The controls on the preservation of structural color in fossil insects

The colors of many animals arise from ordered nanometer-scale variations in tissue structure. Such structural colors-especially those with metallic optical effects-are widespread among modern insects but are preserved rarely in insect fossils. This suggests that a specific set of taphonomic circumstances is required for preservation of structural colors. Here we present the results of the first systematic investigation of the controls on the preservation of structurally colored tissues in fossil insects. Approximately 700 specimens of beetle taxa known to exhibit metallic structural colors were studied from seven Lagersttten: Randecker Maar (early Miocene), Clarkia (early Miocene), Enspel (late Oligocene), Florissant (late Eocene), Eckfeld (middle Eocene), Messel (middle Eocene), and Green River (middle Eocene). The quality of preservation of metallic colors varies among, and within, these biotas; colors are well preserved in most specimens from Clarkia, Enspel, Eckfeld, and Messel, but are typically poorly preserved in specimens from Randecker Maar and absent in Florissant and Green River. These differences are independent of taxonomy and the age and depositional context of the biotas. Instead, variation among biotas is attributed to differences in their late diagenetic history, in particular the maximum depth to which sediments were buried and the nature of fluid flow, as well as recent weathering. Variations in the quality of structural color preservation among specimens from individual biotas are independent of precise stratigraphic and sedimentological context. These intrabiota variations usually result from differences in the extent of microbial degradation of the cuticle and of recent weathering, but also the mode of curation of specimens. The last of these has important implications for curatorial practice.