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Fossils are evidence of organisms that lived in the remote past, a stone-hard historical record of life. The fossil record will never be complete. Geologic events have obliterated much of it. The rest of the record is slanted toward species that had hard parts, lived in dense populations with wide distribution, and persisted for a long time.

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250,000 known species

Considering the current range of biodiversity, there must have been many millions more, but we will never know all of them.

The odds are against finding evidence of an extinct species, because fossils are relatively rare.

When an organism dies, its remains are often obliterated quickly by scavengers.

Organic materials decompose in the presence of moisture and oxygen, so remains that escape scavenging can endure only if they dry out, freeze, or become encased in an air-excluding material such as sap, tar, or mud.

Remains that do become fossilized are often deformed, crushed, or scattered by erosion and other geologic assaults.

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Understand life’s historical context

Features can provide information about environment

Banded iron started forming about 2.4 billion years ago

Earth’s atmosphere and ocean contained very little oxygen, so almost all iron on Earth was in a reduced form

Reduced iron dissolves in water, and ocean water contained a lot of it

Oxygen released into ocean by early photosynthetic bacteria quickly combined with dissolved iron

Oxidized iron compounds are completely insoluble in water, began to rain down on the ocean floor in massive quantities

Compounds accumulated in sediments that would eventually become compacted into banded iron formations

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Organisms become covered by sediments, mud, or ash

Groundwater seeps into remains

fills space around and inside

Minerals dissolved in  water gradually replace minerals in bones and other hard tissues

Mineral particles that crystallize and settle from groundwater in cavities and impressions form detailed imprints of internal and external structures

Sediments slowly accumulate on top of site, exert increasing pressure, transform the mineralized remains into rock

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Consist of:

mineralized bones

teeth

shells

seeds

spores

hard body parts

Trace fossils:

footprints

impressions

nests

burrows

trails

eggshells

feces