The oldest fossil record of a living true eel lineage (Protanguillidae, Anguilliformes) from Mexico reveals the gradual acquisition of modern eel traits

We present an integrative taxonomic study describing the first fossil representative of the family Protanguillidae from the Danian deposits near Palenque, Chiapas, Mexico. Osteological analysis and parsimony-based phylogenetic reconstruction using morphological characters support the designation of a new genus and species as a protanguillid by the presence of several features, including the autogenous premaxillae, the presence of metapterygoid and symplectic bones, the participation of pterosphenoid on the posterior margin of orbital margin, the reduced number of vertebrae, and the configuration of hypural complex. The new taxon differs from Protanguilla palau by the shape of last branchiostegal ray, the disposition of anterodorsal branch of subopercle, and the insertion of unpaired fins. To investigate the evolutionary history of true eels, we conducted a total-evidence tip-dating analysis combining 14 mitochondrial genes with a morphological matrix, incorporating both Cretaceous and Cenozoic fossil taxa. The results support a gradual acquisition of the diagnostic characters of Anguilliformes throughout the Cretaceous. A Bayesian topological test further supports the species here described as a surviving lineage of Cretaceous eels with {dagger}Libanechelyidae bultyncki, closely related to the modern eels. This discovery expands the paleobiogeographic range of the Protoanguillidae and provides new insight into the origin and diversification of true eels.