Agnostida

Agnostida is an extinct order of marine arthropods belonging to the class Trilobitomorpha. Agnostids are characterized by their small size, typically less than 5 millimeters, and a highly reduced number of segments. Their most distinctive feature is the near-complete effacement of pleural furrows on the thorax and pygidium, making them appear almost completely smooth. They possessed a distinctive two-lobed cephalon and pygidium, which were often morphologically similar in size and shape, a phenomenon known as isomerism. This similarity is thought to have aided in a unique mode of enrollment, where the entire animal could be tightly tucked into a protective ball.

Fossil evidence indicates that agnostids were globally distributed during the Cambrian and Ordovician periods. They are considered particularly useful index fossils for stratigraphy due to their widespread occurrence and rapid evolutionary changes. Their exact ecological role is still debated, but they are generally believed to have been planktonic or nektonic, likely filter feeders or detritivores. The order is divided into several suborders, including Agnostina and Eodiscina, distinguished by differences in their cephalic and pygidial structures. The extinction of agnostids occurred by the end of the Ordovician, marking the end of a significant lineage within the trilobites.