An overview of hymenopteran cocoons as a tool to interpret ichnospecies of Fictovichnus (Pallichnidae) and other fossil cocoons of wasps

Abstract

Hymenopteran cocoons are complex structures constructed from silk by larvae and exhibit a wide range of morphologies, compositions, and textures. The recognition of the most relevant characters of modern cocoons is important for the accurate identification of trace fossils attributed to wasps, which are included in the ichnogenus Fictovichnus. Characters assessed in this study are length, equatorial diameter, diameters near the extremes, shape, color, texture, surface morphology, and occurrence (isolated or clustered). We mapped these characters onto a hymenopteran phylogeny, revealing that the distribution of most of them has no evident phylogenetic signal. In many cases, there is more than one character state in a single family, whereas others appear distributed among several groups. Ellipsoidal and ovoid cocoons, showing membranous texture are the most basal and common characters. Bilobated, subconical and fusiform shapes, clustering, nipple and pores seem to be autapomorphies for certain groups. Crabronidae, Pompilidae, Scolioidea and Thynnoidea construct hard coriaceous cocoons, which may show distinctive surface morphologies and would have the highest preservation potential in paleosols. Data presented herein show that both Fictovichnus sciuttoi and Fictovichnus aragon were correctly attributed to Crabronidae or Pompilidae, although Scolioidea and Thynnoidea cannot be ruled out because of the shape and coriaceous texture of their cocoons. According to the low phylogenetic signal of Hymenoptera cocoons found herein, it would be impossible to refine the affinities of these ichnospecies to particular taxa. The simple morphology of Fictovichnus gobiensis precludes a definitive attribution, either to wasps or to coleopterans, even after the new data presented herein.