The spondylium and related structures in the clitambonitidine brachiopods

Development of the ventral muscle field has been studied in 12 genera of clitambonitidine brachiopods: Eremotoechia, Clitambonites, Vellamo, Pahlenella, Lacunarites, Oslogonites, Gonambonites, Estlandia, Anchigonites, Kullervo, Antigonambonites, and Raunites. The last two have a pseudospondylium instead of spondylium, which links them to polytoechiids. The spondylium of the 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 triplex and simplex are identical. The hypothetical ancestor of clitambonitidines with spondylium was presumably a protorthid-like brachiopod probably of the mid-Cambrian age. On the contrary, polytoechiids, as well as Antigonambonites and Raunites, may have been derived from a late Cambrian billingsellid with dental plates. The polyphyly of clitambonitidines follows from development of their ventral muscle field.