Observations on the ontogeny of the upper Cambrian trilobite Peltura scarabaeoides westergaardi Henningsmoen, 1957

A partial ontogeny (meraspides 1-6, 10, 11, and holaspid) is described for P. scarabaeoides westergaardi, based upon intact and exceptionally well preserved specimens from Västergötland, Sweden. Previous studies of the ontogeny of Peltura have been based upon disarticulated sclerites only. Close comparisons are made with the known early stages of P. scarabaeoides scarabaeoides. During ontogeny the glabella first lengthens and then becomes considerably wider during late ontogeny, the visual surface becomes distinct only in later stages, and the eye becomes increasingly close to the glabella. The body remains very spiny until relatively late in ontogeny, though the hook-shaped pleural spines typical of the adult have developed by meraspid degree 10. Axial nodes, prominent in the early stages disappear after meraspid 6, and no trace of the tuberculate sculpture of the meraspid stages is present in the adult. A triangular gap between the anterior edge of the first thoracic pleura and the rear margin of the cephalon has developed by degree 10; this presumably facilitated enrolment. A note is appended on the visual surface of P. scarabaeoides scarabaeoides, previously known only from internal moulds.