Ontogeny of Agnostida

Evidence about ontogeny of the miomerid trilobite groups Eodiscina and Agnostina is summarizied and unified in terminology. There appear to be strong similarities in the juvenile development of both suborders. They both have prolonged meraspid degree 0 and 1 stages, including two to six molts. The degree 0 stage tends to shorten from six to four molts in Eodiscina, and probably from three to two in Agnostina, while degree 1 tends to lengthen whithin Eodiscina. During degree 0 and 1 moltings, none of the segments are liberated into the thorax, and one segment is added into the transitory pygidium. The liberation of thoracic segments marks the beginning of the meraspid degrees and the holaspid period. This developmental scenario contrasts with that of the classical polymerid—in the latter case, the liberation of thoracic segments is accompanied by insertion of new segments in the transitory pygidium. In the Eodiscina, during meraspid degree 2, the polymerid scheme is present.