Developmental and functional controls on enrolment in an ancient, extinct arthropod

Three-dimensional models reveal how the mechanics of exoskeletal enrolment changed during the development of a model organism for insights into ancient arthropod development, the 429-million-year old trilobite Aulacopleura koninckii. Changes in the number, size and allocation of segments within the trunk, coupled with the need to maintain effective exoskeletal shielding of soft tissue during enrolment, necessitated a transition in enrolment style about the onset of mature growth. During an earlier growth phase, enrolment was sphaeroidal, with the venter of the trunk fitting exactly against that of the head. In later growth, if lateral exoskeletal encapsulation was to be maintained trunk length proportions did not permit such exact fitting, requiring an alternative, non-sphaeoridal enrolment style. Our study favours adoption of a posture in later growth in which the posterior trunk extended beyond the front of the head. This change in enrolment accommodated a pattern of notable variation..., , Blender files use the 3-D graphics and animation software Blender (https://www.blender.org/). Add-on animations nodes (https://animation-nodes.com) can be used to visualize the results and assess different parameters. See README for more details.