ABSTRACT
The oldest known stocks of rhynchonelliform brachiopods demonstrate that several primitive patterns of articulation had evolved by early Cambrian times. Welldifferentiated propareas on both valves of the ‘inarticulated’ obolellides, and the presence of a strophic shell in chileides suggest that an axis of rotation around which opening and closing of the valves took place was well defined originally prior to the development of hard structures, achieved by fusion of mantle lobes and periostracal tissue along the posterior margin. The appearance of primitive skeletal articulatory structures on the brachiopod shell – including paired denticles or teeth and sockets as in obolellides and the earliest protorthides, oblique grooves and ridges along the posterior margin in both valves of kutorginides, and an arcuate plate (anterise) in naukatides – closely reflects the transition from an hydraulic process of shell opening to a mechanism employing diductor muscles. All these newly evolved structures served originally to prevent sliding and torsion of the valves during opening and closing.
