Ontogeny of the Late Cambrian trilobite Olenus asiaticus Kobayashi, 1944 from the Machari Formation of Korea

Ontogeny of the Late Cambrian trilobite Olenus asiaticus Kobayashi, 1944 was examined based on the material from the Glyptagnostus reticulatus Zone of the Machari Formation in Yongwol area, Korea. Protaspides are differentiated into anaprotaspid and metaprotaspid stages. Anaprotaspides are represented by circular dorsal shields with faintly segmented axis, while metaprotaspides have five axial rings, eye ridges, and a differentiated protopygidium. Most of meraspides, represented by disarticulated cranidia, are further divided into six morphological groups. The Group 1 is assumed to represent the meraspid degree 0, whereas the remaining groups are subdivided in convenience to describe the morphological development of cranidia. Morphological changes within the meraspid stages are gradual in general. The glabella is parallel-sided or tapers rearwards in the early meraspides, whereas it tapers forwards in the later forms. The preglabellar field appears in the Group 3 and becomes progressively longer in the later meraspid stages. The holaspid cranidia are distinct in having a subquadrate glabella with three pairs of lateral glabellar furrows.