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Agnostida are an order of extinct arthropod which have classically been seen as a group of highly modified Trilobite, though some recent research has doubted this placement. Regardless, they appear to be close relatives as part of the Artiopoda. They are present in the Lower Cambrian fossil record along with trilobites from the Redlichiida, Corynexochida, and Ptychopariida orders, and were highly diverse throughout the Cambrian. Agnostidan diversity severely declined during the Cambrian-Ordovician transition, and the last agnostidans went extinct in the Late Ordovician.
The systematic position of the order Agnostida within the class Trilobita remains uncertain, and there has been continuing debate whether they are trilobites or a stem group. The challenge to the status has focused on Agnostina partly due to the juveniles of one genus have been found with legs differing dramatically from those of adult trilobites, (1987). 9788200075110 ISBN 9788200075110 suggesting they are not members of the clade, of which trilobites are a part. Instead, the limbs of agnostids closely resemble those of stem group crustaceans, although they lack the proximal endite, which defines that group. The study suggested that they were likely the sister taxon to the crustacean stem lineage, and, as such, part of the clade, Crustaceomorpha. (2025). 9781420037548, Taylor and Francis Group. ISBN 9781420037548 Other researchers have suggested, based on a cladistic analyses of dorsal exoskeleton features, that Eodiscina and Agnostida are closely united, and the Eodiscina descended from the trilobite order Ptychopariida. (2025). 9781420037548, Taylor and Francis Group. ISBN 9781420037548 A 2019 study of adult specimens with preserved soft tissue from the Burgess Shale found that agnostidans shared morphological similarities to trilobites and other related artiopodans like Nektaspida, and their placement as stem-crustaceans was unsupported. The study recovered agnostidans as the sister group to other trilobites within the Artiopoda.
They are sometimes preserved within the voids of other organisms, for instance within empty hyolith conchs, within , Selkirkia and under the carapaces of Sidneyia, presumably in order to hide from predators or strong storm currents; or maybe whilst scavenging for food. In the case of the tapering worm tubes Selkirkia, trilobites are always found with their heads directed towards the opening of the tube, suggesting that they reversed in; the absence of any moulted carapaces suggests that moulting was not their primary reason for seeking shelter.
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