized root traces that provide information about ancient soils (paleosols) and water table levels.
Invertebrate activity, such as insect burrows, termite nests, and crayfish burrows, also constitutes a significant portion of the continental record. These are found in deposits ranging from ancient desert dunes to lacustrine (lake) and alluvial (river) deposits. The scarcity of these fossils reflects the generally erosional nature of terrestrial environments, where impressions are often destroyed before rapid burial and lithification can occur.