Taphonomy and taxonomy of a vertebrate microsite in the mid-Cretaceous (Albian–Cenomanian) Blackleaf Formation, southwest Montana

Abstract

The vertebrate fauna of the Cretaceous Blackleaf Formation of southwest Montana remains largely undocumented. A microsite (BL1) discovered in the Flood Member in the Lima Peaks area, Montana, consists of a green siltstone and yields taxa previously unreported from the formation, including several dinosaurs: a hypsilophodont, dromaeosaurid, tyrannosauroid, hadrosaurid and an ankylosaurian. Non-dinosaurian taxa include goniopholidid and Bernissartia crocodilians; Glyptops, cf. chelydrid and other turtles and at least two neopterygiian fish. This diversity corresponds well with the fluvial–deltaic–estuarine environment interpreted for the uppermost unit of the Flood Member. Taphonomic data and sedimentologic relationships suggest that this assemblage represents a floodplain depression accumulation. Comparisons with contemporaneous faunas from around the Western Interior of the USA suggest a remarkably consistent faunal makeup, at least at the family level, existed across western North America in the mid-Cretaceous.

Keywords:

• Cretaceous
• Blackleaf Formation
• chelydrid
• taphonomy
• dinosaur
• microsite

Acknowledgements

We thank Montana State University's Earth Science Department for provision of binocular microscopes, screens and storage of specimens. Thanks also to Drs. Frankie and Bob Jackson for their support and help in field work, Yoshihiro Katsura, graduate students Joey Stiegler, Ben Scherzer and Ashely Poust for their assistance in collection and screening of sediment, and Tobin Hiernonymous and Jason Moore for early fieldwork efforts. We also thank the Beaverhead-Deerlodge National Forest for permission to conduct paleontology research on USFS lands. Our reviewers contributed greatly to improving earlier drafts of this manuscript and aided in identification of the vertebral centrum discussed.