Paper No. 6
Presentation Time: 9:30 AM
THE SEARCH FOR ROOTS: PALAEOPEMPHIX IS NOT A DECAPOD
The rootstock of the decapod Crustacea is nested within the Paleozoic Malacostraca; however, the ancestry is not clearly understood. Several taxa of Permian or Permo-Triassic age have been assigned to the Decapoda must be evaluated when seeking to elucidate the origins of this major class of crustaceans. Palaeopemphix Gemmellaro,1890, was collected from allochthonous blocks of limestone bearing Fusulina s. l. in the valley of the Sosio River, near Palermo, Italy. The genus, to which three species were assigned, was interpreted to be a macruran decapod based upon possession of transverse grooves developed on an elongate carapace and smooth, curved abdominal somites with pointed pleural terminations. Gemmellaro implied that Palaeopemphix was comparable to species within the Pemphicidae and the Mecochiridae; however, Glaessner (1969) concluded that, although its affinities were unknown, Palaeopemphix was probably not a decapod at all. Re-examination of the type material of the three species of Palaeopemphix demonstrates that the genus is characterized by a bivalved carapace, probably with an intercalated rostral plate, weak hingement between left and right valves, a well developed doublure along the ventral margin, a prominent elongate swelling in the anteroventral portion of the carapace, and a groove pattern of two anteroventrally directed grooves and one posteroventally directed groove. Variation of development of the grooves is the primary basis for recognition of species. This combination of characters is decidedly unlike the morphology of known decapod crustaceans and more closely resembles the attributes of the Phyllocarida, a diverse group of malacostracous crustaceans known from Cambrian to Recent occurrences. Within the Phyllocarida, Palaeopemphix appears to represent a new suborder. Thus, the search for the roots of the Decapoda lies elsewhere.