The spondylium and related structures in the clitambonitidine brachiopods

Development of the ventral muscle field has been studied in 12 genera of cl itambonitidine brachiopods: Eremotoechia, Clitambonites, Vellamo, Pahlenell a, Lacunarites, Oslogonites, Gonambonites, Estlandia, Anchigonites, Kullerv o, Antigonambonites, and Raunites. The last two have a pseudospondylium ins tead of spondylium, which links them to polytoechiids. The spondylium of th e studied genera is not derived from convergent dental plates, but develops from the free plate in the early phase of morphogenesis. The early growth stages of spondylium tripler and simplex are identical. The hypothetical an cestor of clitambonitidines with spondylium was presumably a protorthid-lik e brachiopod probably of the mid-Cambrian age. On the contrary, polytoechii ds, as well as Antigen ambonites and Raunites, may have been derived from a late Cambrian billingsellid with dental plates. The polyphyly of clitambon itidines. follows from development of their ventral muscle field.