Brachiopod internal mould

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This fossilized outline of a Palaeozoic brachiopod showcases its defining bilateral symmetry, oriented perpendicular to the hinge line - a trait that sets this group apart from pelecypods (bivalve molluscs), which exhibit bilateral symmetry parallel to the hinge line. Brachiopods, commonly referred to as "lamp shells," feature one valve significantly larger than the other. The larger pedicle valve, also known as the "foot" valve, was either securely attached to or deeply embedded in the seafloor substrate, whereas the brachial valve occupied an uppermost position during life. Fossilized outlines are formed when sediment fills the internal cavity, while external impressions record the exterior of the shell.