THE CONTROL OF TRUNK SEGMENT DEVELOPMENT IN TRILOBITES

The good record of trilobite ontogeny offers multiple opportunities to investigate the evolution of development within the clade. The trunk is the postcephalic body region that is constructed postembryonically in almost all trilobites by the sequential release of segments from a subterminal generative zone. In most trilobites the expression of each new exoskeletal segment coincided with an instar, each of which had to be functionally viable as mature form developed progressively. Because all body parts increased in size from instar to instar, and as anterior segments appeared before those that succeeded them, if all segments started at about the same size and shared a similar growth rate, anterior segments are expected to be longer than those succeeding them. Such gradual relative decrease in segment length along the trunk is witnessed in many mature trilobites, and is notable in several iconic Cambrian forms. However, not all trilobites adhered to this pattern, with departures including abrupt changes in segment relative length at particular points along the trunk, regions in which all segments were of similar length, dynamic patterns of changing relative length during ontogeny, and mismatches between axial and pleural segmental growth. In order to begin to explore how such growth patterns are controlled we conducted a detailed analysis of segment relative length during the late meraspid growth of the 429 million year old Aulacopleura koninckii during which the position of the relatively longest trunk segment migrated posteriorly. We explored analytically whether each segment experienced a constant growth rate that was different among segments, or whether segment growth rates were under regional control. Resolving this may have implications for understanding the basal condition of arthropod trunk segment development.