Cetotheriidae Brandt, 1872

Gol’Din, Pavel, Startsev, Dmitry & Krakhmalnaya, Tatiana, 2014, The anatomy of the Late Miocene baleen whale Cetotherium riabinini from Ukraine, Acta Palaeontologica Polonica 59 (4), pp. 795-814 : 796-797

Family Cetotheriidae Brandt, 1872

Included genera: Cetotherium Brandt, 1843 ; Cephalotropis Cope, 1896 ; Eucetotherium Kellogg, 1931 ; Herpetocetus Van Beneden, 1872 ; Joumocetus Kimura and Hasegawa, 2010 ; Kurdalagonus Tarasenko and Lopatin, 2012 ; Metopocetus Cope, 1896 ; Mixocetus Kellogg, 1934a ; Nannocetus Kellogg, 1929 ; Piscobalaena Pilleri and Siber, 1989 .

Emended diagnosis (modified from Bouetel and Muizon 2006).—Toothless mysticetes characterised by (i) strongly telescoped facial bones, with the anterior margin of the nasal located at or posterior to the level of the antorbital notch, the ascending processes of the maxilla extending posteriorly to the level of the postorbital process of the frontal, and the combined posterior edges of the nasal, premaxilla and maxilla being wedge-shaped and extending almost to the tip of the occipital shield (X-shaped vertex); (ii) ascending processes of the maxillae with concave lateral margins (in dorsal view), contacting medially or approximating each other at their posterior apices; (iii) a dorsoventrally thickened distal portion of the lateral process of the maxilla; (iv) a distally widened supraorbital process of the frontal; (v) a shallow glenoid fossa; (vi) a supraoccipital shield extending no further anteriorly than the line joining the postorbital processes of the frontals; (vii) a paroccipital process extending posterior to the posterior edge of the occipital condyle in dorsal view; (viii) a transversely narrow sigmoid process of the tympanic bulla lacking an inflated base; (ix) a well-developed anterior process and lateral tuberosity of the periotic; (x) a short posterior process of the tympanoperiotic with a flattened distal surface broadly exposed on the posterolateral wall of the skull; and (xi) a mandible having an angular process extending posterior to the condyle, a condyle oriented obliquely to the long axis of the body, and a small and laterally bent coronoid process.

Remarks.—Our diagnosis of Cetotheriidae is based on several recent phylogenetic analyses ( Bouetel and Muizon 2006; Steeman 2007; Marx 2011), the revision of a wide range of material from Belgium ( Steeman 2010), and (re-) descriptions of Herpetocetus spp. ( Whitmore and Barnes 2008), Herpetocetinae indet. ( Boessenecker 2011), Joumocetus shimizui ( Kimura and Hasegawa 2010) and Kurdalagonus mchedlidzei ( Tarasenko and Lopatin 2012) . Mixocetus elysius Kellogg, 1934a matches the diagnosis of Cetotheriidae according to points i–iv, vi, vii, x—listed here, but the poor state of preservation of the holotype raises doubts regarding its identification. Equally difficult to determine are the affinities of Plesiocetopsis hupschii Van Beneden, 1859 , since the holotype is unavailable for study (see Steeman 2010).

Titanocetus sammarinensis ( Capellini, 1901) , “ Mesocetus ” argillarius Roth, 1978 , Diorocetus hiatus Kellogg, 1968 and eschrichtiids have previously been considered to form part of, or be closely related to, cetotheres ( Bisconti 2006, 2012; Steeman 2007). However, none of these taxa have a posterior process of the tympanoperiotic that is broadly exposed on the posterolateral wall of the skull. Besides cetotheres, the only mysticetes to show this feature are neobalaenids ( Fitzgerald 2012): Caperea marginata ( Gray, 1846) and Miocaperea pulchra Bisconti, 2012 . However, the taxonomic position of neobalaenids is still controversial, and they have variously been proposed to ally with balaenids ( Steeman 2007; Ekdale et al. 2011; Bisconti 2012; Churchill et al. 2012), balaenopteroids ( Sasaki et al. 2005; McGowen et al. 2009; Marx 2011) or cetotheriids (e.g., Fordyce and Marx 2012; Marx et al. 2013). In addition, we note that Caperea marginata has a rectangular tympanic bulla in medial and lateral views, a specific trait shared by Aglaocetus patulus Kellogg, 1968 and other genera included by Steeman (2007) in the family Aglaocetidae .