Paleoecological utility of insect trace fossils in dinosaur nesting sites of the Two Medicine Formation (Campanian), Choteau, Montana

Abstract

Insect trace fossils, such as burrows, pupation chambers and nests, can provide broad paleoecological insights by helping to define paleohydrology, effects of seasonality or conditions of associated paleosols. Insect traces adjacent to nesting sites of the dinosaur Troodon formosus in the Cretaceous (Campanian) Two Medicine Formation near Choteau, Montana, demonstrate such paleoecological utility. One outcrop in particular contains an abundance of insect burrows and pupation chambers in a calcareous paleosol. Most trace fossils are interpreted as apocritan (wasps and bees) burrows, brooding chambers and cocoons. Apocritans prefer to construct burrows and brooding chambers in well-drained soils during relatively dry conditions (avoiding wet seasons). Their trace fossils are consistent with previous inferences of semi-arid conditions and seasonality for the Two Medicine Formation. Moreover, apocritan nesting is likely to have occurred in the same places and conditions as dinosaur nests: well above the local water table and during dry seasons. Such trace fossils hold the potential for more precise definitions of paleoecological factors in dinosaur nest sites. For example, within the Two Medicine Formation, the Celliforma ichnofacies is commonly associated with eggs of T. formosus and Continuoolithus, but not with those of Maiasaura peeblesorum, perhaps indicative of subtle nesting site preferences.

Keywords:

• Hymenoptera
• wasps
• ichnology
• cocoon
• Mesozoic
• trace fossils

Acknowledgements

We thank J. Horner of the Museum of the Rockies for his support, as well as Pete Polette (of PPP fame), Pat Druckenmiller and various field crews of Camp Makela Paleontology Field School for helping Varricchio with the beginnings of this research in the 1990s. Frankie Jackson (Montana State University), Jason Moore (Texas A&M University), Yoshi Katsura (Gifu Prefecture Museum) and Ashley Poust (Montana State University) gave Martin valuable feedback and assistance in the field; Jackson also reviewed the preliminary manuscript and provided much helpful feedback. We appreciate Kalmar Stevenson for taking measurements of the pupae and Darrin Strosnider for preparing thin sections. We also thank Jorge Genise (Museo de Paleontológica, Trelew, Argentina) for his discussions with Martin on preliminary results of the research and his thorough review of the manuscript. Andrew K. Rindsberg (The University of West Alabama) provided advice on ichnotaxonomy. Ruth Schowalter and Jing Varricchio are appreciated for their patient support of their respective spouses during both fieldwork and manuscript preparation. Research was in part supported by NSF grant #0847777 (EAR) to D. Varricchio.