ABSTRACT
Evolutionary steps leading to the basal crustaceomorphs are traced. In the most plesiomorphic Cambrian arthropods, all appendages except the first antenna were similarly shaped throughout the body. The endopod was a simple stem, with some 15-20 identical short segments and a conical tip. The exopod was a thin rounded flap devoid of setae. No appendages were adapted to assist in feeding, and substrate sediment with its inclusions was simply engulfed for food. Successive steps leading to a basal crustaceomorph level included endopod segment reduction and diversification, formation of exopod setae and segments, and later on, specialization of the first three limbs for swimming. With the agnostids, we enter the evolutionary stairway to modern crustaceans previously traced from fossil evidence in Upper Cambrian ‘Orsten’ concretions. In a number of side lineages, the 1st or 2nd appendage evolved into a clumsy grasping organ, indicating the need for limb specialization when sediment feeding was succeeded by other modes of feeding. However, in contrast, ancestors of the crustaceomorphs stuck to a fairly plesiomorphic morphology, which preserved an evolutionary flexibility.
