Article: The external surface of Dictyonella and of other pitted brachiopods

The external surface of Dictyonella is characterized by a smooth depressed umbonal region and a pitted ornament. The exposed part of the smooth plate, here termed the colleplax, is enlarged by resorption of the outer shell. A model is developed to explain the mode of function of the colleplax and its relationships to the soft tissue. The apparently complicated surface pattern of Dictyonella and other pitted brachiopods is shown to be produced by simple radial growth modified by various factors. The surface cells of Dictyonella may have up to nine puncta on the cell floor and variations in depth of the cells partly reflect preservation of the cavernous calcareous shell, which is apparently composed of a thin primary layer and a fibrous secondary layer. An Ordovician species, D. planicola sp. nov., possesses an ornament intermediate between typical Silurian Dictyonella and the presumed ancestral Eichwaldia. In other brachiopods. pitted surfaces may also be relaled to caeca, or to temporary marginal caeca in impunctate slocks. Those of other genera bear no relation to endopuncta and may be grouped as (a) complete or incomplete superficial pits possibly containing organic substances; (b) anteriorly directed shallow aaiticutes housing marginal setae; and (c) smaller and more steeply inclined arrugiae containing other sensory bristles.