ORIGIN OF MAXILLOPODA

The Maxillopoda Dahl (1956) was defined primarily by a 5-6-5 body plan; that is, a head supporting five pairs and a thorax six pairs of appendages, and an abdomen of five segments, the first of which may bear a pair of genital limbs and the last of which, the telson, bears caudal rami. Representatives as diverse as ostracods, cirripeds, orstenocarids and skaracarids appear in the Cambrian, so we are dealing with a radiation that in the most part took place over 500 million years ago. A progenetic origin of the maxillopodans, from an ur-malacostracan stock, has been proposed (Newman, W. A. 1983. Crustacean phylogeny-Crustacean Issues, Vol. 1, pp. 105-119). However, new information, including the recently discovered Cambrian forms Skaracarida and Orstenocarida, the structure of spermatozoa, and preliminary results from the sequencing of ribosomal RNA, suggests that the Maxillopoda are more deeply rooted in the crustaceans than previously supposed, likely closer to the ur-crustacean than to the ur-malacostracan.