Morphogenesis and phylogenetic relationships of clitambonitidines, Ordovician brachiopods

Clitambonitidine brachiopods have been studied since the 19th century. They constitute a peculiar group of brachiopods, often endemic to the Baltic, and limited to the Ordovician. According to the latest classification in the Treatise, clitambonitidines embrace 32 valid genera and one new Korinevskia gen.n. The study of juvenile shells of clitambonitidines resulted in the discovery of growth patterns of their ventral muscle attachment structure (spondylium). Spondylium evolved not by convergence of dental plates as previously believed, but is derived from free spondylium, which lack the supporting median septum characteristic for protorthids. Spondylium bearing (without dental plates) clitambonitidines dorsal articulation structures resembles also those of Middle Cambrian arctohedrids (portorthid brachiopods). The Clitambonitidina is not monophyletic: Clitambonitoidea and Polytoechoidea are derived distantly related ancestors.