Агностиды - маленькие трилобиты, существовавшие начиная с раннекембрийского и кончая позднеордовикским временем. У большинства агностид глаза отсутствовали; швы краевые (гипопариевые) или отсутствовали; число туловищных сегментов равно двум (Agnotus, Hypagnostus) или трем (Eodiscus). Цефалон и пигидий равновеликие (отсюда название - изопигиевые трилобиты).

 Scientists have long debated whether the agnostids lived a pelagic or a benthic lifestyle. Their lack of eyes, a morphology not well-suited for swimming, and their fossils found in association with other benthic trilobites all suggest a benthic (bottom-dwelling) mode of life. They are likely to have lived on areas of the ocean floor that received little or no light and fed on detritus that descended from upper layers of the sea to the bottom. In contrast, their wide geographic dispersion in the fossil record is uncharacteristic of benthic animals, suggesting a pelagic existence. The thoracic segment appears to form a hinge between the head and pygidium allowing for a bivalved ostracodan-type lifestyle. Furthermore, the orientation of the thoracic appendages appears ill suited for benthic living. However, recent work suggests that some agnostids were benthic predators, engaging in cannibalism and possibly manifesting pack-hunting behavior.

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