Dinosaur tracks from the Frontier Formation, Montana: preservation, distribution and palaeoecological significance for the middle Cretaceous terrestrial ecosystems of North America

ABSTRACT

The Coniacian-Santonian stages in western North America are characterized by a sparse fossil record. We present here the first account of dinosaur tracks from nine sites in the Frontier Formation (Coniacian-Santonian) of southwestern Montana. Tracks are largely preserved in distal alluvial facies as sandstone casts, with a single example of shallow epirelief impression interpreted as undertrack. Sandstone casts show significant relief, kinematic features (i.e., scale marks), and variable morphologies, arguing for a strong substrate control on their preservation. Putative producers are assigned to ornithopod, ankylosaurian, and theropod dinosaurs in decreasing order of abundance. This record reflects a composition similar to those of other middle Cretaceous formations in North America. Moreover, ankylosaurian tracks of the Frontier Formation represent the first known from Coniacian strata in North America. The incorporation of body and trace fossils yielded by the Frontier Formation indicates the presence of a fauna with North American endemic elements. The establishment of Coniacian-Santonian dinosaurian palaeocommunities akin to those observed in Campanian and Maastrichtian formations may reflect the trend towards provincialism attested for the end of the Mesozoic in North America.

KEYWORDS:

• Dinosaur tracks
• Frontier Formation
• middle Cretaceous
• North America

Acknowledgments

We wish to thank S. Williams, J. Scannella, and E. Metz for their support in the field and for curating and providing us with access to the fossil material at the Museum of the Rockies. We are also indebted to, W. Freimuth, I. Aguilar, J. Wilson, S. Choi, A. Poust, and the 2019 and 2021 field crews for their help in prospecting and collecting dinosaur tracks. Finally, this research could not have been possible without the assistance of the staff of US Forest Service and Bureau of Land Management that provided assistance and accesses to the areas studied in this paper.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/08912963.2023.2184692.