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The Carboniferous is the Paleozoic geological period between the Devonian and the Permian, approximately 359–299 million years ago. Its subdivisions include the Mississippian and Pennsylvanian. The name refers to the importance of coal-bearing rock successions, rather than implying that every Carboniferous environment was a coal swamp.^[ref]

Coal records the accumulation and burial of plant material in settings where production and preservation exceeded decay. Carboniferous fossils and the sediments around them can therefore be used together: a plant compression constrains the vegetation, while the enclosing deposit supplies evidence about the environment of preservation. A fossil assemblage is a sample from particular places and preservation conditions, not a census of all life during the period.

The Devonian is a geological period after the Silurian and before the Carboniferous. Its fossil record is especially important for studying early jawed vertebrates, the origin of tetrapod limbs and the diversification of land plants.

The pectoral fin of Tiktaalik combines fish-like features with structures informative about the origin of a weight-supporting vertebrate limb. Such a character combination supports reconstruction of a transition without requiring that the sampled individual was itself a direct ancestor.^[ref]

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 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.