Rhynchonellids

Rhynchonellids are a widespread and morphologically distinctive order of extinct articulate brachiopods. These marine invertebrates flourished from the Devonian to the end of the Cretaceous period, though their ancestors can be traced back to the Ordovician. Their most striking characteristic is their strongly plicate shell, meaning the valves are ornamented with prominent radial folds and troughs. This plication is typically most pronounced anteriorly and diminishes towards the umbo. The shell shape is generally triangular to ovate, with a prominent, beak-like umbo. A distinctive feature is the absence of a true deltidium in the pedicle valve, which is common in other rhynchonellide orders.

Rhynchonellids possessed a robust, calcareous shell composed of calcite. They were likely epifaunal organisms, attaching themselves to the substrate via a pedicle or living as free-lying individuals. Their feeding mechanism involved a lophophore, a ciliated feeding apparatus. The fossil record of rhynchonellids is extensive, reflecting their abundance and wide distribution across various marine paleoenvironments. Their distinctive shell morphology makes them important index fossils for dating sedimentary rocks from the Devonian to the Cretaceous. Many genera and species have been described, showcasing considerable variation in shell size, degree of plication, and overall shape within the order. Their extinction at the end of the Cretaceous is a part of the K-Pg extinction event.