Atrypida

Atrypida is an extinct order of articulate brachiopods that thrived during the Paleozoic Era, from the Ordovician period to the Devonian period. Characterized by their impunctate or finely punctate shell structure (meaning their shells lack, or possess only very fine, pores), and typically spirally arranged brachidia (the internal, skeletal support structures for the lophophore, the feeding organ), atrypids are important index fossils for dating and correlating geological strata from their time period. They are generally considered to have been suspension feeders, filtering organic particles from the water column.

Atrypids exhibited a wide range of morphological variation, although a typical atrypid shell is semicircular or subcircular in outline, with a pronounced fold and sulcus (a ridge and groove) on the shell valves. The pedicle opening, through which the stalk attaching the brachiopod to the substrate passed, is often prominent. The shells are often ornamented with radial ribs or growth lines.

The order is subdivided into several suborders and families based on detailed characteristics of their internal structures and shell morphology. Their evolutionary history is complex and subject to ongoing research, but they represent a significant component of Paleozoic marine benthic communities. Their relatively rapid evolution and widespread distribution make them valuable tools for biostratigraphy. The extinction of the Atrypida at the end of the Devonian period was part of the major Late Devonian extinction event that significantly impacted marine life.