The physical evidence of past life, known as a fossil, encompasses more than just the preserved skeletal remains of ancient organisms. Fossils represent any long-lasting record of a biological entity, including the remnants of the organism itself or the traces of its activities. This preserved evidence allows scientists to reconstruct ancient ecosystems and the life histories of extinct animals and plants. Nests, as structures built by an organism, document behavior rather than anatomy. Understanding this classification requires distinguishing between the two major types of preserved evidence.
Defining Body Fossils and Trace Fossils
Fossils are fundamentally categorized into two groups based on the type of biological material preserved. Body fossils are the preserved remains of the organism’s actual body parts, such as bones, teeth, shells, or leaves. These remains provide direct anatomical evidence of the creature and its physical structure. The fossilized bones of a dinosaur or the hardened shell of an ancient clam are prime examples of body fossils.
The second category is the trace fossil, which preserves the evidence of an organism’s activity rather than its body. This group includes features like footprints, burrows, trails, and coprolites, or fossilized feces. Trace fossils, unlike body fossils, are rarely transported from their original location, meaning they document the exact spot where an activity occurred in the ancient environment. A nest is classified as a trace fossil because it is a structure constructed by an animal, reflecting a deliberate behavior.
Ichnofossils: Where Nests Fit in the Fossil Record
The scientific term for a trace fossil is ichnofossil, derived from the Greek word ikhnos meaning “track” or “trace.” Nests are a type of ichnofossil that provide an indirect record of reproductive behavior. Preservation often occurs through taphonomy, the process describing how an organism or its trace is preserved until discovery. For nests built in soft sediment, the original structure is often preserved when it is rapidly infilled by a different type of sediment, such as sand or mud.
This infilling material can solidify and retain the shape of the original nest, distinguishing it from the surrounding substrate. For example, fossilized dinosaur nests show a difference in texture and sedimentary structure between the nest and the host rock. While the eggshells found within the nest are technically body fossils, the structure that held them—the nest itself—is the ichnofossil. Insect nests, such as those made by wasps or termites, are also common ichnofossils, often preserved as cemented structures in ancient soils.
Behavioral Insights from Fossilized Nests
Studying fossilized nests provides a unique window into the paleoethology, or ancient behavior, of extinct organisms. The complexity of a nest can reveal information about the level of parental care, a behavior not preserved in body fossils alone. For instance, the fossil of the theropod dinosaur Citipati was discovered sitting in a brooding posture over a clutch of eggs, demonstrating bird-like parental care. The shape and arrangement of eggs within a nest, known as a clutch, suggest specific laying strategies and incubation methods.
The characteristics of the eggshells themselves, such as their porosity, can indirectly indicate whether the nest was open or buried underground. Low-porosity eggshells, like those of many sauropods and early theropods, suggest the eggs were buried. Conversely, high-porosity eggshells, seen in more advanced theropods, indicate eggs laid in open nests. The presence of numerous nests in a small area, such as colonial nesting sites, reveals information about the social behavior and reproductive strategies of the ancient animals.