INSECTS USING ONE ANOTHER DURING THE LATE CRETACEOUS: TRACE FOSSIL EVIDENCE OF PARASITOID (OR WAS IT CLEPTOPARASITE?) BEHAVIOR IN THE TWO MEDICINE FORMATION, MONTANA

Parasitoids resemble parasites by devoting part of their life cycle in or on host organisms, but they normally kill their hosts. This behavior is exceedingly common in insects, particularly ichneumonid wasps. However, the timing of parasitoid evolution in wasps or other insects is still uncertain. For instance, ichneumonid wasps date to the Middle Jurassic, but whether these insects or their immediate descendants were parasitoids is unknown. Fortunately, trace fossils give additional clues of parasitoid presence and establish minimum times for when this or similar behaviors evolved. In our study we found persuasive ichnological evidence of parasitoids in the Two Medicine Formation (Campanian) of Montana. These trace fossils were in multiple outcrops of calcareous paleosols in the Willow Creek Anticline, a place better known for its dinosaur eggs and nests. Suspected parasitoid trace fossils consist of small cocoons inside larger ones, smaller-diameter holes in apices of larger cocoons, and smaller-diameter burrows connecting to larger cocoons and brooding chambers. Larger “host” cocoons are within a narrow size range (19-23 mm long, 10-12 mm wide), are identifiable as Rebuffoichnus isp., and were likely made by apocritans (wasps). Smaller, enclosed cocoons are also regularly sized (5-7 mm long, 3-4 mm wide). In one sample, host:enclosed cocoon volumes had a 11.7:1 ratio (n = 18). Disparate sizes but predictable proportional differences in cocoons resemble those of modern parasitoid wasps, which are often smaller than their host insects. Holes in cocoons and small burrows are analogous to those made by parasitoids exiting cocoons. Alternative explanations for our observations are inquilines (“squatters”) that used spaces provided by larger cocoons for pupation, or cleptoparasitism, in which smaller insects stole food resources in brooding cells. Regardless, these trace fossils supply tantalizing evidence of insect-on-insect parasitoid behavior in the Mesozoic.