An Alternative Phylogeny of Peracarid Crustaceans

Abstract

In order to properly understand the origins of the Order Amphipoda, their phylogenetic position within the Superorder Peracarida must 1st be established. A hypothetical ancestral peracarid is derived from Dahl's hypothetical ancestral eumalacostracan, which was proposed as lacking a carapace. The independent derivation of a carapace for special needs on at least 2 occasions is a central feature of a new phylogenic arrangement for the peracaridan orders. This phylogeny is based on an analysis of 29 character states using Hennigian methods. Two lineages are immediately apparent: a heterogeneous line containing the Amphipoda, Mysidacea and Thermobaenacea; and 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; this new arrangement is more in accord with the known fossil record. Future examinations of the phylogeny of peracarid orders should be based on a scheme for the Superorder, enabling plesiomorphic features of individual orders to be properly defined. The polyphyletic nature of the Peracarida is discussed; at least 3 independent lines of peracaridan radiation might have arisen from a pre-Carboniferous syncarid-like ancestral eumalacostracan.