Article: Isopod trackways from the Crayssac Lagerstatte, Upper Jurassic, France
Christian Gaillard, Pierre Hantzpergue, Jean Vannier, Anne-Laurence Margerard and Jean-Michel Mazin
Well-preserved arthropod trackways are described from the laminated limestones of the Crayssac Lagerstatte (south-west France, Lower Tithonian). They occur in sediments deposited in the temporary coastal mudflats of intertidal to supratidal zones. The trackways are referred to Pterichnus isopodicus isp. nov., and are interpreted as the locomotion traces of isopods. Different trackway morphotypes are recognized and clearly resulted from variations in the original consistency of the sediment. Sinuous trackways may correspond to vagrant activity on wet mud whereas numerous straight ones indicate a more rapid crawling on a soft-to-firm substratum (e.g. tidal flat during emersion). The preferred orientation of trackways indicates that isopods were crawling in a direction perpendicular to shoreline as a result of possible taxis induced by sediment wetness and/or by a migratory behaviour controlled by tidal rhythm. Unusually long emergence of the sediments may have favoured the preservation of dense networks of trackways. An isopod identity is supported by the general morphology of the tracks and the association of trackways with isopod body fossils. Archaeoniscus, which occurs abundantly in Late Jurassic deposits of England and France, was probably the trace-maker.