Evolution and systematics

Lampshells were a dominant life form during the Cambrian period (570-500 million years ago [mya]). They flourished during the Paleozoic (570-240 mya) and Mesozoic eras (240-65 mya). The brachiopods reached the zenith of their evolutionary diversification during the Ordovician period (500-435 mya). At that time, the articulate lampshells appeared and underwent their most important phase of evolution. Since the close of the Paleozoic era, however, when their populations were decimated by mass extinctions from unknown causes, their numbers have been steadily decreasing.

Fossil remains have shown that articulates were once widely distributed and abundant throughout the marine world. Although over 30,000 species are known from fossil records between the Cambrian period and the present day, only 250 to 325 species are thought to be extant. The majority of present-day brachiopod species are assigned to the class Articulata rather than the class Inarticulata. The two classes are distinguished primarily by the way in which the two valves are attached along the rear line of the organism, and their method of contact. As of 2003, there are three living orders of articulates and four extinct ones.

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