Euarthropods from the Early Ordovician Cabrières Biota, including a new aglaspidid and freshly moulted trilobite material

Early Ordovician fossil lagerstätten provide insight into the transition between the Cambrian Explosion and Ordovician Radiation biodiversification events, with euarthropods typically being important and abundant constituents of these animal communities. One such lagerstätte is the late Floian Cabrières Biota of France, where euarthropods described to date include asaphid, raphiophorid, and phacopid trilobites, as well as phyllocarid crustaceans and rare specimens of chelicerates possibly aligned to Chasmataspidida. In this paper, we re-evaluate the non-trilobite euarthropods of the Cabrières Biota, describe freshly-moulted trilobite material, and introduce a probable Aglaspidida from this locality. This aglaspidid is distinct from other members of the family Tremaglaspididae, and reveals a palaeobiogeographical trend in Ordovician aglaspidids that were restricted to cold, polar environments. Trace fossils preserved on the aglaspidid carcass provide data on taphonomic processes at the site. Further contextual information useful for interpreting Cabrières Biota euarthropod material is the presence of multiple specimens of flattened and wrinkled trilobite pygidia, interpreted as having been preserved shortly after moulting before remineralisation was complete. Serial wrinkles in such specimens can resemble the segmentation of euarthropod bodies, and one specimen previously described as a possible chelicerate may actually be a freshly moulted trilobite fragment. Finally, specimens previously described as phyllocarids are revealed to be shells of the rostroconch mollusc Ribeiria sp., indicating that phyllocarids were not present at this locality. The Cabrières Biota euarthropod assemblage currently consists of trilobites, an aglaspidid, a possible chasmataspidid, and appendage fragments.