The euthycarcinoid arthropods from Montceau-les-Mines, France: functional morphology and affinities

Abstract : New three-dimensionally preserved specimens of two euthycarcinoid arthropods, namely Schramixerxes gerem and Sottyxerxes multiplex, allow complete description of both the dorsal and ventral sides of the exoskeleton. The functional morphology is tentatively interpreted for the first time. In S. gerem, the ‘thirteenth somite', or ‘monosomite' is fully described and re-interpreted as the main articulatory process of the body, between the cephalic region and the preabdomen. The morphology and arrangement of the two parts of the process clearly indicate that the anterior cephalic region of the body could move laterally and bend ventrally, while posterior somites could only move ventrally. Unlike several other euthycarcinoid species, the ventral side of the head area exhibits one or two plates instead of mandibles; such distinct morphologies are indicative of different feeding mechanisms and behaviours among euthycarcinoid arthropods. Possible homologies with the labrum of Hexapoda support the hypothesis that euthycarcinoids have hexapod affinities.