Shell structure, ontogeny and affinities of the Lower Cambrian bivalved problematic fossil Mickwitzia muralensis Walcott, 1913

Exceptionally preserved carbonate- and shale-hosted Mickwitzia muralensis from the Lower Cambrian Mural Formation, southern Canadian Rocky Mountains, complement one another to yield an unusually complete account of its ontogeny, ecology and phylogenetic relationships. The shell of M. muralensis is composed of dense phosphatic layers interspersed with porous organic-rich layers. At the insertion of shell-penetrating tubes, shell layers deflect inwards to produce inwardly pointing cones. The tubes are interpreted as having hosted setae that were secreted by outer-epithelial follicles. Follicular setae also occurred at the mantle margin, where they were oriented within the plane of the shell as in modern brachiopods. During ontogeny, the initial setae oriented in the plane of the shell occurred before the first shell-penetrative setae. In the juvenile and early-mature stages of shell secretion, a posterior opening was present between both valves and was used for the protrusion of an attachment structure. In the late-mature shell, this opening became fixed in the ventral valve. Based on the posterior margin and the shell microstructure, a close relationship between Mickwitzia and the paterinids is proposed with differences interpreted as heterochronic. The shell-penetrative setal apparatus of M. muralensis is distinct from that previously described of Micrina, though both types are conceivably homologous to adult and juvenile setae of modern brachiopods.