Article: A new type of delthyrial cover in the Devonian brachiopod Mucrospirifer

A new type of delthyrial cover, stegidial plates, is described from very well-preserved specimens of Mucrospirifer mucronatus (Conrad) from the Traverse Group (Middle Devonian) of Michigan. Stegidial plates are unlike other delthyrial covers, such as deltidial plates and pseudodeltidia, because they are not integral parts of either valve: they are truly independent plates. As such, they can be compared only with the stegidium of some Upper Devonian spiriferides. The stegidium is re-interpreted in the light of its probable homology with stegidial plates: stegidial structures as a whole are regarded as unique within the phylum.