Articulatory structures

Functional morphology of articulatory structures and implications for patterns of musculature in Cambrian rhynchonelliform brachiopods... 

Until recently, the presence or absence of articulatory structures along the posterior margin of the valves of brachiopods was used as a primary character in the discrimination of two first order taxonomic units within the Phylum, generally recognised as being of Class rank (e.g. Huxley, 1869 Williams and Rowell, 1965). Stemming from... 

By contrast, there has been relative neglect of the analyses and implications of the presence of edentulate articulatory structures in brachiopods that were classified previously as articulates or inarticulates . A number of Cambrian stocks of brachiopods classified previously as inarticulates now fall within the articulated clade of rhynchonelhforms (Williams et al., 2000). This paper reviews the functional background to such conclusions and the implications for the hinge/muscle system development of the brachiopods. 

All these hypothetical steps can be traced in the obolellates, which evolved simple articulatory structures soon after the development of hemiperipheral growth in the dorsal valve. Together with the closely placed ventral diductors of the obolellates, which operated a low angle to the commissural plane, so too did the articulatory structures work mostly to prevent anterior shifts of the dorsal valve. Another distinctive trend in obolel-late evolution was the maintenance of the position of the diductor muscles sub-parallel to the commissural plane. With an increased relative convexity of both valves, one result was the evolution of high ventral muscle platforms in naukatides. 

Ventral Articulatory Structure. 0 Absent I Teeth 2 Linear Ridge 3 Linear Furrow. 

---