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 in the number of mature trunk segments, well known to
characterize the development of this species. It suggests how an animal
whose early segmental development was remarkably precisely controlled was
able to realize marked variation in mature segment number that was
related, apparently, to life in a physically challenging, reduced oxygen
setting.