The Brachiopoda are found from the Cambrian Rocks up to the present day, and present us with an example of a group which appears to be slowly dying out. Nearly four thousand extinct species have been described, and the class appears to have attained its maximum in the Silurian epoch, which is, for this reason, sometimes called the "Age of Brachiopods." Numerous genera and species are found also in both the Devonian and Carboniferous formations. In the Secondary Rocks Brachiopoda are still abundant, though less so than in the Palaeozoic period. In the Tertiary epoch a still further diminution takes place, and at the present day we are not acquainted with more than a hundred living forms. Of the families of Brachiopoda, the Productidae, Strophomenidae, and Spiriferidae are the more important extinct types. Of the genera, the most persistent is the genus Lingula, which commences in the Cambrian Rocks, and has maintained its place up to the present day, though it appears to be gradually dying out.
According to Woodward: - "The hingeless genera attained their maximum in the Palaeozoic age, and only three now survive (Lingula, Discina, Crania) - the representatives of as many distinct families. Of the genera with articulated valves, those provided with spiral arms appeared first, and attained their maximum while the Terebratulidae were still few in number. The subdivision with calcareous spires disappeared with the Liassic period, whereas the genus Rhynchonella still exists. Lastly, the typical group, Terebratulidae, attained its maximum in the Chalk period, and is scarcely yet on the decline."
Of the families of the Brachiopoda, the Productidae and Strophomenidae are exclusively Palaeozoic. The Spiriferidae are mainly Palaeozoic, but extend into the Lias, where they finally disappear. The Lingulidae commence in the Cambrian period, and have survived to the present day. The Rhyncho-nellidae, Craniadae, and Discinidae commence in the Silurian period, and are represented by living forms in existing seas. The Thecidiidae extend from the Trias to the present day; and the Terebratulidae appear to commence in the Upper Silurian, and are well represented by living forms.