REEVALUATION OF THE BRACHYURAN (CRUSTACEA, DECAPODA) BAUPLAN

The traditional view of brachyuran (Crustacea: Decapoda) morphology is that only a few morphotypes of the theoretically available possibilities are occupied. This view was first codified in the hypothesis of Guinot (1977) that crabs were separated into three sections, the podotremes, heterotremes, and thoracotremes, depending on the nature and position of the genital openings. Later work expanded this single character classification to include aspects of the abdomen and the nature of the pereiopods, among other features. This scheme has been largely retained ever since as paleontologists and neontologists have placed new taxa into those three groups, albeit with some modifications. Discovery of new fossil material has shown that there is considerable variability within the podotreme groups, the most primitive crabs, and the heterotremes, which are more derived crabs. Several new forms from the Cretaceous cannot be placed in the conventional bauplan of existing sections within the Brachyura and call into question the relatively conservative view of brachyuran radiation presently held. Among the early groups of crabs, more morphotypes existed than has previously been thought, thus indicating that the crabs may have experienced bursts of radiation into a variety of forms during the Mesozoic before settling into a smaller number of forms later. Drivers for this radiation are not known but may include widespread coral reef habitat in the Late Jurassic, various competitive biotic events during the Jurassic and Cretaceous, and high sea levels during the Cretaceous. In addition, this pattern of early radiation into a diversity of forms before settling into a more limited number of body plans seems to be common in the Arthropoda at the subphylum (class) and other supra-ordinal levels; it may be being repeated at the subordinal level within the Brachyura. Research supported by NSF grant EF-0531670 to Feldmann and Schweitzer.