Functional morphology and evolution of xiphosurids

Aspects of the morphology, evolution and systematics of the Xiphosurida are treated. The ancestral forms lacked specialization for ploughing, and their chilaria were evidently developed as prosomal walking legs. The corresponding tergite (of the pregenital segment) was probably separate from the main prosomal shield in the early xiphosurids as well as in the eurypterids. From this stem two main groups seem to have evolved. One consists of the synziphosurids, large-eyed eurypterid-like hunters with striking opisthosomal tagmosis. The other consists of the burrowing and ploughing xiphosurids, in which the opisthosomal tergites were subject to progressive fusion ending with a single opisthothoracic tergal shield in the Late Palaeozoic. The last prosomal appendages evolved into the chilaria, if this did not happen earlier, and the corresponding free tergite disappeared. Probably in Carboniferous time the limulines came into existence through a sudden displacement of the prosomal/opisthosomal boundary.