Phenotypic plasticity of colour-patterned Plectodonta (Strophomenida, Brachiopoda) from the Lower Devonian of the Barrandian area (Czech Republic)

Colour patterned shells of the strophomenide brachiopod Plectodonta mimica (Barrande, 1879) are described. The shells coming from the Kotys Limestone (Lochkov Formation, Lochkovian) bear equidimensional spots spaced on shell surface in a divaricate pattern, in concentric bands or in combination of both, indicating high phenotypic variability. Coloration has been observed on ventral but rarely also in dorsal valves. Divaricate pattern of spots indicates that origin of spots is likely connected with neurosecretory function of the mantle. Similar origin is suggested for the truncated rugellae arranged in checkerboard pattern that are developed in some strophomenid brachiopods, e.g., Cymostrophia, Gunnarella, Ptychoglyptus, Sowerbyella (Rugosowerbyella). The camouflage function is not suggested for the colouration of Plectodonta because its ventral valve was likely faced downward.