A fossil is any remnant, impression, or trace of an animal, plant, or other organism from a past geologic age preserved within the Earth’s crust. Fossil formation is rare, requiring rapid burial in sediment, ice, or resin to protect the remains from decay. The totality of these preserved remains is known as the fossil record, which serves as the primary source of information for the science of paleontology. By studying these ancient relics, scientists reconstruct the history of life on Earth, understanding biological evolution and past environmental conditions.
Mold and Cast Fossils
Mold and cast preservation is a common method of fossilization, especially for organisms with hard parts like shells or exoskeletons. The process begins when an organism is buried in soft sediment, such as mud or sand, which then hardens around the body. Over time, groundwater dissolves the original organic material, leaving a hollow space in the rock that perfectly mirrors the shape of the former organism. This negative impression is called an external mold.
The resulting empty cavity can then be filled by new sediment or minerals that precipitate from circulating groundwater. Once this infilling material hardens, it creates a solid, three-dimensional replica of the original organism’s exterior, which is known as a cast. Sediment sometimes fills an internal cavity, such as the inside of a snail shell, creating an internal mold or steinkern. These molds and casts preserve the external form but lack the fine cellular or tissue details of the original hard part.
Trace Fossils
Trace fossils, or ichnofossils, are not the preserved remains of the organism itself. They are the fossilized evidence of an organism’s activity and behavior, providing indirect clues about how ancient life functioned. These traces give scientists a window into the life of a creature, revealing details such as its gait, speed, and social interactions.
The preserved activities are diverse and include footprints and trackways left by animals moving across soft sediment. Other common examples are burrows and borings, which are tunnels organisms made while dwelling or feeding. Fossilized feces, called coprolites, are also trace fossils, offering data about the diet of an extinct animal. Since trace fossils are preserved where the activity occurred, they provide information about the ancient environment, such as oxygen levels or water salinity.
True Form and Original Material Fossils
True form fossils represent the direct preservation of an organism’s remains, sometimes with little chemical alteration. This rare preservation requires conditions that prevent natural decay driven by bacteria, scavengers, and chemical breakdown. One effective method is preservation in a low-temperature environment, such as the freezing of woolly mammoths in Arctic permafrost.
These frozen specimens can retain soft tissues, hair, and organs, allowing for the study of original biological material. Another mechanism involves natural encapsulation in a protective substance like amber, which is fossilized tree resin. Small insects, spiders, and occasionally tiny vertebrates become trapped in the sticky resin, which then hardens into a durable polymer.
This process can preserve delicate structures in exquisite detail, sometimes even at the cellular level. Natural asphalt seeps, such as the La Brea Tar Pits, have similarly preserved the bones of thousands of animals by trapping them and protecting the remains from decomposition.
Petrified Fossils
Petrified fossils are those that have been “turned to stone” through mineral deposition. This process usually occurs through two related mechanisms: permineralization and replacement. Permineralization involves mineral-rich groundwater seeping into the porous tissues of a buried organism, such as the tiny spaces in bone or wood. Minerals like silica, calcite, or iron compounds then precipitate out of the water, filling these voids and hardening the remains.
Replacement is a distinct process where the original organic material is dissolved and simultaneously substituted, atom by atom, by a new mineral. While permineralization leaves some original material embedded in a mineral matrix, replacement leaves none, though it preserves the fine structure. Petrified wood is a result of silicification, where silica replaces the wood’s cellulose, perfectly preserving the tree rings and cellular structure.