Evolution of the earliest smooth spire-bearing atrypoids (Brachiopoda: Lissatrypidae, Ordovician–Silurian)

Calcified lophophore supports present in the oldest of the spire-bearing brachiopods, the Lissatrypidae which range in age from middle Ordovician (Caradoc) through middle Silurian (Wenlock) time, demonstrate complex evolutionary patterns and early divergence. The smooth-shelled brachiopods Protozyga, Idiospira, and Cydospira, which had medially or dorso-medially directed spiralia, evolved from the primitive early Caradoc atrypoid Manespira n. gen., which had a spiralium of less than one whorl and a whole or partial jugum. Early divergence produced four separate lineages, the Protozyginae n. subfam., Septatrypinae, Cyclospirinae, and Zygospiridae (which led to the ribbed atrypoids). Each of these groups developed distinctive brachidia, probably a reflection of early experimentation in filter-feeding strategies. In late Ordovician and early Silurian time, the ribbed Atrypidae, which had evolved from the Zygospiridae, expanded rapidly to dominate the shallow benthos of tropical seas: they accomplished this by perfecting large shell sizes, increasing the number of whorls in the spiralia, orienting the spiralia dorsally and separating the jugum into discrete processes. The smooth-shelled atrypoids, Lissatrypidae, usually played a secondary role, though they were prominent in the Silurian. Two new developments took place in the Silurian: evolution of the thicker-shelled Lissatrypinae (by mid-Llandovery), and the unique Glassiinae, with medially directed spiralia (by Wenlock time). Both of these declined during the Devonian, but the Glassiinae were the last survivors of the smooth atrypoids in late Devonian (Frasnian) time, with a new genus, Peratos (type species P. arrectus n. sp.), ending the lineage