WALOSZEK, D.; University of Ulm: Evolutionary History of Crustacean Segmentation and Tagmosis: A Fossil-Based Perspective

Evolution of crustacean segmentation, the formation of jointed body segments, the formation of groups of segments (tagmosis), and the formation of jointed articles along segmental appendages cannot be viewed separately from historical traits and functional constraints. Much of this system originated from ground patterns established in the earliest segmented animals that bore sclerotic stiffenings of the dorsal parts of the chitin-bearing cuticle (= arthrodization) and similar organization along their appendages, i.e. articles connected by pivot joints (= arthropodization): the Arthropoda sensu stricto. Various Cambrian arthropods described in the last 20 years have updated our views of early arthropod morphology and phylogeny considerably. One important source was the discovery of the minute Lower to Upper Cambrian �Orsten� 3D arthropod fossils from Sweden and other sites worldwide. Such well-preserved fossilized animals serve as empirical evidence of ancient morphologies (as opposed to hypothetical models) and provide us with an insight into the morphogenetic development of segments and their equipment, particularly the limbs, at evolutionary levels ranging from basal members of Euarthropoda (Agnostus pisiformis) to derivatives of the stem-lineage (e.g., Martinssonia elongata) and crowngroup of Crustacea (e.g., Rehbachiella kinnekullensis with 30 successive larval stages). Another fascinating data source are the flattened fossils of the Lower Cambrian Chengjiang lagerstaetten in China that continue to yield spectacular new finds. This and the �Orsten� are the major sources of this review of the structural and functional development toward the crown-group of Crustacea, the Eucrustacea that stresses particular examples of recently investigated forms.