Fossilization

How fossils form. An organism or evidence of it gets buried in sediments or volcanic ash; water and dissolved inorganic compounds infiltrate it; then chemical changes and pressure from accumulating sediments above transform it to stony hardness.

Asteroid

Rocky, metallic body, a few to 1,000 kilometers across, hurtling through space.

Analogous Structures

Body parts that once differed in evolutionarily distant lineages but converged in structure and function as responses to similar environmental pressures.