Agnostida

Agnostida is an order of minute, typically eyeless arthropods traditionally classified among the trilobites, which flourished in Cambrian and Ordovician seas from approximately 521 to 444 million years ago. Members of the order are among the smallest trilobites known—most adults measured only a few millimeters, rarely exceeding 15–20 mm—and are distinguished by a body in which the head shield (cephalon) and tail shield (pygidium) are of nearly equal size and broadly similar appearance, joined by a thorax of only two (or, in the suborder Eodiscina, two to three) segments. Because agnostids evolved rapidly, dispersed widely, and were largely indifferent to sedimentary facies, their remains constitute some of the most important index fossils in geology: several internationally ratified boundaries of Cambrian series and stages are defined by the first appearance of particular agnostid species, making the order fundamental to the modern global timescale of the Cambrian Period.

Morphology

Agnostids are