Primary phosphate mineralization in mandibular elements of middle paleozoic archaeostracans from Spain

Abstract Archaeostracans are an early fossil crustacean group linked to malacostracans. The earliest archaeostracans are late Cambrian, but the group diversified mostly in the Silurian and Devonian. Complete specimens are known from konservat-lagerstätten, but loose mandibular elements are relatively abundant in decalcified, acid resistant residues of middle Palaeozoic carbonates. These mandibular elements have been assumed to be originally heavily sclerotized and secondarily phosphatized, but have received little attention in the scientific literature. In the present study, we describe isolated mandibular elements, gnathal lobes (likely belonging to a single archaeostracan species), from the middle Palaeozoic of the Cordoba Province, Spain, and provide evidence that they were originally mineralized with calcium phosphate. Our results are in accordance with recent evidence that malacostracans use calcium phosphate to strengthen mechanically challenged areas of their cuticle, particularly the wear prone mandibular surfaces, and support the hypothesis of a dual calcium phosphate/carbonate mineralization system evolving early in arthropods.