| Summary: | Monodontidae is a small family of odontocete cetaceans currently distributed in the Artic and North Atlantic Ocean and it includes two species: Monodon monoceros and Delphinapterus leucas. The fossil record of Monodontidae is still rather limited, including Denebola brahycephala from the late Miocene Almejas Formation (Mexico) and Bohaskaia monodontoides from the early Pliocene Yorktown Formation (Virginia, Usa). A new fossil odontocete neurocranium, discovered in early Pliocene sediments of the Arcille succession (Grosseto, Italy), is here referred to the family Monodontidae by exhibiting the medial exposure of the maxillae making up the anterior and lateral margins of the external bony nares, the anterior positioning of the orbit, and the subvertical occipital shield. The specimen, kept in the collection of the Museo di Storia Naturale dell’Università di Pisa with catalogue number I17602, represents the first record of Monodontidae within the Mediterranean Basin. This fossil is described and identified as a new genus and new species, completely different from the two monodontid fossil species known to date. MSNUP I17602 represents a small-sized monodontid characterized by: a swelling of the premaxillary sac fossae, a subcircular outline of the external bony nares, a rounded and quite elevated vertex, and triangular-shaped nasal bones. After the exam of all the skulls of D. leucas and M. monoceros kept in Italian museums, it has been possible to identify some morphological features of MSNUP I17602 in common with the two extant species. These similarities have also been supported by the results of the geometric morphometric analyses performed choosing two-dimensional landmark configurations in dorsal and lateral view. However, several autapomorphies exhibited by MSNUP I17602 make it impossible to attribute the specimen to one or the other monodontid genus already described. According to the result of the phylogenetic analysis, MSNUP I17602 falls in a derivate position within Monodontidae, belonging to the clade ...
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