An alternative phylogeny of peracarid crustaceans

In order to properly understand the origins of the Order Amphipoda, their phylogenetic position within the Superorder Peracarida must first be established. A hypothetical ancestral peracarid is derived from Dahl's hypothetical ancestral eumalacostracan, which was pro­ posed as lacking a carapace. The independent derivation of a carapace for special needs on at least two occasions is thus a central feature of a new phylogenetic arrangement for the peracaridan orders. This phylogeny is based on an analysis of 29 character states using Hennigian methods. Two lineages are immediately apparent: 1, a heterogeneous line con­ taining the Amphipoda, Mysidacea, and Thermosbaenacea; and 2, a homogeneous "mancoid" line leading from the Isopoda through the Spelaeogriphacea and Tanaidacea to the Cumacea. The mancoid line as proposed is inverted relative to earlier phylogenetic schemes; however, this new arrangement is more in accord with the known fossil record. It is suggested that future examinations of the phylogeny of peracarid orders be based on a scheme for the Superorder, thus enabling plesiomorphic features of individual orders to be properly defined. Lastly, the polyphyletic nature of the Peracarida is discussed and it is suggested that at least three independent lines of "peracaridan" radiation might have arisen from a pre-Carboniferous syncarid-Iike ancestral eumalacostracan. This study began as an inquiry into the origins and distribution of the malacostracan Order Amphipoda. The group is represented by more than 7,000 de­ scribed species, of which at least 70% are marine (Barnard, personal communi­ cation). Of use to the following discussion, pertinent facts about amphipods include: a) they are found in nearly all environments from fresh and brackish waters to the deep sea, from the poles to the tropics; b) most amphipod families are very widespread, being represented in the shallow seas surrounding all con­ tinents; c) embryos are carried in a brood pouch and have direct development, thus excluding a dispersive planktonic larval stage; and d) fossils are rare, the oldest being Upper Eocene and looking very much like a modern gammaridean. In addition, because of the supposed young age of the group and its high species richness, amphipods have been often characterized (Bousfield, 1973) as "explo­ sive" speciators. Barnard (personal communication), however, has in press pa­ pers hypothesizing a late Paleozoic presence of modern kinds of amphipods in Pangaean freshwaters. To properly place amphipodan origins it is first necessary to discuss their phy­ logenetic position within the Superorder Peracarida. The latter is one of five superorders within the Class Malacostraca which includes such diverse organisms as the crabs, krill, and stomatopods, along with a host of interesting lesser-known forms. All adult malacostracans have the following features in common: a) the head bears five pairs of appendages (antennules, antennae, mandibles, maxillules, and maxillae); b) the body is composed of well-differen tiated thoracic and ab­ dominal parts; c) the thorax consists of eight somites; d) the abdomen consists of six (rarely seven) somites and telson; e) the male genital aperture is located on the eighth thoracic somite whereas the female genital aperture is on the sixth thoracic somite; f) the pereopod endopod is developed into the walking leg; and g) the digestive tract is distinctive in that the posterior part of the foregut is differentiated into anterior chewing and posterior filtering parts.