Orthids

Orthids are a diverse and extinct group of brachiopods belonging to the class Articulata. They flourished from the Middle Ordovician to the Late Permian periods. Orthids are characterized by their generally biconvex, equivalve shells, meaning both valves are of similar size and convexity. Their shells are typically punctate, containing microscopic pores. The shell material is usually impunctate, and the internal surface of the shell displays prominent radial ribs and a distinct muscle scar pattern.

The ventral valve, or pedicle valve, often possesses a well-developed delthyrium, an opening through which the pedicle extended for attachment. The dorsal valve, or brachial valve, typically has a simple hinge line. Orthids are a cosmopolitan group, found in marine sediments worldwide. Their ecological roles varied, with some likely infaunal, burrowing into soft substrates, while others were epifaunal, living on hard surfaces.

Their taxonomic classification has been subject to revision, but they are considered a foundational group within the articulate brachiopods. Orthid fossils are important index fossils for correlating rock strata from the Paleozoic Era. Their widespread distribution and relatively rapid evolution make them valuable for understanding ancient marine environments and the history of life on Earth. The group's extinction occurred during the Permian-Triassic mass extinction event, a significant turning point in Earth's history.