INSECT TRACE FOSSILS AND THEIR PALEONTOLOGICAL IMPORTANCE IN THE TWO MEDICINE FORMATION (CAMPANIAN), CHOTEAU, MONTANA

Late Cretaceous pupation chambers, cocoons, and burrows attributable to wasps, bees, and beetles are preserved in calcareous paleosols of Argentina, Uruguay, Mongolia, and North America. Such trace fossils and their paleosols provide important insights on local paleohydrology, vegetative cover, tropic relationships, and seasonality in Cretaceous ecosystems. When associated with non-avian dinosaur nesting sites, insect trace fossils also may give clues about dinosaur nesting preferences. Our study follows previous reports of insect trace fossils in calcareous paleosols near dinosaur nesting sites of the Two Medicine Formation (Campanian) near Choteau, Montana. Paleosols and their trace fossils are in the paleontologically classic Willow Creek Anticline, known for its abundant dinosaur nests. We examined burrows, brooding chambers, and cocoons collected from several localities across this area. After measuring and describing these trace fossils, we determined their ichnotaxa and evaluated their paleoecological relationship to host paleosols. Most insect trace fossils are prolate ellipsoid cocoons with helical ridges, identified as Rebuffoichnus sciuttoi and attributed to burrowing wasps. Cocoons are 21.4 ± 2.8 mm long, 11.4 ± 1.4 mm wide, with a mean volume of 1558 ± 595 mm³ (n=162). Cocoons are entire or truncated; some of the latter imply successful emergence of adult insects. A few cocoons are joined to small-diameter burrows, suggesting the presence of parasitoids. The abundance of insect-nesting trace fossils and their recurrence in calcareous paleosols at multiple sites point toward well-drained soils near fluvial or lake floodplains. These traits typify the Celliforma ichnofacies, which is linked to mostly arid terrestrial environments with seasonal flooding. Anastomosing rivers and freshwater lakes deposited the middle subfacies of the Willow Creek Anticline; the Celliforma ichnofacies suggests this interval of lacustrine deposition was punctuated by seasonal droughts. Troodon nesting sites and eggshells (ootaxon Continuoolithus) are often associated with the Celliforma ichnofacies. One occurrence of the ootaxon Spheroolithus documented above these horizons signifies subtle nesting preferences and possible niche partitioning between Troodon and Maiasaura.