Post-embryonic development of Dalmanitina, and the evolution of facial suture fusion in Phacopina

A large sample of post-embryonic specimens of Dalmanitina proaeva elfrida and D. socialis from the Upper Ordovician (Sandbian to Katian) Prague Basin allows for the first reasonably complete ontogenetic sequence of Dalmanitoidea (Phacopina). The material provides an abundance of morphological information, including well-preserved marginal spines in protaspides and meraspides, and hypostome external surfaces throughout. The development of D. proaeva elfrida is unusual due to variability in timing of the first trunk articulation. While broadening our developmental understanding of Phacopina, a diverse group of phacopid trilobites, we are also able to study the evolution of their specializations in exoskeletal molting behavior. Adult phacopines, unlike most other trilobites, had fused facial sutures. This means that rather than molting through the Sutural Gape mode, characterized by opening of the facial sutures and separation of the librigenae, they disarticulated the entire cephalon in S...