SEGMENT-SPECIFIC GROWTH RATES IN TRUNK DEVELOPMENT OF THE TRILOBITE AULACOPLEURA KONINCKII

Individual segment length data for much of the ontogeny of a single trilobite species, Aulacopleura koninckii, has been compiled from a dataset of the best preserved specimens in order to investigate morphometrically how the trilobite trunk developed. During the meraspid phase the sagittal length of individual segments maintained a constant growth rate per molt in accordance with Dyar’s Rule. However, each segment possessed a different and unique growth rate value, the magnitude of which increased posteriorly from the anteriormost thoracic segment. In the holaspid phase, posterior segments' growth rate, estimated over a given increment of overall size, continued to be greater than that of anterior segments. In addition, as some posterior segments eventually caught up and exceeded the length with the anterior segments, the position of the longest segment shifted posteriorly during meraspid and holaspid growth. Both these observations suggest that the meraspid segment growth scheme may have extended into the holaspid phase without marked modification. Although trunk segments in A. koninckii are homonomous in shape and were all generated sequentially at the anterior of the terminal trunk piece, the constant-rate growth suggests a certain degree of modulated expression among these iteratively produced developmental modules. However, as each segment’s per-molt growth rate increased posteriorly in a regular fashion, segments collectively developed according to a regular pattern throughout the thorax. This is the first detailed study of the how development of individual segments constructed the trunk in any trilobite. Whether aspects of this pattern are general to all trilobites remains to be established. Our results are consistent with analyses of other aspects of the growth of A. koninckii, which consistently suggest precise and tight regulation of the development of the size and shape of individual sclerites. This contrasts with the unusual variation in the number of thoracic segments among mature specimens of A. koninckii, and also with the high number of anomalously developed articulations between trunk segments that we have recorded. Thus, while the development of some characters was apparently very tightly controlled in A. koninckii, other characters presented some degree of developmental instability.