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The Cenozoic is the geological era from 66.00 million years ago to the present. It comprises the Palaeogene, Neogene and Quaternary. Its subdivisions organize geological and climate records; their boundaries are defined stratigraphically, with numerical ages calibrated separately.^[ref]

The Cretaceous is the youngest Mesozoic period, approximately $143.1$ to $66$ million years ago in the dated chart used here.^[ref] A numerical boundary estimate is a calibration of the geological time scale, not an assertion that every local geological process changed simultaneously. Stratigraphy establishes correlations; radiometric dating supplies numerical age constraints.

The Eocene is a Palaeogene epoch following the Paleocene, approximately $56$ to $33.9$ million years ago.^[ref] Its strata are used in paleoclimate reconstruction, where oxygen isotope ratios and other material properties must be calibrated against the physical conditions that produced them. The stratigraphic label identifies time, rather than one globally uniform sediment type.

The Holocene is the present Quaternary epoch, following the Pleistocene, with its beginning near $0.0117$ million years ago.^[ref] Its relatively short duration makes dating conventions important when comparing high-resolution paleoclimate records. A shared epoch label does not imply uniform climate throughout its duration or across different regions.

The Jurassic is the Mesozoic period between the Triassic and Cretaceous, approximately $201.4$ to $143.1$ million years ago.^[ref] Its position in a rock sequence can be established by correlated stratigraphic evidence even where no directly datable volcanic layer occurs. Numerical calibration and recognition of the interval in local strata are therefore distinct tasks.

The Mesozoic is the geological era from about 251.902 to 66.00 million years ago. It contains the Triassic, Jurassic and Cretaceous periods and precedes the Cenozoic. The chart gives uncertainty on the older numerical boundary.^[ref]

The Miocene is the older Neogene epoch, approximately $23.04$ to $5.333$ million years ago.^[ref] Dating a rock to this interval places it before the Pliocene; interpreting the environment in which it formed additionally requires observations such as sedimentary rock structure, composition and fossils. An age label and an environmental reconstruction answer different questions.

The Mississippian is the earlier subperiod of the Carboniferous, following the Devonian and ending at the beginning of the Pennsylvanian. It spans roughly 359 to 323 million years ago. The corresponding body of rocks is the Mississippian subsystem.^[ref]

For example, a Mississippian fossil is older than a Pennsylvanian fossil, even though both may be labelled Carboniferous. This subdivision makes broad comparisons more informative without requiring an exact numerical age for each specimen. A numerical boundary age is an estimate on a calibrated time scale; the named subdivision also depends on agreed stratigraphic definitions.

The Neogene is the geological period from 23.04 to 2.58 million years ago in the consulted chart. It contains the Miocene and Pliocene, between the Palaeogene and Quaternary.^[ref]

The Oligocene is a geological epoch within the Palaeogene, between the Eocene and Miocene, approximately 34 to 23 million years before the present. It is also the corresponding interval of rock strata in chronostratigraphic classification.

The boundaries are defined using reference sections and correlation criteria, while their numerical ages are calibrated by dating and can be refined. A rock is therefore not assigned to an epoch solely because a rounded age estimate falls inside a convenient numerical interval. Fossils, stratigraphic relationships and independent dating must be interpreted together.^[ref]

The Palaeogene is the geological period from 66.00 to 23.04 million years ago in the consulted chart. It contains the Paleocene, Eocene and Oligocene, preceding the Neogene within the Cenozoic.^[ref]

The Paleocene is the earliest Palaeogene epoch, approximately $66$ to $56$ million years ago.^[ref] It follows the Cretaceous and precedes the Eocene. Correlating the same interval between separated sedimentary rocks requires independent stratigraphic evidence; thickness alone is insufficient because deposition rates, erosion and missing intervals differ between sites.

The Pennsylvanian is the later subperiod of the Carboniferous, following the Mississippian and preceding the Permian. It spans roughly 323 to 299 million years ago. The corresponding rocks form the Pennsylvanian subsystem: a time interval and the rocks deposited during it are related but distinct uses of the name.^[ref]

The word is useful when comparing fossils from different regions. A fossil described as Pennsylvanian belongs to this interval rather than to an undifferentiated stretch of the Carboniferous. The classification provides a shared temporal framework; it does not imply that every locality had the same environment or preserved the same organisms.

The Pleistocene is the Quaternary epoch preceding the Holocene, approximately $2.58$ to $0.0117$ million years ago.^[ref] Marine oxygen isotope ratios provide evidence of changing ice volume and deep-water temperature across this interval; combining many sediment-core records reduces local noise but still requires an age model.^[ref]

The Pliocene is the younger Neogene epoch, approximately $5.333$ to $2.58$ million years ago.^[ref] It is included in the long marine oxygen isotope ratio stack used to compare earlier ocean conditions with those of the Pleistocene. Aligning records makes their temporal patterns comparable; it does not turn one isotope measurement into a unique independent temperature measurement.^[ref]

The Quaternary is the current geological period, beginning 2.58 million years ago. It contains the Pleistocene and Holocene epochs. Repeated major glacial cycles make its marine and continental records important for studying climate variability.^[ref]

The Triassic is the earliest Mesozoic period, approximately $251.9$ to $201.4$ million years ago.^[ref] It precedes the Jurassic. In a local sedimentary rock sequence, a missing part of this interval may reflect nondeposition or later erosion; the absence of a named interval in one section does not imply that no time elapsed there.