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Intact Fossil

A fossil that forms when decomposition doesn’t happen, preserving the organism intact; extremely rare and useful.

Compression Fossil

A fossil formed when sediments accumulate on an organism, cementing it into rocks; common for soft-bodied organisms like plants and invertebrates.

Downsides to Fossil Evidence

Fossils are rare, represent only small parts of Earth's history, and are subject to various biases.

Habitat Bias

The tendency for organisms living underground to be more likely to fossilize.

Taxonomic and Tissue Bias

Organisms with hard parts are more likely to fossilize; some tissues fossilize more easily than others.

Temporal Bias

Recent fossils are more common than ancient fossils.

Abundance Bias

The fossil record is better for common species; for example, house cat fossils are more likely to be found than tiger fossils.

Precambrian Era

The time from Earth's formation (4.6 billion years ago) to the appearance of most modern animal groups (541 million years ago).

Origin of Prokaryotic Life

The emergence of the first simple life forms, such as bacteria, during the Precambrian Era.

Cyanobacteria

Organisms that heavily produced oxygen, contributing to the oxygenation of the atmosphere and oceans.

Phanerozoic Eon

The interval from 541 million years ago to the present, encompassing three major eras.

Paleozoic Era

The era beginning with the appearance of major animal lineages and ending with the mass extinction of multicellular life.

Mesozoic Era

The era that follows the Paleozoic, known for the dominance of dinosaurs and gymnosperms.

Cenozoic Era

The current era characterized by the dominance of flowering plants, birds, and mammals.

Adaptive Radiation

A process where a single lineage rapidly produces many descendant species with diverse adaptive forms.

Ecological Opportunity

The availability of more resources or new types of resources that can lead to adaptive radiations.

Morphological, Physiological, or Behavioral Innovation

The evolution of new key traits that can drive adaptive radiations.

Cambrian Explosion

A significant adaptive radiation event where early soft-bodied animals diversified into hard-bodied, multi-segmented animals with brains.

Hox Genes

Genes crucial for animal development that contributed to the diversification during the Cambrian Explosion.