Last modified: Wed 4/16/97 1900 PDT
 Canon Terms |
Object | Classes | IndexFossil; FossilAssemblage |
| Instances | (none) | ||
| Event | Classes | GeologicalTime; GeologicalEra; GeologicalPeriod; GeologicalEpoch | |
| Instances | EarthHistory; PrecambrianEra; PaleozoicEra; MesozoicEra; CenozoicEra; ArcheozoicPeriod; ProterozoicPeriod; CambrianPeriod; OrdovicianPeriod; SilurianPeriod; DevonianPeriod; CarboniferousPeriod; PermianPeriod; TriassicPeriod; JurassicPeriod; CretaceousPeriod; TertiaryPeriod; QuaternaryPeriod |
Earth history is the behavioral trajectory of the Physical Agent we call the Earth, a complex event covering a span of time some 4.6 billion (109) years long.
Earth history is conventionally structured into a sequence of eras. Each era is structured as a sequence of periods, and each period is structured as several regionally specific sequences of epochs. These subdivisions are more nomenclatural conveniences than natural divisions of Earth history based on documentable events (although such events are often used as indices for the correlation of this chronology with geological observations). The focal point for this nomenclature of so-called geological time is the evolution of different assemblages of organisms whose existence is inferred from the fossil record. The particular fossil organisms that are nomenclaturally relevant are referred to as index fossils.
The nomenclature for the eras and periods into which Earth history is conventionally organized are given in the following table. Dates in MYA (million years ago) and durations in MY (million years) are also given.