Machaerid
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Machaeridians are a little-studied class of fossil animals. They are known from the Ordovician to the early Permian. The group consist of three distinct families: the plumulitids, turrilepadids and lepidocoleids. The machaeridians are characterized by having serialized rows of calcitic shell plates. the plumulitids are flattened from above and looks much like the coat of mail armour of chitons. The two other families are laterally compressed and some lepidocoleids have fomred a dorsal hinge, which make these machaeridians look like a string of bivalves. The systematic affinity of the group has been uncertain for more than 150 years and scientists have been assigning the fossils to groups like the echinoderms, barnacles, annelids and mollusks. Particularly more recently it has been demonstrated that the group is related to annelids and demonstrates that this group evolved a distinct mineralized armour independent of other groups, like the mollusks or brachiopods within the Lophotrochozoa.