|Origin of the Maxillopoda; urmalacostracan ontogeny and progenesis|
Newman, W.A. (1983). Origin of the Maxillopoda; urmalacostracan ontogeny and progenesis, in: Schram, F.R. (Ed.) Crustacean phylogeny. Crustacean Issues, 1: pp. 105-119
In: Schram, F.R. (Ed.) (1983). Crustacean phylogeny. Crustacean Issues, 1. A.A. Balkema: Rotterdam, The Netherlands. ISBN 90-6191-231-8. 372 pp., more
In: Schram, F.R. (Ed.) Crustacean Issues. Balkema/CRC Press/Taylor & Francis: Rotterdam. ISSN 0168-6356, more
The Maxillopoda have been inferred to have a 5-6-5 body plan derivable from the 5-8-8 plan of the malacostracans. Since crustaceans in general have anamorphic development, it has also been inferred that the reduction was achieved by progenetic paedomorphosis; that is, precocious maturity of a larval form. The peneids and euphausiids are the only malacostracans having a relatively complete anamorphic larval sequence. It is complicated by numerous apomorphies including loss of naupliar feeding and the acquisition of the caridoid facies. Maxillopodans, how-ever, typically have feeding nauplii, and there is no trace of the caridoid facies at any stage of development. Maxillopodans appear in the fossil record in the Cambrian, long before the first eumalacostracans, which first appear in the Devonian as caridoids. Therefore, at least a precaridoid malacostracan ancestry for the maxillopodans is indicated. Virtually all non-eucarid malacostracans, including the primitive leptostracan phyllocarids, have epimorphic or nearly epimorphic development. Therefore, the larval development of precaridoid malacostracans must be reconstructed. By removing caridoid apomorphies from the eucarid sequence, and integrating what remains with knowledge of the most anamorphic development known in crustaceans, that of the cephalocarids, a reasonable approximation of precaridoid, urmalacostracan development can be reconstructed. The urmalacostracan ontogenetic sequence suggested here consists of approxi-mately 14 stages, beginning with a feeding nauplius and including the first adult or complete body plan. The stages are divided almost equally between naupliar and 'caripodid' sequences. No proto-zoeal stages appear between them because the feeding nauplii eliminate their need. The 11th or 'premancoid' caripodid of the urmalacostracan larval sequence has the essential urmaxillopodan features including a 5-6-5 body plan.