Ontogeny of an Early Cambrian eodiscoid trilobite from Henan, China

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Abstract
Numerous phosphatized specimens from Xichuan, Henan, China permit a detailed study of the morphology and ontogeny of the Early Cambrian eodiscoid trilobite Neocobboldia chinlinica Lee. Several morphological changes of pygidia are seen to accompany growth, by means of which 13 quantitatively defined stages with a probable growth rate of 1.20, and 4 developmental periods (anaprotaspid, metaprotaspid, meraspid, and holaspid) have been recognized for this species. The anaprotaspis is the first description for eodiscoids. It shows great but gradual changes in morphology with progressive maturity. Morphometric and qualitative analyses indicate that three thoracic segments were formed regularly and then released from the pygidium during the meraspid period; and more than one instar might have occurred within a stage. The exoskeleton consists of two well-preserved layers, both bearing fine ornament that showed gradual change with growth as well. The free cheek with visual surface attached and the hypostoma show many of the fundamental characters of eodiscoids. Some morphological differentiation in cranidia may reflect a sexually dimorphic condition. Statistical and taphonomic analyses indicate that these specimens of N. chinlinica may represent a complete population. The larvae may have lived in swarms in a warm shallow water environment.