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Isotaphonomy in concept and practice: an exploration of vertebrate microfossil bonebeds in the Upper Cretaceous (Campanian) Judith River Formation, north-central Montana

Published online by Cambridge University Press:  21 February 2017

Raymond R. Rogers ,
Matthew T. Carrano ,
Kristina A. Curry Rogers ,
Magaly Perez  and
Anik K. Regan
Raymond R. Rogers
Affiliation:
Geology Department, Macalester College, 1600 Grand Avenue, Saint Paul, Minnesota 55105, U.S.A. E-mail: rogers@macalester.edu.
Matthew T. Carrano
Affiliation:
Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Post Office Box 37012, Washington, D.C. 20013, U.S.A. E-mail: carranom@si.edu
Kristina A. Curry Rogers
Affiliation:
Geology Department and Biology Department, Macalester College, 1600 Grand Avenue, Saint Paul, Minnesota 55105, U.S.A. E-mail: rogersk@macalester.edu
Magaly Perez
Affiliation:
Geology Department, Macalester College, 1600 Grand Avenue, Saint Paul, Minnesota 55105, U.S.A. E-mail: rogers@macalester.edu.
Anik K. Regan
Affiliation:
Geology Department, Macalester College, 1600 Grand Avenue, Saint Paul, Minnesota 55105, U.S.A. E-mail: rogers@macalester.edu.
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Abstract

Vertebrate microfossil bonebeds (VMBs)—localized concentrations of small resilient vertebrate hard parts—are commonly studied to recover otherwise rarely found small-bodied taxa, and to document relative taxonomic abundance and species richness in ancient vertebrate communities. Analyses of taphonomic comparability among VMBs have often found significant differences in size and shape distributions, and thus considered them to be non-isotaphonomic. Such outcomes of “strict” statistical tests of isotaphonomy suggest discouraging limits on the potential for broad, comparative paleoecological reconstruction using VMBs. Yet it is not surprising that sensitive statistical tests highlight variations among VMB sites, especially given the general lack of clarity with regard to the definition of “strict” isotaphonomic comparability. We rigorously sampled and compared six VMB localities representing two distinct paleoenvironments (channel and pond/lake) of the Upper Cretaceous Judith River Formation to evaluate biases related to sampling strategies and depositional context. Few defining distinctions in bioclast size and shape are evident in surface collections, and most site-to-site comparisons of sieved collections are indistinguishable (p≤0.003). These results provide a strong case for taphonomic equivalence among the majority of Judith River VMBs, and bode well for future studies of paleoecology, particularly in relation to investigations of faunal membership and community structure in Late Cretaceous wetland ecosystems. The taphonomic comparability of pond/lake and channel-hosted VMBs in the Judith River Formation is also consistent with a formative model that contends that channel-hosted VMBs were reworked from pre-existing pond/lake assemblages, and thus share taphonomic history.

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Articles
Information
Paleobiology , Volume 43 , Issue 2 , May 2017 , pp. 248 - 273
Copyright
Copyright © 2017 The Paleontological Society. All rights reserved 

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