First reliable fossil record of the family Isonychiidae (Insecta, Ephemeroptera) from Cretaceous Kachin amber

The mayfly family Isonychiidae is represented by the single extant genus Isonychia Eaton, 1871 and has long lacked a reliable fossil record, limiting insights into its evolutionary history. Here, we describe Fujiwaranychia chiyokoae gen. et sp. nov., a female imago preserved in mid-Cretaceous Kachin amber from northern Myanmar. This specimen represents the first reliable fossil evidence for Isonychiidae and confirms the family's presence in Laurasia during the Cretaceous. The new genus shares several morphological features with extant Isonychia, including forewing venation and a similar tarsal formula, but differs in the absence of cubital cross veins in forewings and the presence of an unforked media posterior vein in the hind wing. These differences justify its placement in a separate, extinct genus within Isonychiidae. The discovery extends the known history of the family by nearly 100 million years and reveals a previously unrecognized degree of morphological diversity. The fossil provides a critical calibration point for future phylogenetic studies and underscores the paleontological value of amber inclusions in tracing the evolutionary history of mayflies.