The discovery of an Early Miocene chaeomysticete from the Pietra da Cantoni Group in Piedmont (north-western Italy) allowed for the creation of Atlanticetus n. gen. and Atlanticetus lavei n. sp. The new species is represented by a partial skeleton including the earbones and shows anatomical resemblance with Atlanticetus patulus (new combination) from the western North Atlantic. The Early Miocene age of the new specimen supports the view that it represents the oldest record of Chaeomysticeti from the Mediterranean. A new phylogenetic analysis showed that both A. patulus and A. lavei belong to a radiation of basal thalassotherian taxa. The basal thalassotherians are found being monophyletic to the exclusion of Cetotheriidae and Balaenopteroidea. The reconstruction of ancestral characters at selected nodes informs that the group including Atlanticetus and living balaenopterid taxa independently evolved rostra wide at base, anterolateral expansion in the tympanic bulla and a peculiar arrangement of the endocranial foramina of the periotic exhibiting a noteworthy phenomenon of convergent evolution in feeding and hearing functions with Balaenopteridae. The paleobiogeographic analysis showed that the North Pacific was the center of origin of Balaenomorpha (crown mysticetes), Thalassotherii and Balaenoidea. The recurrent invasion of the Mediterranean by balaenomorph mysticetes occurred from both the North Atlantic and North Pacific.

Anatomy, photography and measurements. The specimen studied is in the collection of the Museo Paleontologico Territoriale Astigiano  (MPTA), Asti, Piedmont (north-west Italy) with the accession number 13301. The specimen includes a partially complete skeleton that was mechanically prepared and fixed using Paraloid. Measurements were taken with digital calipers Sourcingmap (300 mm) and Tacklife D02 (150 mm) both with error margin to the nearest 0.01 mm. Photography was made by using a Nikon D750 full frame DLSR with Tamron 90 mm f/2.8 Di VC macro lens. Light was provided by a Speedlight flash Nikon D700 with softbox mounted on the DSLR. Anatomical terms follow Mead & Fordyce (2009), Ekdale et al. (2011), and Martìnez-Cáceres et al. (2017). The comparative analysis included all the mysticete taxa listed in the Supplementary Information file that comprise most of the described species of living and fossil mysticetes belonging to all the baleen whale radiations.

Anatomical abbreviations. Anatomical abbreviations as follows: aam, area for articulation with malleus; aat, anterior attachment site for tympanic bulla; al, anterolateral lobe; apmx, ascending process of maxilla; ap, attachment pedicle of the compound posterior process of the tympanoperiotic; app, anterior process of periotic; atc, anterodorsal crest; atl, atlas; cp, conical process; ct, crista transversa; elf, endolymphatic foramen; Eo, Eustachian outlet; eof, endocranial opening of facial canal; fi, fossa incudis; fnp, facial nerve path on posterior process of periotic; fms, fossa for stapedial muscle; fo, foramen ovale; fpsq, falciform process of squamosal; hppt, hamular process of pterygoid; hye, hiatus epytympanicus; iam, internal acoustic meatus; inr, involucral ridge; inv, involucrum; ipp, inner posterior prominence; ipn, interoprominential notch; ir, involucral ridge; lf, lateral furrow; lof, lateral opening of the facial canal; lt, lateral tuberosity; mr, main ridge; opp, outer posterior prominence; ow, oval window; pch, pars cochlearis; per, periotic; plf, perilymphatic foramen; pllf, fossa for perilymphatic foramen; ppp, posterior process of periotic; prg, promontorial groove; rw, round window; sma, suprameatal area of periotic; sp, sigmoid process; stm, stylomastoid fossa; ttg, tensor tympani groove; tyb, tympanic bulla; tyc, tympanic cavity; vg, vascular groove.

Institutional abbreviations. MGB, Museo Geopaleontologico ‘Giovanni Capellini’, Università degli Studi di Bologna, Bologna, Italia; MPTA, Museo Paleontologico Territoriale Astigiano, Asti, Italia; RBINS, Royal Belgian Institute of Natural Sciences, Brussels, Belgium; USNM, National Museum of Natural History, Smithsonian Institution, Washington DC, USA.

Phylogenetic analysis. The phylogenetic analysis was performed based on the morphological dataset available from the Supplementary Information file that includes 355 characters scored in 88 taxa. Inference of phylogenetic trees was made by using TNT 1.5 software (Goloboff & Catalano 2016) by 2000 replications of Tree Bisection Reconnection algorithm (TNT command mult 2000=tbr) retaining 10 trees each. A treespace of 20000 potential cladograms was allowed in the tree buffer. Protocetidae was set as outgroup. An analysis using implied weights (default parameters: function k = 3.0) was also performed by using all the new technology algorithms as implemented in TNT and hitting the shortest trees 10 times at least. Then a traditional search was done based on the trees stored in the RAM. The available RAM was 16 MB with 6% occupied by the buffer. The total memory required for the data was 0.56 MB. Node support was calculated in TNT by bootstrap (1000 replicates, traditional search with default parameters), symmetric resampling (1000 replicates, absolute frequencies) and Bremer support (suboptimal value set to 10). Character mappings onto the preferred cladograms were performed through MESQUITE 3.6 (Maddison & Maddison, 2019).  The agreement between the phylogenetic pattern and the stratigraphic ages of the taxa was calculated by the Stratigraphic Consistency Index (SCI; Huelsenbeck 1994). The reconstruction of the biogeographic states at the nodes of the phylogenetic tree was made by MESQUITE 3.61 (Maddison & Maddison 2019) using  maximum likelihood algorithms with default Mk1 model as implemented in the software. The geographic occurrences and the stratigraphic ages of the specimens used in the analysis are available in the Supplementary Information of this paper.