Ontogeny of the ptychaspidid trilobite Quadraticephalus elongatus from the Furongian (late Cambrian) Hwajeol Formation, Korea

Abstract

The development of trilobite pygidium involves both articulation process at the frontal part and the formation of new segments at the rear end, and hence the development of meraspid pygidium accompanies 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 of Q. elongatus, the convexity of the cranidium of Q. elongatus increased in a slower rate

the yoked free cheek of Q. elongatus split to form a ventral median suture in a later developmental stage

and, a rim-like ridge, which disappeared in the early holaspid pygidium of A. subglobosa, maintained in the holaspid period of Q. elongatus. These morphological changes with growth imply that a paedomorphism was involved in the evolution of Q. elongatus. Eleven stages are recognized for the meraspid pygidial development, which began with an accumulation phase during which the number of segments within the pygidium increased from 3 to 7, followed by an equilibrium phase with 7 segments, and ended up with a depletion phase during which the number of segments within the pygidium decreased down to 4. During the depletion phase, the pygidial length did not increase or even slightly decreased. The onset of epimorphic phase, in which the total number of trunk segments does not increase anymore, precedes the onset of holaspid period during ontogeny, demonstrating that the developmental mode of Q. elogatus is protomeric.