A new species of lamprey from Cretaceous semisaline environment in China

A new fossil species of lamprey, Caeruleum gracilis sp. nov. is described from the Early Cretaceous Jiufotang Formation in China Hebei province, North China. Caeruleum gracilis sp. nov. is characterised by a slender body shape and a dorsal fin originating above the second branchial pouch. Given the intricate life history of lampreys, their presence in saltwater and freshwater, and their convoluted and ambiguous evolutionary trajectory, it is imperative to investigate whether the living environment of fossil lamprey species encompassed varying levels of salinity. This inquiry will facilitate our understanding and tracing of the earliest adaptations of lampreys to freshwater and saltwater environments. Based on salinity analysis, the environment of the Jiufotang Formation may be attributed to brackish conditions. Therefore, the lampreys Caeruleum gracilis sp. nov. and Caeruleum miraculum from the Jiufotang Formation may represent the one of earliest lampreys records that acquired adaptations to freshwater environments.