Article: Formation and function of protegular pitting in some North American acrotretid brachiopods

SEM examination of well-preserved protegula of acrotretid brachiopods from the Ordovician, Silurian, and Lower and Middle Devonian of Ontario, the Lower Devonian of the Yukon Territory, and the Silurian of Oklahoma, has led to the formulation of a modified selective resorption model for the formation of protegular pits in this group. This model contrasts with the bubble raft model of Biernat and Williams (1970) and proposes that the development of protegular pits is due to resorption by the mantle, with the complexity, degree, and configuration of pitting dependent on the length of the larval stage. Protegular pitting may be terminated at any of a number of stages and is thought to be related to the necessity for the larval stage on one hand to develop a protective rigid shell and on the other to remain buoyant. Thus the resorption of calcium phosphate is probably a weight-controlling mechanism during the planktic stage of the larval acrotretid—a mechanism directly comparable to the use of frets or of a Gitterwerk in airframe construction. The necessity for juvenile acrotretid brachiopods to remain afloat is probably related to unknown environmental factors that made it desirable to postpone settling.The valve previously considered to be the pedicle valve of Opsiconidion arcticon was misassigned. Pedicle valves of species of Opsiconidion are now known to be highly conical, to bear well-defined pedicle openings at the apex, and to lack muscle scars. They bear conical protegula with ultrastructure identical to that of the brachial valves.