Fossils offer a glimpse into Earth’s ancient past, serving as records of life that once thrived millions of years ago. While many envision fossilized bones, teeth, or shells, paleontology also studies another category of evidence. These forms provide insights into the activities and behaviors of ancient life, rather than just their physical remains.
Distinguishing Trace Fossils
A trace fossil, also known as an ichnofossil, represents the preserved evidence of an organism’s activity rather than its body. This contrasts with body fossils, which are the actual preserved physical remains, such as bones, shells, or teeth. For example, a dinosaur bone is a body fossil, while a dinosaur footprint is a trace fossil. This distinction is important for paleontologists because trace fossils capture an organism’s behavior during its lifetime, offering a window into ancient ecosystems.
Trace fossils provide indirect evidence of life, showing how organisms moved, fed, or rested. Unlike body fossils, which can be transported away from where an organism lived, trace fossils are typically preserved in the place where the activity occurred. This “in situ” preservation means they offer more accurate information about the ancient environment an animal inhabited. The study of these activity records is called ichnology.
Common Types of Trace Fossils
Trace fossils encompass a diverse range of preserved behaviors, providing details about ancient life.
Tracks and trails: These include footprints left by walking animals, such as the three-toed impressions of dinosaurs or the tracks of early hominins. Trails can also be less regular marks, like those left by crawling worms or snails across soft sediment. These can reveal an animal’s gait, speed, and even social behavior.
Burrows and borings: These are tunnels or holes excavated by organisms in soft sediments or hard substrates like wood or shells. These structures offer clues about dwelling, feeding, or hiding behaviors.
Coprolites: Fossilized feces provide direct evidence of an ancient organism’s diet by preserving indigestible remnants like bone fragments or plant matter.
Gastroliths: Also known as “stomach stones,” these are smooth, polished stones found in the abdominal regions of some fossilized vertebrates, indicating their use in grinding food.
Stromatolites: Layered structures formed by microbial mats, reflecting biological activity that shaped sediments.
How Trace Fossils Form
The formation of trace fossils relies on a specific sequence of events that preserve the impression or infilling of an organism’s activity. Initially, an organism creates a mark in soft sediment, such as mud or sand. This impression must then be quickly covered by additional layers of sediment before it can be eroded or destroyed. Rapid burial is important to protect the delicate trace.
Over time, these layers of sediment undergo lithification, a process where they are compacted and cemented together, gradually turning into solid rock. The original impression becomes a mold, and the infilling sediment forms a cast, preserving the shape of the ancient activity. Trace fossils are often best preserved in fine-grained sediments like sandstone and limestone. Marine environments are particularly conducive to trace fossil formation due to the presence of minerals in water that aid preservation and the likelihood of rapid sediment deposition.
Insights from Trace Fossils
Trace fossils offer valuable information, extending beyond what body fossils alone can provide. They are indicators of ancient animal behavior, revealing how organisms moved, fed, reproduced, or interacted with their environment. For instance, trackways can show whether an animal walked or ran, its stride length, and even if it traveled in groups. Bite marks on fossilized shells provide evidence of ancient predator-prey relationships.
These fossils also serve as paleoenvironmental indicators, helping scientists reconstruct past conditions like water depth, sediment consistency, and oxygen levels. Because trace fossils are preserved in their original location, they accurately reflect the environment where the organism lived. They can even indicate the presence of soft-bodied organisms, such as worms, which rarely leave body fossils due to their lack of hard parts. Studying trace fossils helps paleontologists gain a more comprehensive understanding of ancient ecosystems and the broader history of life on Earth.