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

Cretaceous Ctenostome Bryozoans of Wyoming and South Dakota
Part of the Lecture Notes in Earth System Sciences book series (LNESS, volume 143)


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.


Ctenostome bryozoans Cretaceous Western Interior Seaway Lithoimmuration 



We are grateful to Neal Larson (Black Hills Institute of Geological Research) for arranging fieldwork in South Dakota and allowing us to borrow specimens from his collection. John Sime (The College of Wooster) assisted during fieldwork and searched for material in the USGS collections, while Luke Larson, Jamie Brezina and Mike Ross also facilitated fieldwork. Bill Schopf and Anatoliy Kudryavtsev provided access to the laser Raman spectroscope in the Department of Earth and Planetary Sciences, UCLA. This work was also supported in part by the Luce and Wengerd Funds at The College of Wooster.


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© Springer-Verlag Berlin Heidelberg 2013

Authors and Affiliations

  1. 1.Department of GeologyThe College of WoosterWoosterUSA
  2. 2.Department of Earth SciencesNatural History MuseumLondonUK

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