Palaeoecology, Preservation and Taxonomy of Encrusting Ctenostome Bryozoans Inhabiting Ammonite Body Chambers in the Late Cretaceous Pierre Shale of Wyoming and South Dakota, USA

The straight ammonite Baculites is locally abundant in the Pierre Shale deposited across the Western Interior Seaway (WIS) of North America during the Late Cretaceous (Campanian-Maastrichtian). Fossils of Baculites are commonly preserved with body chambers filled by fine-grained sediment. Removal of the ammonite shell occasionally exposes the undersurfaces of runner-like bryozoan colonies embedded within this sediment. These bryozoans encrusted inner shell surfaces of empty body chambers of ammonites that apparently floated for some interval after death, before dropping to the seafloor where they were grazed by chitons, crustaceans and limpets and further encrusted by brachiopods and other bryozoans. SEM studies show that the runner-like bryozoans, previously identified in the literature as ‘pyriporoid’ or ‘pyriporid’ cheilostomes, are in fact ctenostomes belonging to a new genus and species, Pierrella larsoni. The ctenostome identity of these Pierre Shale bryozoans is evident from not only distorted zooid shapes and the lack of a calcareous skeleton, but also the remarkable preservation of some zooids with setigerous, or pleated, collars. The mode of preservation of these ctenostomes is enigmatic, although epibiont shadowing due to dissolution of surrounding shell played a role in most instances. As there is only minor overgrowth by organisms with mineralized skeletons, it is clear that the colonies are not preserved by bioimmuration. Most or all of the encrusted ammonites are concretionary so it is possible that rapid growth of authigenic calcite, which has been shown to be present by laser Raman spectroscopy, preserved the ctenostome zooids through a process of lithoimmuration before complete decay of their body walls.