Machaeridians are an enigmatic group of multisclerite-bearing benthic marine invertebrates, known from Lower Ordovician to Pennsylvanian rocks. They receive almost no attention, but are very common fossils, especially in faunas of Middle Ordovician to Early Devonian age. Evidence that machaeridians form a monophyletic group includes many shared derived morphological features of their sclerites, and their highly characteristic mode of sclerite formation. Machaeridian sclerites are calcitic, and feature an outer sclerite layer formed by marginal accretion, and an inner layer formed by serial addition of calcite elements in linear series radiating from the sclerite umbo. The class Machaeridia is understood herein to comprise the order Turrilepadomorpha Pilsbry, 1916, including the families Turrilepadidae Clarke 1896 and Plumulitidae Jell 1979, and the order Lepidocoleomorpha Schallreuter 1985, including the family Lepidocoleidae Clarke 1896. The latter family is unique in the possession of a dorsal tongue and groove hinge by which opposing sclerites of a given body segment articulated. All machaeridian species have distinct dextral and sinistral sclerite morphologies. All lepidocoleid species, and isolated members of some other families, are enantiomorphic, with some right-handed and some left-handed individuals. Recent classifications have united machaeridians with other problematic groups, including tommotiids, sachitids, and hercolepadids. Such proposed relationships have very weak evidential bases, and should be rejected.
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