A new Monodontidae (Cetacea, Delphinoidea) from the lower Pliocene of Italy supports a warm-water origin for narwhals and white whales

A new taxon of monodontid cetacean, Casatia thermophila, gen. et sp. nov., is here described on the basis of a partial skull from lower Pliocene (5.1–4.5 Ma) marginal-marine deposits of Tuscany (central Italy). This new taxon belongs to Monodontidae based on the presence of a medial exposure of the maxillae anterior and lateral to the external bony nares; it mainly differs from all other named monodontids by the presence of a median depression of the premaxillae anterior to the premaxillary sac fossae and by a medial margin of the premaxillary-maxillary suture that does not parallel the anterolateral profile of the external bony nares. Our phylogenetic analysis, the first including all taxa of Monodontidae, recovers Casatia as a crown monodontid, more closely related to Delphinapterus than to Monodon and sister group of an unnamed taxon from the North Sea. The holotype of Casatia represents the first and only fossil monodontid from the Mediterranean Basin. Taking its place beside abundant fossils of strongly thermophilic marine vertebrates, such as the bull shark Carcharhinus leucas, the tiger shark Galeocerdo cuvier, and the extinct sirenian Metaxytherium subapenninum, Casatia thermophila represents the strongest evidence supporting the hypothesis that monodontids once thrived in low-latitude, warm-water habitats. On the basis of our phylogenetic reconstruction, early relatives of the extant monodontids might have adapted independently to the high-latitude, cold-water environments they currently master. The definitive disappearance of the Neogene thermophilic monodontids could be attributed to the cooling episode that accompanied the onset of long-term Northern Hemisphere glaciation around 3 Ma.