Article: A phenetic strategy model for dalmanellid brachiopods

A detailed morphological study, through a sequence of nearshore to offshore sedimentary facies, of the Ordovician dalmanellid Onniella and the Silurian Isorthis, suggests that there is a positive correlation between environment and morphotype. A phenetic strategy model is proposed in which offshore dalmanellid populations are characterized by deep, rounded sulcate shells, V-shaped deep pedicle muscle fields, strong crural fossettes, strongly impressed dorsal adductor muscle fields (i.e. quadripartite), large non-restricted brachial valve sockets, and coarse costellae. By contrast, nearshore dalmanellid populations are characterized by transverse, shallow, non-sulcate shells with U-shaped shallow pedicle muscle fields, weak crural fossettes, weakly impressed dorsal adductor muscle fields, restricted brachial valve sockets, buttressed brachiophores, and fine costellae. There are exceptions to the model but generally the positive correlation, which is explained in terms of functional necessity, has repercussions on previously proposed species phytogenies and their use as biostratigraphic indicators.