Ontogeny of the Ptychaspidid TrilobiteQuadraticephalus elongatusKobayashi, 1935 from the Furongian (Late Cambrian) Hwajeol Formation, Korea

The development of the trilobite pygidium involves both an articulation process at the frontal part and the formation of new segments at the rear end, and hence the development of the meraspid pygidium entails complicated morphological changes. This study deals with the ontogeny of the Furongian (late Cambrian) ptychaspidid trilobite,Quadraticephalus elongatus(Kobayashi, 1935), from the Hwajeol Formation of the Taebaek Group, Taebaeksan Basin, Korea, with a special focus on the segmentation process during the meraspid pygidial development. Compared to the ontogeny of a ptychaspidid trilobite,Asioptychaspis subglobosa(Sun, 1924), which is assumed to be an ancestral species ofQ. elongatus, the convexity of the cranidium ofQ. elongatusincreased in a slower rate; the yoked free cheek ofQ. elongatussplits to form a ventral median suture in a later developmental stage; and, a rim-like ridge, which disappeared in the early holaspid pygidium ofA. subglobosa, was maintained in the late holaspid period ofQ. elongatus. These morphological changes with growth imply that paedomorphosis was involved in the evolution ofQ. elongatus. Eleven stages are recognized for the meraspid pygidial development, which began with an accumulation phase during which the number of segments increased from three to seven, followed by an equilibrium phase with seven segments, and ended up with a depletion phase during which the number of segments within the pygidium decreased to four. During the depletion phase, the pygidial length did not increase or even slightly decreased. The onset of the epimorphic phase, in which the total number of trunk segments does not increase anymore, precedes the onset of the holaspid period, demonstrating that the developmental mode ofQ. elongatusis protomeric.