Crustacea Phyllopoda and Malacostraca: a reappraisal of cephalic and thoracic shield and fold systems and their evolutionary significance

Abstract

In 1909 Caiman presented the carapace hypothesis, according to which all Crustacea possess or once possessed a carapace fold, formed from the posterior margin of the cephalon. In those Crustacea which have no such free fold but a cephalothoracic shield covering the cephalon and part or the whole of the thorax, this shield was interpreted as a carapace fold fused to the dorsum of the thorax and often overhanging its sides, for example, in lobsters and natantians. When a carapace fold or carapace shield is missing, as, for example, in Anostraca or Syncarida and behind the maxilliped segment in amphipods and isopods, this has been understood to imply that a carapace fold or shield has been lost. The carapace hypothesis has been universally accepted and has influenced the interpretation of Crustacean evolution and inter-relationships. A re-examination of numerous Crustacea, including embryos and larvae, with the application of a variety of histological standard techniques, proved that no cephalic carapace fold is formed in any malacostracan presumed to possess it, or in any notostracan, spinicaudate or laevicaudate branchiopod examined. In the Branchiopoda, when a dorsal fold is formed, it is always attached to a trunk segment. Free dorsal folds in the Malacostraca are always attached to the posterior margin of a thoracic segment, and dorsal cephalothoracic shields are never formed as a result of the fusion of a cephalic carapace fold to the dorsum of the thorax. Instead, the formation of a cephalothoracic shield in the Malacostraca is always due: (i) to the fusion of the 1st thoracic segment to the cephalon in connection with the differentiation of maxillipeds; and (ii) to the obliteration of lateral and dorsal external segment borders in the whole or part of the dorsum of the thorax apparently because of the formation of continuous and unsegmented branchiostegal folds. These folds are always continuous with those of the maxilliped segment and the cephalon. In the Anaspidacea, Amphipoda and Isopoda, which have no branchiostegal folds behind the maxilliped segment, the external segmentation is retained in all segments behind it, and the same is the case with the segments behind the cephalothoracic shield in, for example, mysids and cumaceans. The rule that the number of thoracic malacostracan segments covered by a cephalothoracic shield is identical to the number of segments contributing to the formation of branchiostegal folds has no exceptions. In the light of the failure of the carapace hypothesis, the retention of an external thoracic segmentation represents a plesiomorphic condition and the formation of an only partly segmented or an unsegmented thorax a derived one. The Syncarida, with up to eight free thoracic segments in the Palaeocaridacea and Bathynellacea, then appear to reflect an ancestral condition and are possibly not too far removed from malacostracan ancestors. This permits a new approach to eumalacostracan phylogeny and higher systematics. The reason for many misunderstandings concerning dorsal folds and shields appears to be technical. Observations leading to claims that a carapace foldsensuCaiman exists, or existed, in all Crustacea were to a large extent based on studies of wholemounts, often made after maceration in potassium hydroxyde solutions. This leads to the dissolution of soft tissues so that the dorsal integument comes to consist of only a single sheet of cuticle attached anteriorly to the cephalic region. This may easily give the false impression of representing a true carapace fold. An examination of serial sections of the intact cephalothorax reveals the true topographic inter-relationships.