Rensselaeria, genus of extinct brachiopods (lamp shells) found as fossils in Lower Devonian marine rocks (387 to 408 million years old). The shell is large and elongated. Its surface markings include fine costae (i.e., lines that radiate from the narrow apex of the shell to the distal, or terminal, margins) and arcuate (bowlike) growth lines. Because of its restricted time range, relative abundance, and ease of recognition, Rensselaeria serves as a reliable guide, or index, fossil for the Lower Devonian.