Development of an Ordovician trilobite was more variable than in its later relatives

New discoveries in trilobite research: a large sample of developmental stages of trilobite genus Dalmanitina from the Ordovician of the Prague Basin allowed Lukáš Laibl and his colleagues to describe ontogeny of basal members of the suborder Phacopina. Adult phacopines are recognised by their fused facial sutures, which in other trilobites are non-fused and used during the moulting process. This means that rather than molting through the sutural gape mode, characterized by opening of the facial sutures and separation of the librigenae, adult phacopines disarticulated the entire cephalon in so called Salter’s mode of molting. Early developmental stages of Phacopina, however, moulted by sutural gape mode because they retained unfused facial sutures. The fusion of facial sutures occured during the postembryonic development and its developmental timing was intraspecifically fixed in most of the members of the group. However, this was not the case for Dalmanitina as in this genus facial suture fusion occurred variably in ontogeny. This confirms the hypothesis that basal members of major clades have less contrained developmental programs. The material also provides insight into how linked morphologies and behaviors evolved, potentially suggesting the timing of facial suture fusion in Phacopina moved earlier during development and became more intraspecifically fixed over geological time.

Drage H.B., Laibl L., Budil P. (2018): Postembryonic development of Dalmanitina, and the evolution of facial suture fusion in Phacopina. Paleobiology 44, 638-659 (DOI)