Article: Mantle-body arrangement along the hinge of early protrematous brachiopods: evidence from Crozonorthis

The earlier discovery of mantle canals lining the interareas of protrematous brachiopods and the implication that these areas were lined with mantle and not simply outer epithelium is supported by evidence from Crozonorthis. In this genus the ventral interareas show a clear external differentiation, reflecting a lining of mantle laterally and of outer epithelium medianly. Moreover, this morphology provides clear evidence, contrary to popular opinion, that setae could develop along the growing margin of a protrematous interarea. A well-defined junction, between parts adjacent to the delthyrium which are smooth and lateral parts with perforations that housed successive generations of setae during life, marks the position where, on the interior, the inner epithelium separated from the outer epithelium to form the body wall.