Encrusting Sclerobiont Paleoecology and Bioerosion Of Oysters in the Type Campanian (Upper Cretaceous) of Southwestern France

The Campanian Stage of the Upper Cretaceous was established by Henri Coquand in 1857 based on a sequence of richly fossiliferous shallow water carbonates in the Charente and Charente-Maritime departments of southwestern France. One of the most common macrofossils is the gryphaeid oyster Pycnodonte vesicularis (Lamarck, 1806), which often forms extensive shell beds. This bivalve lived primarily on soft marly substrates, forming hard substrate islands. They frequently supported sclerobiont communities comprising encrusters (diverse cheilostome and cyclostome bryozoans, foraminiferans, oysters, bivalves, sabellid and serpulid polychaetes, calcareous sponges), borers (the sponge borings Entobia, the worm borings Maeandropolydora and Caulostrepsis, the barnacle borings Rogerella, the phoronid borings Talpina, the predatory borings Oichnus), the ballistic crustacean trace Belichnus, and grazers (Gnathichnus and Radulichnus). Collections of Pycnodonte vesicularis from the Late Campanian Biron, Barbezieux and Aubeterre formations (in ascending stratigraphic order) were assembled to study the systematics and paleoecology of the sclerobionts, and describe the bioerosion ichnofauna. These chalky marls record a sequence from deeper to shallower shelf environments. P. vesicularis shells from the deeper-water Biron Formation are relatively large and complete, with their encrusting fauna mostly intact and on exterior surfaces, suggesting rapid burial. Shells from the overlying shallower-water and highly bioturbated Barbezieux and Aubeterre formations are typically heavily bioeroded with fragmentary encrusters, pointing to a complex history of colonization on shell exteriors and interiors. The diversity of sclerobionts increases upwards as the depositional environments shallowed, especially for the bryozoans. Part of this diversity increase may be because of the longer seafloor residence time of the shallower shells, and part may be due to the growing surficial complexity of the bioeroded shell substrates, but most of this diversity increase appears to reflect a rising biological productivity with the shallowing seas. The bioerosion ichnodiversity increases stratigraphically upwards with the shallowing paleoenvironments