Early oysters and their supposed Permian ancestors

The origin of oysters is a much debated palaeontological issue. Recent morphological studies indicate that oysters are characterized by a particular ribbing pattern, the so-called antimarginal ribs which are characterized by a proper morphogenetic pattern. Therefore antimarginal ribs can be used as a diagnostic character in phylogeny. The earliest recognized bivalve displaying diagnostic features of oysters (sinistral attachment to the substrate, typical hinge and ligament structure, and antimarginal ribs) is Ostracites cristadifformis Schlotheim, 1820 first appearing in the Anisian of Europe. This common species was later attributed to Enantiostreon and is here transferred to Umbrostrea. Similar, but dextrally attached forms are found in the Permian. The best represented one is Prospondylus acinetus (Prospondylidae), which retains an initial byssate stage, but is characterized by clearly antimarginal ribs. Some enigmatic forms appearing in the Late Permian are here classed as Prospondylidae gen. indet. These bivalves are characterized by having been cemented since earliest postlarval stage and by antimarginal ribs, therefore being morphologically intermediate between P. acinetus and Umbrostrea cristadifformis. We propose an evolutionary line P. acinetus–Prospondylidae gen. indet.–U. cristadifformis leading to early oysters.