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The record of ontogenetic development of the 429 million year old trilobite Aulacopleura koninckii is exceptional for its completeness, and offers insights into factors regulating growth now yet realized in any other fossilized arthropod. An assessment of the physical and biotic environment of the locality that has yielded almost all well-preserved specimens belonging to this species suggests that A. koninckii was opportunistic and thrived in conditions of fluctuating oxygen availability, blooming when oxygen levels dropped below those required to support diverse benthos. Both the overall form and the ontogenetic trajectory of this animal are consistent with carefully controlled, incremental growth that maximized surface area for gas exchange in the trunk region at each developmental stage. Despite clear evidence that the growth of both size and shape was targeted, A. koninckiishowed remarkable variation in the numbers of thoracic segments at maturity, and also a curiously labile pattern of the development of articulations between segments. Analysis of the growth of individual segments shows that the trunk segment growth was under regional, as opposed to segment-specific control, and this may have been the basal condition in arthropod postembryonic trunk development. |