A fossil represents any preserved remains, impression, or trace of a once-living organism from a past geological age. The process of fossilization is a rare occurrence, depending heavily on specific conditions and the materials surrounding the organism after death. Certain natural settings and geological processes are far more conducive to preserving biological material over millions of years.
How Fossils Form: The Right Conditions
For an organism to become a fossil, specific environmental conditions must be present to prevent its rapid destruction. A primary requirement is quick burial beneath layers of sediment, which protects the remains from scavengers and decomposition. Organisms living in environments where sediment actively accumulates, such as lakes, river deltas, or ocean basins, have a higher chance of preservation.
Environments with low oxygen levels are also beneficial for fossilization. Anoxic conditions slow the decomposition of organic matter by bacteria, allowing more time for preservation to occur. Additionally, the presence of minerals in groundwater is important, as these can infiltrate and replace the organic material of the deceased organism.
Sedimentary Rocks: The Ultimate Preservation Ground
Sedimentary rocks are the most common medium for fossil preservation because they form under conditions that favor the safeguarding of ancient life. These rocks originate from the accumulation of sediments, such as sand, mud, and silt, which are deposited in layers. The gentle formation process of sedimentary rocks, occurring at relatively low temperatures and pressures, typically does not destroy delicate biological structures.
The transformation of sediment into rock involves compaction and cementation. Compaction happens as overlying material presses down, squeezing sediment grains closer together and reducing pore space. Subsequently, cementation occurs when minerals, transported by water, fill the remaining spaces between grains, binding them into solid rock. Common cementing agents include silica, calcite, and iron oxides. This process allows for the detailed preservation of organisms within the rock layers.
Many fossils found in sedimentary rocks are formed through permineralization, where minerals fill the pores of organic materials like bone or wood. Groundwater containing dissolved minerals, such as silica, calcite, or iron, seeps into the porous tissues. As the water evaporates, these minerals precipitate and crystallize, effectively turning the remains into stone while retaining the original shape and even microscopic details. Another common method involves molds and casts, where an organism’s hard parts leave an impression (a mold) in the surrounding sediment. If this void is later filled by minerals, it creates a three-dimensional replica of the original organism, known as a cast.
Beyond Rock: Unexpected Preservation Methods
While sedimentary rocks account for most fossils, other unique environments can also lead to exceptional preservation. Amber, fossilized tree resin, is renowned for preserving small organisms like insects. When sticky resin flowed from trees, it could trap insects, spiders, or plant material. The resin then hardened, preventing decay by isolating the organism from oxygen and destructive microorganisms.
Ice also offers a powerful preservation method, particularly in cold climates with permafrost. The consistently freezing temperatures halt decomposition, allowing for the preservation of soft tissues, hair, and even internal organs. Woolly mammoths and rhinoceroses discovered in Siberian permafrost are prime examples, providing direct insights into the appearance and biology of extinct animals.
Tar pits, such as the La Brea Tar Pits, represent another unusual preservation site. Animals became trapped in the sticky asphalt, and their bones were preserved due to the lack of oxygen and bacterial activity within the tar. While soft tissues typically do not survive in tar pits, the bones of large mammals like saber-toothed cats and dire wolves are exceptionally well-preserved, offering a rich record of Pleistocene megafauna.
When Preservation Fails: Factors That Destroy Fossils
Many factors can prevent an organism from fossilizing or destroy existing fossils. Decomposition by bacteria and fungi, along with scavenging by other animals, rapidly breaks down organic remains before burial can occur.
Environmental forces like erosion can also destroy potential fossils or expose buried ones to destructive elements. Acidic soils can dissolve hard parts like shells and bones, preventing their mineralization. Furthermore, the formation of igneous and metamorphic rocks typically obliterates any organic material. Igneous rocks form from molten rock at high temperatures, incinerating remains, while metamorphic rocks form under intense heat and pressure that deform or destroy existing fossils.