The shell‐structure, growth and functional morphology of some Lower Devonian athyrids from northwest Spain

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Abstract
In the Emsian (Lower Devonian) of northwest Spain three congeneric species of athyrids have been recognized principally by their, as yet, unique shell microstructures. The shell remains thin throughout ontogeny, with hardly any internal thickening, while the external lamellae are long and two or four times as thick as the values, being composed of many layers of secondary fibers. A growth model for these shells is presented, including the recognition and differentiation of true valve growth as compared to growth of lamellae. Functional morphology is discussed and a new explanation presented for the function of the visceral forearm. The possible growth and disposition and lophophore are described and, taken together with morphological and environmental factors, an anterior inhalant water circulation system is proposed.