Hidden taxonomic and taphonomic diversity revealed by mayflies (Ephemeroptera: Hexagenitidae) from mid-Cretaceous Kachin amber.

Hexagenitidae is an extinct family of Ephemeroptera with a fossil record spanning from the Early Jurassic to the Early Cretaceous. However, in mid-Cretaceous Kachin amber, one of the most diverse insect-bearing biotas known to date, only a single species, Hexameropsis elongatus Lin et al., has been described, and it is known solely from male adults. In this study, we describe three well-preserved species of Hexagenitidae from Kachin amber, including one new genus and species based on a female imago, Crehkahtihtengia dongi gen. et sp. nov., a new species, Hexameropsis fehrmannorum sp. nov., and a putative representative of Hexameropsis Tshernova & Sinitshenkova, highlighting that the diversity of this family in Kachin amber is greater than previously recognized. Ecologically, Hexagenitidae in Burmese amber likely represents rhithral mayflies associated with running-waters habitats, in contrast to the predominantly lacustrine members of this family known from Jurassic and Cretaceous compression fossils. We conducted a taphonomic analysis of the new mayfly specimens using light microscopy, fluorescence microscopy, scanning electron microscopy (SEM), energy-dispersive and wavelength-dispersive X-ray spectroscopy (EDS and WDX), X-ray micro-computed tomography (Micro-CT), and Raman spectroscopy. Our study provides evidence that clay minerals were involved in the structural preservation of insect fossils in amber. We found the co-preservation of calcite, quartz, pyrite, and hydrous phyllosilicates (likely chamosite/berthierine or other clay minerals) within each individual fossil specimen, in which these phases infilled internal body voids and likely helped to maintain body shapes. This multi-phase assemblage indicates a complex, multi-stage depositional history within a single inclusion and highlights the diversity of mineralization-based preservation pathways in Kachin amber. Together with previous work on the taphonomy of Kachin amber inclusions, these results suggest that mineralization products associated with preservation of biological structures in amber may reflect evolving fluid compositions, fluctuating redox conditions, and dynamic transformations of pore space within the amber and at its interface with the surrounding environment.