New evidence for the co-occurrence of two genera of Paleoparadoxiidae (Mammalia, Desmostylia) from the Middle Miocene of Japan: insights into taxonomic status and paleodiversity in Desmostylia

Desmostylia, an extinct order of marine mammals, includes two major families: Paleoparadoxiidae and Desmostylidae. Within Paleoparadoxiidae, three genera—Archaeoparadoxia, Paleoparadoxia, and Neoparadoxia—have been identified, with Paleoparadoxia being the only genus found on both coasts of the North Pacific Rim. In Akan, Hokkaido, Japan, one of the largest Paleoparadoxia fossil assemblages in the world has been discovered from the Middle Miocene Tonokita Formation. Previous studies suggested the presence of two congeneric species of Paleoparadoxia, while recent taxonomical revisions raise the possibility that two genera, Paleoparadoxia and Neoparadoxia, were present in Akan. Here, we describe three paleoparadoxiids specimens from Akan, which consist of two partial crania and a mandible. Based on morphological comparisons and phylogenetic analysis, we identified these specimens as Paleoparadoxia sp. and Neoparadoxia sp. This represents the first record of two genera of Paleoparadoxiidae in the same locality and horizon, with the potential to provide valuable insights into cladogenesis and morphological diversification within this family. In addition, the Akan specimens exhibit mosaic characteristics of Paleoparadoxia and Neoparadoxia, suggesting that a reassessment of their morphological features for taxonomic identification and phylogenetic analysis is required. To better understand species-level diversity patterns in Desmostylia, we conducted stage-binned analysis and richness curve analysis. Our analysis revealed three significant points in their diversification history: (1) Desmostylidae reached peak diversity at the Oligocene-Miocene boundary, coinciding with a glacial event; (2) Paleoparadoxiidae achieved peak diversity during the Middle Miocene global warming event; (3) both families declined in diversity and went extinct during the Middle to Late Miocene global cooling event. These findings indicate that desmostylian diversity could have been closely linked to climatic events, with the differing peak diversities of Paleoparadoxiidae and Desmostylidae reflecting their respective preferences for warmer and cooler climates. Our analysis provides a valuable baseline for understanding the diversity and evolutionary history of Desmostylia.