Unique morphology and ecology lifestyle of a new craniiform brachiopod from the Upper Ordovician of Estonia (Baltica)

Unlike most other brachiopods, craniiforms lack a prominent pedicle, cement to a hard substrate by their ventral valve, and thereby can be easily distinguished in paleontological materials. In this study, we describe a new genus and species from the Porkuni Regional Stage of Northeastern Estonia (Baltica), corresponding to the lower part of the Hirnantian Stage of the Upper Ordovician. Pocillocrania rubeli gen. et sp. nov. was a small brachiopod, about 3–4 mm in size, that lived on dorsal (abfrontal) surfaces of reticulate colonies of stenolaemate bryozoans and, like other craniids, was cemented by its ventral valve. The ventral valve of P. rubeli n. gen. n. sp. has a cup-shaped form with a steep, straight posterior slope. The anterior part of the dorsal valve is concave and was probably deeply inserted into the ventral valve. The external surface of the dorsal valve bears peripheral hollow spines, which sometimes bifurcate. Almost all ventral valves, which have been studied, are attached to the reticulate vertical colonies of bryozoans Parachasmatopora porkuniensis Lavrentjeva, 1985. The cup-shaped ventral valve of P. rubeli n. gen. n. sp. was likely an adaptation to inhabiting living organisms. Due to silicification, both valves of P. rubeli preserve well-defined muscle scars, allowing reconstruction of the musculature of this new craniid species as consisting of five pairs of muscles, which participate in the movement of the valves. A morpho-functional analysis based on data from extant craniids suggests that the lophophore of P. rubeli n. gen. n. sp. was of the simple spirolophous type and consisted of two arms raised into the mantle cavity, each forming one spiral coil. Thus, despite its unusually shaped ventral valve, the musculature and lophophore of P. rubeli n. gen. n. sp. exhibit a structure typical of craniids.