Development in Aulacopleura koninckii - Glimpses into the Evolution of Body Organization in an Early Arthropod Clade
A concentration of well preserved, articulated juvenile and mature exoskeletons of the Czech middle Silurian trilobite Aulacopleura koninckii provides an unparalleled opportunity to explore the nature of exoskeletal growth and morphological variation during trilobite development. A new dataset selected via a comprehensive survey of the highest quality specimens offers improved resolution of original morphology by all measures considered. It confirms that meraspid size increase was exponential and the meraspid growth rate per instar was strikingly constant. The degree of variation in both size and shape among later meraspid instars was remarkably constant, and suggests that an active process channeled the meraspid development of both these attributes. The degree of precision of the growth control is remarkable given this species' well-known variance in holaspid trunk segment number. Size-related changes in the shape of the dorsal exoskeleton and of the segment-invariant cephalon were detected in the meraspid stage, but in the holaspid phase marked allometry was detected only in the trunk region, with the pygidium showing notable expansion in relative size. Meraspid cranidial allometry was subtle, with significant changes in instar form detectable only after several molts. Analyses suggest that this synarthromeric trilobite developed progressively without sharp transition throughout meraspid and holaspid ontogeny, except for the synchronous cessation of trunk segment appearance and release at the onset of the holaspid phase. We suggest that the remarkably tight control of the development of sclerite size and shape, and the gradual, "track-like" ontogeny of A. koninckii may have resulted from development in conditions of reduced oxygen availability in which physiological viability operated within narrow morphological limits.