A nektaspid arthropod from the Early Cambrian Sirius Passet fauna, with a description of retrodeformation based on functional morphology

The Sirius Passet fauna from the Lower Cambrian of North Greenland yields a diverse suite of poorly sclerotized arthropods. A new nektaspid from the f auna described here differs from typical naraoiids in having six thoracic s egments and in being isopygous. Soft parts are poorly known, but posterior limbs are preserved in some specimens. Although specimens are invariably hi ghly flattened, details of the axial articulation suggest that the animal c ould enrol. The widths of the articulating half rings can be used to recons truct both the degree of rotation at each articulation during enrolment and the height at which each point along the half ring is above the fulcrum, a nd this in turn may be used to make a full three-dimensional reconstruction of the exoskeleton. This method of retrodeformation is in principle applic able to the reconstruction of other taxa, especially trilobites, found in s hales.