Koskinoid perforations in brachiopod shells: function and mode of formation

Clusters of tiny perforations called 'koskinoid' have been interpreted as having accommodated byssus-like fibrils for attachment in Devonian Uncites and the Orthotetacea, which range from the Silurian through the Permian. The mode of formation of the tiny holes has inspired three hypotheses: (1) The fibrils protruded at the commissure and then were surrounded by growing shell. (2) The fibrils protruded from a juvenile foramen at the beak, which then was covered by shell that caused them to separate and migrate. (3) The fibrils penetrated their own shells like pedicles of living brachiopods are known to penetrate calcite. The first two hypotheses assume that the fibrils were produced by a rudimentary pedicle within the shell; the third postulates that the fibrils are diverticula from the outer epithelium of the mantle. The facts that the clusters are not confined to the beak region, but occur scattered over the ventral valve, and that they also penetrate the interara, favor the third theory. All Orthotetacea except Thecospira have the koskinoid perforations; all Davidsoniacea except Morinorhynchus lack them. This suggests that the two genera should trade places in the classification.