TRACE FOSSIL EVIDENCE FOR INSECT-ON-INSECT PARASITOID BEHAVIOR IN THE UPPER CRETACEOUS TWO MEDICINE FORMATION OF CHOTEAU, MONTANA

Parasitoid behavior in modern insect species is diverse but most often expressed through larval forms of holometabolous insects that consume and kill another holometabolous insect. Of modern insects, many hymenopterans are parasitoids; most of these species lay their eggs on larvae of other species and in some situations bury these larvae in brooding chambers as a form of provisioning. Other parasitoids lay eggs on cocoons in brooding chambers, and some parasitoids will burrow into a brooding chamber to oviposit on a pupa. Well-fed parasitoids later emerge from pupal cases or brooding chambers as adults, leaving exit holes in pupal cases or burrows in the surrounding sediment. Consequently, a combination of body and trace fossil evidence can be used to construct hypotheses of when parasitoid behavior happened in the geologic past, a behavior that is currently poorly documented. For example, diversification of hymenopterans during the Jurassic implies that parasitoid behavior evolved in some of their lineages then, but otherwise no other evidence supports such an assertion. Persuasive examples of Late Cretaceous insect-on-insect parasitoid behavior are provided by trace fossils associated with pupal cases from the Two Medicine Formation (Campanian) of Choteau, Montana. Pupal cases are prolate ellipsoids with a mean length and width of 20.7 and 10.4 mm, respectively (n=336) and preserved as steinkerns. Pupal cases are commonly within vertical burrows, a co-occurrence that is consistent with hymenopteran brooding chambers. However, noticeably smaller-diameter (2-3 mm) burrows also connect with or emanate from a few pupal cases. These trace fossils indicate former habitation of the pupal cases, but the striking size difference between the burrows and cases show that burrows were made by a different species of insect. Because modern parasitoids are typically smaller than their hosts, a parasitoid insect is a possible tracemaker for the burrows. The shear abundance of pupal cases in the Two Medicine also suggests formerly bountiful opportunities for host selection by insect parasitoids. Similar use of both body and trace fossil evidence will hopefully aid researchers in future investigations of similar occurrences of parasitoid behavior and its evolutionary development in insects.