Why the fossil record is a lottery, not a library
If every creature that ever lived left a trace, we would be wading knee-deep in fossils. Instead, the chance of an animal becoming a fossil is less than 1%. To survive the ages, a skeleton must navigate a brutal obstacle course of scavengers, acid, and decay.
The fossil record is not a complete census of ancient life, but a highly filtered subset. For a bone to endure, it must first possess the right anatomy. Organisms with hard parts, such as teeth, shells, or bones, have a significant advantage. In contrast, soft-bodied creatures like jellyfish or slugs almost always decompose entirely, leaving no physical remnant behind.
Even with hard parts, the window for preservation is incredibly narrow. The primary requirement is rapid burial to protect the remains from the elements. This typically occurs near moving water, such as rivers or floodplains, where sediment can quickly cover a carcass. In arid environments, wind-blown sand from dunes can serve a similar purpose. Without this swift interment, scavengers will likely scatter or consume the remains, and the forces of erosion, corrosion, and decalcification will erase the skeleton.
If burial is successful, the chemistry of the surrounding sediment becomes the next gatekeeper. High temperatures and acidic soils—those with a low pH—are particularly destructive. Such environments can dissolve hydroxyapatite, the calcium phosphate mineral that provides bone its structure. Even in stable conditions, the organic proteins like collagen eventually break down, and physical pressure can crush or dissolve the inorganic minerals.
Ultimately, most fossils we recognize, including famous dinosaur bones, are not the original biological material. Instead, they are 'stone photocopies' created through processes like replacement or permineralization, where the original bone is minerally modified over millions of years.
Source: Why Don't All Skeletons Become Fossils?
