Article: Shell structure and inferred growth, functions and affinities of the sclerites of the problematic Micrina

The stratiform laminae of Micrina sclerites originally consisted of rheomorphic successions of monolayers of micrometric-sized, apatitic tablets, presumably interleaved with chitin and glycosaminoglycans (GAGs). Paired laminae enclose slot-like chambers swelling into lobes distally that originally contained GAGs and deposits of spherulitic and prismatic apatite. The laminae are pervaded by apatitic tubes, apparently secreted by microvillous setoblasts and containing, at the surface, chitinous setae. Internal markings suggest that the triangular (sellate) sclerite supported a pair of muscles and the planospiral (mitral) sclerite, a medial muscle and gonadal sacs flanked by a pair of crescentic muscle bases. Both sclerites were secreted by a mantle with a circumferential fold. The sellate and mitral sclerites are homologized with the anterior and posterior shells of Halkieria and could have become the dorsal and ventral valves of the ancestral brachiopod by a sequence of transformations. These include: the folding of the halkieriid body axis; accelerated mixoperipheral growth of the anterior (dorsal) shell to enclose, with the posterior (ventral) shell, a mantle cavity lined with modified ciliated epithelium of the foot; reduction of sclerite-secreting epithelium to the locus of the brachiopod pedicle epithelium; and the anterior (dorsal) spread of gonadal lamellae.