A trace fossil (ichnofossil) is the preserved record of an ancient organism’s biological activity, not the remains of the organism itself. These remnants capture evidence of how life interacted with its environment. The study of these ancient activities is a specialized branch of paleontology called ichnology, which focuses on interpreting the behaviors that created these enduring markings. Ichnology reveals that the fossil record extends beyond mineralized bones or shells, encompassing the movements, feeding habits, and shelters of organisms.
Differentiating Trace Fossils from Body Fossils
The distinction between the two main categories of fossils rests on what is preserved: anatomy or action. Body fossils consist of the physical remains of an organism, such as mineralized bones, teeth, shells, or petrified wood. They represent the organism’s anatomy and structure, often requiring hard parts for long-term preservation.
Trace fossils, in contrast, are indirect evidence, representing the preserved behavior of an organism while it was alive. A dinosaur bone is a body fossil, but its footprint is a trace fossil. This difference is significant because a single organism can leave behind many trace fossils, but only one body. Trace fossils can document the presence of soft-bodied organisms that may never have been preserved as body fossils. The behavior recorded can also be analyzed separately from the biological affinity of the tracemaker, which is often impossible to identify precisely.
Catalog of Common Types
Trace fossils are classified based on the behavior (ethology) they represent. One recognizable type is the locomotion trace (Repichnia), which includes footprints, trackways, and continuous trails left by moving animals. Dinosaur trackways record stride length and foot shape, which scientists use to estimate walking speed and posture.
Another category includes dwelling and resting traces, which reflect stationary behaviors. Domichnia are dwelling structures like burrows and borings, such as U-shaped burrows created by ancient worms or crustaceans that served as long-term shelters. Cubichnia are resting traces, which are impressions left on the sediment surface when an organism, like a trilobite, temporarily settled into the soft substrate.
The final group encompasses feeding and waste traces, documenting how organisms acquired and processed nutrients. Coprolites are fossilized feces that can contain undigested material like bone fragments or plant matter, providing direct evidence of an animal’s diet. Gastroliths, or stomach stones, are polished stones swallowed by certain dinosaurs and other vertebrates to aid in the mechanical breakdown of food.
Preservation and Formation
The preservation of fleeting behaviors into permanent rock structures requires specific geological conditions. The process, part of taphonomy, often begins with the organism interacting with a soft, fine-grained substrate, such as mud or fine sand. The impression must then be rapidly buried by a new layer of sediment before erosion or biological activity can destroy the structure.
A trace fossil is often preserved through the formation of a mold and a cast. A mold is the original impression or cavity left in the sediment, such as a foot depression on a muddy surface. A cast is formed when this empty mold is subsequently filled with different sediment, which hardens and creates a three-dimensional replica of the original trace. The quality of the substrate is a determining factor, as damp, cohesive mud is more likely to retain a sharp impression than loose, dry sand.
Insight into Ancient Life
Trace fossils offer insights into ancient life that body fossils cannot, specifically regarding behavior and environmental conditions. Paleoichnologists use these traces to reconstruct the paleoenvironment, interpreting conditions like water depth, salinity, and sedimentation rates. Recurring assemblages of trace fossils, known as ichnofacies, correlate with specific depositional environments. For instance, complex, horizontal grazing traces are typical of deep-sea environments, while simple vertical burrows characterize high-energy, shallow-water settings.
The contribution of ichnofossils is the evidence they provide for paleobehavior, illustrating how organisms moved, fed, and rested. Trackways reveal the presence of an animal and the mechanics of its locomotion, such as whether a dinosaur walked bipedally or quadrupedally. Analyzing the shape and infilling of burrows can determine if an organism was a stationary filter feeder or a mobile deposit feeder.
Trace fossils also extend the fossil record by providing timing evidence for life forms that lacked hard parts. Since soft-bodied organisms rarely form body fossils, their existence in the Precambrian and early Paleozoic eras is often documented solely by the burrows and trails they left. This evidence pushes back the known appearance of complex behaviors, like active burrowing, hundreds of millions of years before the first body fossils of the tracemakers appeared.