Citation: Nobile, F.; Collareta, A.; Perenzin, V.; Fornaciari, E.; Giusberti, L.; Bianucci, G. Dawn of the Delphinidans: New Remains of Kentriodon from the Lower Miocene of Italy Shed Light on the Early Radiation of the Most Diverse Extant Cetacean Clade. Biology 2024, 13, 114. https://doi.org/10.3390/ biology13020114 Academic Editor: Zuozhi Chen Received: 31 December 2023 Revised: 29 January 2024 Accepted: 1 February 2024 Published: 11 February 2024 Copyright: © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). biology Article Dawn of the Delphinidans: New Remains of Kentriodon from the Lower Miocene of Italy Shed Light on the Early Radiation of the Most Diverse Extant Cetacean Clade Francesco Nobile 1,2 , Alberto Collareta 1,3, * , Vittore Perenzin 4 , Eliana Fornaciari 5 , Luca Giusberti 5 and Giovanni Bianucci 1,3 1 Dipartimento di Scienze della Terra, Università di Pisa, Via Santa Maria, 53, 56126 Pisa, Italy; francesco.nobile@phd.unipi.it (F.N.); giovanni.bianucci@unipi.it (G.B.) 2 Dottorato di Ricerca Geoscienze e Ambiente, Università di Pisa, Via Santa Maria, 53, 56126 Pisa, Italy 3 Museo di Storia Naturale, Università di Pisa, Via Roma 79, 56011 Calci, Italy 4 Museo Civico Archeologico, Via Lorenzo Luzzo, 23, 32032 Feltre, Italy; sassruisfeltre@gmail.com 5 Dipartimento di Geoscienze, Università di Padova, Via Giovanni Gradenigo, 6, 35131 Padova, Italy; eliana.fornaciari@unipd.it (E.F.); luca.giusberti@unipd.it (L.G.) * Correspondence: alberto.collareta@unipi.it Simple Summary: Nowadays, the infraorder Delphinida consists of oceanic dolphins and porpoises plus a handful of riverine and (sub-)Arctic forms. Overall, the delphinidans account for more than half of the living cetacean species, thus comprising the core of present-day marine mammal diversity. The fossil record indicates that a critical phase of the evolutionary history of Delphinida occurred during the Early Miocene (c. 23.0–16.0 million years ago) when the extinct genus Kentriodon first appeared and became widespread worldwide. Our paper deals with a new delphinidan fossil from northeastern Italy, namely, an incomplete skull with ear bones dating back to 20.4–19.0 million years ago. This new specimen is recognized herein as a representative of Kentriodon and specifically as the first unambiguous member of this genus from the Euro-Mediterranean region. Our new find represents the best candidate for being the most ancient member of Kentriodon. The evolutionary success of Kentriodon (which lasted until the Late Miocene, less than 11.3 million years ago) may have been favored by the evolution of a peculiar biosonar system exploiting narrow-band high-frequency sounds, which in turn would have been hardly detectable by large-bodied, predatory toothed whales. Furthermore, Kentriodon was seemingly characterized by a proportionally larger brain compared to many coeval toothed whales, which in turn may evoke enhanced behavioral capabilities. Abstract: Nowadays, the infraorder Delphinida (oceanic dolphins and kin) represents the most diverse extant clade of Cetacea, with delphinids alone accounting for more than 40% of the total number of living cetacean species. As for other cetacean groups, the Early Miocene represents a key interval for the evolutionary history of Delphinida, as it was during this time span that the delphinidans became broadly distributed worldwide, first and foremost with the widespread genus Kentriodon and closely related forms. Here, we report on a new odontocete find from Burdigalian (20.4–19.0 Ma) deposits of the Friulian-Venetian Basin of northeastern Italy, consisting of the par- tial cranium of a small delphinidan with associated ear bones (right periotic, stapes, malleus and tympanic bulla). Osteoanatomical considerations and comparisons allow us to assign the studied specimen to the genus Kentriodon. This is the first confirmed record of Kentriodon from Europe as well as from the whole proto-Mediterranean region. Stratigraphic and phylogenetic considerations suggest that our new specimen may represent the geologically oldest member of Kentriodon. The evolutionary success of Kentriodon may correlate with the emergence of narrow-band high-frequency echolocation as a possible strategy to escape acoustic detection by large marine predators such as the squalodontids. In addition, the relatively high encephalization quotient of Kentriodon spp. may have provided these early dolphins with some kind of competitive advantage over the coeval non-delphinidan odontocetes. Biology 2024, 13, 114. https://doi.org/10.3390/biology13020114 https://www.mdpi.com/journal/biology