Probable insect cocoon or pupation chamber in a channel‐fill sandstone bed of the Lower Cretaceous Jinju Formation, South Korea

摘要

> An ellipsoidal chamber which is composed of a very thin organic layer and emplaced in an enlargement continuing upwards into a vertical burrow is found in thin sections of a fluvial channel‐fill sandstone bed of the Lower Cretaceous Jinju Formation in South Korea. Micromorphological analysis demonstrates that the ellipsoidal chamber is attributable to Fictovichnus . It can be interpreted as a free‐standing wall of a wasp cocoon attributable to Fictovichnus sciuttoi or Fictovichnus aragon , or an organic lining of chafer beetle or weevil pupation chamber attributable to Fictovichnus gobiensis . The analysis further shows that the vertical burrow and enlargement appear to reflect chafer and weevil larval behaviour, but the arrangement of the chamber within the enlargement appears to be the same as a wasp cocoon inside a cell. Such uncertainties of ichnotaxonomic assignment and putative producer suggest that the Jinju specimen may reflect an aberrant pupation behaviour of coleopterans or hymenopterans. This Early Cretaceous specimen may be the earliest record of an insect pupation structure probably associated with a burrow. Its atypical morphologies and very rare occurrence in a channel sandstone bed without other evidence of pedogenesis suggest that the Jinju specimen was constructed in incipiently and briefly exposed sediment, unlike typical insect nesting structures constructed in soil.