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.

Type species: Cetotherium rathkii Brandt, 1843; south coast of Taman Peninsula; Tortonian, Late Miocene of the Eastern Paratethys.

Included species: Type species and Cetotherium riabinini Hofstein, 1948.

Diagnosis (Fig. 3).—Small cetotheres (3 to 4 m long; condylobasal length of skull ∼1 m) differing from other members of the family in having a narrow rostrum, a roughly triangular (pointing dorsally) exposure of the posterior process of the tympanoperiotic on the posterolateral wall of the skull, a triangular occipital shield with a low and transversely wide external occipital crest, and a tympanic bulla as high and wide anteriorly as it is posteriorly; Cetotherium differs from all cetotheres except Kurdalagonus in having an anteroposteriorly short and dorsoventrally high zygomatic process of the squamosal and a transversely wide postglenoid process; differs from all cetotheres except “Cetotherium” mayeri (sensu Riabinin 1934) in having a mandible with a straight, as opposed to laterally curved, distal portion; differs from Herpetocetus and Nannocetus in having a robust, bulbous paroccipital process; differs from Herpetocetus, Metopocetus, and Nannocetus in having a ventrally open (as opposed to partially floored) facial sulcus on the posterior process of the periotic; differs from all cetotheriids except Eucetotherium helmersenii (Brandt, 1871), Herpetocetus, and Nannocetus in having the postglenoid process oriented ventrally to ventromedially in posterior view; differs from Herpetocetus, Joumocetus, Metopocetus, Nannocetus, and Piscobalaena in having the proximal portion of the premaxilla not covered by the maxilla, and ascending processes of the maxillae approximating each other posteriorly without ever making contact.

Remarks.—Cetotherium has in the past been used as a wastebasket taxon for several species not belonging to this genus. In particular, the diagnoses of Brandt (1873), Spassky (1954), and Mchedlidze (1970) define a wider group including all cetotheriids.

Geographic and stratigraphic range.—Black Sea region; Tortonian, Late Miocene of the Eastern Paratethys.

Type and only specimen: NMNH-P 668/1, partial skeleton including a nearly complete skull.

Type locality: City of Nikolaev, southern Ukraine.

Type horizon: Late Sarmatian (corresponding to the early Tortonian, Late Miocene) of the Eastern Paratethys (Radionova et al. 2012).

Diagnosis (Fig. 4).—Species of Cetotherium differing from C. rathkii in having: (i) a triangular nasal that gradually tapers posteriorly (as opposed to being wedge-like and strongly compressed); (ii) the anterior edge of nasal located just posterior to the level of the antorbital notch (not far posterior as in C. rathkii); (iii) an elongated posterior portion of the facial skull; (iv) a V-shaped, notched anterior margin of the palatines; (v) an angular (as opposed to gradually curved) anterior edge of the supraorbital process of the frontal; (vi) the distal portion of the postglenoid process of the squamosal bent medially, protruding dorsally and posteriorly (not posteroventrally as in C. rathkii); (vii) a convex nuchal crest in dorsal view; and (viii) a sigmoid process of the tympanic bulla oriented slightly posteriorly (not perpendicularly to the long axis of the bulla as in C. rathkii).

Description

All parts of the specimen are exceptionally well preserved, with no signs of post-depositional deformation. The skull (Figs. 2, 5-7, Table 1) is about 970 mm long, which constitutes approximately one third of the estimated total body length. In dorsal view, the rostrum is transversely narrow, triangular and gradually tapers along its entire length. In lateral view, the rostrum is straight and directed anteroventrally relative to the braincase. The unfused rostral bones have been transversely displaced, but show no signs of distortion.

Skull

Premaxilla.—In dorsal view, the premaxilla is narrow transversely, but slightly widens near its anterior tip (Fig. 5A). In lateral view, the premaxilla is elevated above the maxilla along most of the rostrum (Fig. 5C). Anterior to the narial fossa, its medial margin is transversely concave. Posterior to the narial fossa, the premaxillae diverge, before converging again and becoming narrow transversely at the level of the antorbital notch. The premaxilla terminates 10–15 mm anterior to the posterior margin of the nasal, possibly as a result of post-mortem displacement. In cross section, the premaxilla is roughly triangular along its anteriormost part, oval along the most distal portion, and transversely compressed adjacent to the bony nares.

Maxilla.—The maxilla is slightly arched dorsoventrally (Fig. 5C) and strongly tapers from its base to the tip of the rostrum in dorsal view (Fig. 5A, B). In ventral view, the palatine process descends ventromedially and contacts the vomer medially to form a longitudinal keel. Parallel and somewhat medial to the lateral border of the maxilla, an alveolar groove (see also Bouetel and Muizon 2006) runs along the entire length of the rostrum (Figs. 5B, 6B). Several rows of sub-parallel, anteroposteriorly oriented palatal foramina and associated sulci occur on the anterior portion of the maxilla, before becoming aligned into a single row and oriented anterolaterally near the base of the rostrum. These palatal foramina are noticeable more abundant on the left side, indicating some degree of asymmetry. The posterior border of the infraorbital plate is interrupted by a notch located at the level of the alveolar groove.

In dorsal view, the posterior third of the maxilla bears several dorsal infraorbital foramina (12 on the left size, 8 on the right), the posteriormost of which is located at the base of ascending process (Fig. 6A). Unlike in Piscobalaena nana, none of the foramina stands out for being particularly large. The posterolateral margin of the maxilla is concave and terminates in a short, anteroposteriorly narrow lateral process oriented perpendicular to the longitudinal axis of the skull. At its apex, the lateral process of the maxilla is thickened dorsoventrally. Medially, it merges into the antorbital process, which runs to the base of the ascending process (Fig. 6A). The ascending process is anteroposteriorly elongate and parallel-sided, tapering only slightly anteroposteriorly. Posteriorly, the ascending processes approximate each other posterior to the nasals, but never actually come into contact.

Nasal.—The nasal is triangular and gradually narrows posteriorly. Its anterior margin is located at the level of the antorbital process of the maxilla, whereas the posterior margin aligns with the base of the postorbital process of the frontal (Figs. 4A, 5A, 6A).

Lacrimal, mesethmoid.—Missing.

Frontal.—The frontal is narrowly exposed on the skull vertex, posterior to the ascending process of the maxilla (Fig. 6A). The gradually descending supraorbital process of the frontal is oriented anterolaterally, widens distally, and has an angular anteromedial margin. There is a low but noticeable transverse crest (orbitotemporal crest?) located on the anterior half of the supraorbital process (Fig. 6A), as well as second, hardly visible crest running further posteriorly. The preorbital process is short and massive. The postorbital process is long and slender, oriented posterolaterally, and closely approximates the apex of the zygomatic process of the squamosal (Figs. 6A, 7A). The temporal fossa is oval in dorsal view and roughly 1.5 times as wide transversely as it is long anteroposteriorly. The fronto-parietal suture descends anteroventrally from the vertex and follows a zigzag pattern in lateral view (Figs. 6A, 7A).

Parietal.—The parietals are narrowly exposed on the vertex, where they are separated from each other by the interparietal (Fig. 6A). In lateral view, the parietal is about as long anteroposteriorly as it is high dorsoventrally, and borders the alisphenoid ventrally (Figs. 7A, 8). The parietal-squamosal suture is keeled. In lateral view, the nuchal crest is rounded and somewhat elevated dorsally.

Palatine.—In ventral view, the palatine extends anteriorly beyond the level of the antorbital notch, and posteriorly to the level of the foramen pseudovale (Fig. 6B). The lateral margins of both palatines are damaged, but a transverse constriction in the posterior third of the left bone suggests a dumbbell shape as in Cetotherium rathkii. Together, the anterior margins of both palatines form a V-shaped, posteriorly pointing notch (Figs. 4D, 6B).

Pterygoid.—The anteroposteriorly elongate and transversely narrow medial lamina of the pterygoid borders the vomer medially, and the basioccipital crest posteriorly (Fig. 6B). The lateral lamina is irregularly shaped, approximating the lateral surface of the alisphenoid anterolaterally (Fig. 8) and the foramen pseudovale posterolaterally. The anteromedial margin of the lateral lamina is covered by the palatine (Fig. 6B). The pterygoid sinus fossa extends anteriorly to the level of the foramen pseudovale and is filled with matrix, thus obscuring the dorsal lamina of the pterygoid. The apex of the pterygoid hamulus is broken.

Vomer.—In ventral view, the vomer extends posteriorly to the anterior margin of the basioccipital crest and covers the basisphenoid-basioccipital suture. The posterior portion of the vomerine crest is low. Anteriorly, the vomer is poorly preserved, but seems to have been clearly exposed on the ventral surface of the rostrum.

Alisphenoid.—The alisphenoid is largely covered by the pterygoid and squamosal. As a result, only its lateral portion is exposed in the temporal fossa, where it borders the parietal, frontal, pterygoid and squamosal (Fig. 8).

Orbitosphenoid.—In ventral view, the orbitosphenoid occurs as a large, triangular element anterior to the facial canal (Fig. 6B).

Squamosal.—In dorsal view, the supramastoid crest is sigmoidal and joins the nuchal crest at a right angle. The zygomatic process is not aligned with the lateral border of the exoccipital and instead clearly separated from the latter by a distinct angle (Fig. 6A). The zygomatic process is directed somewhat anterolaterally, short anteroposteriorly, wide transversely, and uniformly high dorsoventrally, which makes it extremely robust compared to that of other cetotheriids. In lateral view, a clearly developed sternomastoid fossa (Bouetel and Muizon 2006) is present between the supramastoid crest dorsally and the posterior process of the periotic ventrally. Ventrally, the squamosal bears a shallow, anteroventrally facing glenoid fossa. The foramen pseudovale is almost entirely enclosed by the squamosal, and medially bordered by the slender, irregularly shaped falciform process (Fig. 6B). The postglenoid process is anteroposteriorly flattened, wide transversely, and slightly twisted medially (Figs. 4C, 6B). In lateral view, it projects posteroventrally, with a concave dorsal and a convex ventral border (Figs. 4B, 5C). The posterior portion of the postglenoid process is bent posterodorsally and directed medially.

Exoccipital.—The exoccipital forms a pentagonal plate, and is anteroposteriorly thickened where it forms the paroccipital process (Figs. 6B, 7A). A low, undulating crest extends horizontally from the dorsal margin of the foramen magnum. Ventral to the latter, a shallow fossa surrounds the occipital condyle, which thus appears to form a neck. The large paroccipital process is made of rugose bone, teardrop-shaped in lateral view, and extends posteriorly beyond the level of the occipital condyle to form the posteriormost point of the skull. In dorsal view, the supramastoid and nuchal crests do not extend on to the paroccipital process (Fig. 6A).

Supraoccipital.—In dorsal view, the supraoccipital bone is sub-triangular (Figs. 6A, 7B, C) and extends anteriorly to the level of the temporal fossa, but not beyond the apex of the zygomatic process. There are no distinct tubercles. Near the skull vertex, the nuchal and external occipital crests join together to form an elevated area. The nuchal crest is elevated above the occipital shield along almost its entire length and slightly convex in dorsal view. The external occipital crest is high along its anterior portion (Fig. 7B, C) and then becomes lower and transversely wider, before terminating in a wide, flat area dorsal to the foramen magnum.

Basioccipital.—The basioccipital is trapezoidal and partially covered by the posterior portion of the vomer (Fig. 6B). Laterally, the basioccipital contributes to the border of the cranial hiatus. The basioccipital crest is large and bulbous, with a flattened ventral surface. A low median crest represents the posterior extension of the vomerine crest.

Periotic.—Both bones are unprepared and covered by matrix or the underlying tympanic bulla. The composite posterior process of the periotic and tympanic bulla is relatively short and bears a deep (but not tube-like) facial sulcus. The distal surface of the posterior process is clearly exposed on the lateral skull wall as a subtriangular, dorsally directed wedge, interposed between the exoccipital and the squamosal (Figs. 3, 7A).

Tympanic bulla (Fig. 9).—The description is based on the right bulla as preserved in situ. The longitudinal axis of the bulla is oriented slightly anteromedially. In ventrolateral view, the tympanic bulla is oval or slightly kidney-shaped, whereas in ventral view the bone shows a markedly angular anteromedial corner and slightly narrows along its posterior third. The anterolateral portion of the ventral surface of the bulla is transversely concave. The sigmoid process is straight, moderately long, relatively massive, evenly thickened anteroposteriorly along its entire length, and directed slightly posteriorly (Fig. 9B, C). The base of the sigmoid process is located posterior to the centre of the bulla and not inflated, with the bone surface surrounding it being relatively smooth. In lateral view, the bulla is oval in outline. In medial view, there is a distinct but low medial lobe, which is separated from its lateral counterpart by a subtriangular median furrow (Fig. 9D-F). The lateral lobe extends somewhat further posteriorly than the medial one. The main and involucral ridges converge anteriorly.

Mandible.—The distal portion of the mandible is straight (i.e., not bowed laterally) (Fig. 5). The condyle is large, elevated above the mandibular body, and increases in width dorsally (Fig. 10A, C). The dorsolateral portion of the condyle is twisted at an angle of 45° relative to the anteroposterior axis of the mandible (Fig. 10C). The angular process is massive, bulbous and somewhat extended posteriorly (Fig. 10A). Dorsal to the angular process, there is a notch for the internal pterygoid muscle located medial to the subcondylar furrow (Fig. 10B). The coronoid process is long anteroposteriorly, low dorsoventrally, and bent laterally. The postcoronoid crest is low. The mandibular foramen is dorsoventrally narrow and has a notched anterior margin. Damage to the bone has exposed part of the narrow mandibular canal (less than 10 mm in diameter anterior to the base of the coronoid process). An alveolar groove is present along the dorsomedial surface of the mandible and well developed near its distal end. The mandibular symphysis is unfused and marked by a 25 cm long symphyseal groove. The latter initially runs parallel to the anterior edge of the mandibular body, then turns 90° and continues posteriorly parallel to the lower edge of the body. In lateral view, the mandibular body is perforated by a series of mental foramina aligned in a single, longitudinal row.

In cross section, the outline of the mandibular body significantly changes anteroposteriorly (Fig. 11). The body is high, laterally flattened and symmetrical in the symphyseal area. Further posteriorly, it lowers and widens, and the dorsal crest flattens. Forty centimetres from the tip, the bone acquires an asymmetrical profile with a convex lateral surface and a flat medial one. The lower edge is convex and lacks a ventral crest. From this point onwards, the depth of the mandible gradually increases posteriorly. Seventy centimetres from the tip, the dorsal crest becomes sharp, and at 80 cm the mandible again becomes symmetrical, with rounded medial and lateral sides. Posterior to the coronoid process, the height of the mandible lowers, before rising again and reaching its maximum at the level of the condyle.

Measurements of the left and right mandibles reveal an obvious degree of asymmetry (Table 2), with the body of the left mandible being longer, more curved and slightly lower along its entire length. In addition, the proximal portion of the left mandible is longer, as is the base of its coronoid process (by 10 mm). There are no traces of either pathological or diagenetic deformation.

Vertebral column

Four cervical, 10 thoracic, 8 lumbar and 19 caudal vertebrae have been preserved (Figs. 12-14, Table 3), leading us to a total estimate of 46 or 47 (C7, T12, L8, Ca19-Ca20). Except for the cervicals, all of the vertebrae and their associated ribs show well-developed pachyosteosclerosis (see also Hofstein 1948). The epiphyses of C7 and all of the thoracic, lumbar and anterior caudal vertebrae are unfused. In Ca12-Ca17, the epiphyses are fused, with the anterior suture having become partially obliterated in Ca12.

Cervical vertebrae.—C4-C7 are separate, with no signs of intervertebral fusion. The centra are subrectangular in outline and bear wide, slightly curved neural arches with low neural spines. Posterior to C4, the transverse processes are oriented anterolaterally. In C4, the symmetrical diapophysis and parapophysis connect laterally to form a transverse foramen. In C5, only the bases of the transverse processes have been preserved. In C6, the diapophysis is thickened and oriented ventrally, whereas the equally thickened parapophysis is short and oriented laterally. In C7, only the thick and transversely wide diapophysis is present. The prezygapophyses are generally small and the postzygapophyses underdeveloped, which is relatively unusual among mysticetes and may be a result of individual age variation.

Thoracic vertebrae.—Judging from available descriptions of Piscobalaena nana (Bouetel and Muizon 2006) and “Cetotherium” aff. mayeri (Spassky 1954), the thoracic vertebrae appear to have been mounted out of order (Figs. 13A, 15). Five complete vertebrae form part of the mounted skeleton, with the fragmented remains of a further 5 having been reconstructed using plaster. The anteriormost two vertebrae appear to be missing. Facets on the centra for the articulation with the rib capitula are present on the first three of the preserved vertebrae (TA-TC), here interpreted as T3, T4 or 5, and T7 or 8, respectively (Fig. 15). A small metapophysis occurs on the anterior margin of the transverse process of TA. On the subsequent vertebrae, the metapophyses increase in size and migrate medially, approaching the base of the transverse process and, ultimately, the neural arch.

Lumbar vertebrae.—All lumbar vertebrae (8 in total) appear to have been preserved. The centra are oval in anterior view and flattened dorsoventrally, with height: width ratios of 0.65–0.76 (Table 3). L1-L3 are characterised by posteroventrally oriented, narrow (but long) transverse processes, high and posteriorly inclined spinal processes, and massive, laterally compressed metapophyses. In L4, the transverse process is oriented laterally, before becoming oriented anterolaterally from L5 onwards (Fig. 13A). In L1-L4, and especially L2-L3, the anterior margin of the spinal process is markedly concave (Fig. 13B). Posterior to L4, the spinal process develops a straight anterior edge and becomes wider anteroposteriorly, as well as more posteriorly inclined.

Caudal vertebrae.—19 caudal vertebrae have been preserved (Fig. 14). Ca1 differs from the posteriormost lumbar vertebra in having paired ventral processes for a chevron bone, as well as a shortened transverse process. The bases of the metapophyses converge anterior to the neural arch. Ca2 and Ca3 have a square transverse process, which is separated from the centrum by a deep groove in dorsal and anterior view (Fig. 13A); the same groove occurs posteriorly on Ca4. Unlike in most other mysticetes, including “Cetotherium” mayeri-like whales from the Eastern Paratethys and Piscobalaena nana, there are no vertical foramina perforating the transverse process of any of the caudal vertebrae (Brandt 1873; Spassky 1954; Bouetel and Muizon 2006).

Along Ca4-Ca10, the metapophyses, neural arches, and spinal and transverse processes gradually reduce in size, with the latter completely disappearing posterior to Ca7. The centra narrow transversely along their posterior portions, giving them the shape of a flattened cone in dorsal view. In Ca11, the centrum shortens abruptly, thus losing its conical shape. The neural arch is absent, and the metapophysis is small. From Ca12 onwards, the vertebrae are roughly spherical and irregular in shape. From Ca13 onwards, the vertebrae are divided into an anterior and a posterior half by a transverse groove. Posterior to Ca13, the spinal process completely disappears (Fig. 14). Although the posteriormost caudal vertebrae are often missing, the presence of 7 shortened posterior vertebrae with no obvious processes, which in living mysticetes correspond to the fluke region, implies that the vertebral column of Cetotherium riabinini is essentially complete.

Ribs.—Hofstein (1948, 1965) reported that 11 pairs of ribs had been preserved, (it is unclear whether all of them were present on both sides), with 9 of them being complete. Many of the ribs are fragmentary and re-modelled, making it difficult to describe them accurately. However, there are at least 8 to 9 right ribs and 3 left ribs present in the mounted skeleton. The first one or two pairs are absent, which corresponds to the absence of the first two thoracic vertebrae. Almost all of the ribs are extensively thickened. The anteriormost rib is shortened and curved, and followed by a series of sickle-shaped anterior ribs with well-developed capitula and small tubercula. The central ribs are sigmoidal in lateral view (“spiral” according to Hofstein 1948) (Fig. 15). The capitula are absent on the three posteriormost ribs (two according to Hofstein 1948, 1965). The last rib is sickle-shaped, and shorter and thinner than any of the others. None of the preserved ribs show any evidence of articulation with the sternum.

Forelimb

Scapula.—The scapula is roughly triangular, fan-shaped, and relatively short anteroposteriorly (Fig. 16, Table 4). The anterior margin is blunt and rounded, while the posterior margin is elongated. The robust acromion is rounded anteriorly and forms a sharp angle with the anterior margin of the scapular blade. The coracoid process is robust, but short, and slightly curves towards the acromion. The spine is reduced, flattened and shifted anteriorly. The portion of the scapula bearing the glenoid cavity is slightly concave medially. A low ridge bordering the infraspinous fossa posteriorly is developed on the lateral surface of the posteriormost portion of the scapular blade, and in particular near the glenoid cavity.

Humerus.—The humerus is flattened and dumbbell-shaped in lateral view (Fig. 16, Table 4). The posterior edge of the shaft is shorter than the anterior one as a result of the extensive development of the ulnar facet at the distal epiphysis. Both the greater and lesser tubercles are well developed, and divided by a shallow furrow. Only the distal epiphysis of the left humerus is fused to the shaft.

Radius.—The radius is transversely flattened and slightly curved anteriorly in lateral view (Fig. 16, Table 4). Both of the distal epiphyses are fused, unlike their proximal counterparts.

Ulna.—The ulna is transversely flattened and sigmoidal in lateral view. The olecranon is well developed and proximally elongated (Fig. 16, Table 4). Only the distal epiphyses are fused to the shaft.

Carpus, metacarpus, and phalanges.—According to Hofstein (1965), the carpals, metacarpals and most of phalanges have not been preserved. In the mounted skeleton, it is difficult to distinguish genuine bones from those remodelled for display. As far as can be told, the metacarpals and proximal phalanges are dumbbell-shaped, while the distal phalanges are elongated and conical.

Pelvis and hindlimb.—According to Hofstein (1948, 1965), both of the pelvic bones and femora were found with the skeleton. Unfortunately, both elements are currently missing. As noticed by Hofstein (1965: 26), the pelvic bones were elongated, while the femora “looked like oval plates”, with a diameter of up to 5 cm.

Remarks.—Unlike the adult holotype of Cetotherium rathkii, that of C. riabinini represents a juvenile or sub-adult. Nevertheless, we consider it unlikely that the morphological differences between them are simply the result of ontogenetic variation. In living baleen whales, the orientation of the squamosal generally does not change during ontogeny (PG, personal observation). By contrast, the zygomatic process can expand during growth-at least in living odontocetes. In this light, the ontogenetically young C. riabinini seems more anatomically “adult” (peramorphic) than C. rathkii, with differences in the morphology of the squamosal (and, possibly, the facial region) likely reflecting the specific anatomy of the jaw attachment.

“Cetotherium” klinderi, another Neogene fossil mysticete known from the Black Sea region, was mainly defined based on the distinctive anatomy of a tympanic bulla from Chişinău (Kishinev), Moldova. The latter clearly differs from that of other species of Cetotherium in having a widened posterior portion in medial view, as well as a strongly inflated involucrum (Brandt 1873: pl. 12: 4). “C.” klinderi can thus be excluded as a potential senior synonym of C. riabinini. Note that a second specimen of “C.” klinderi from Nikolaev reported by Brandt (1873) is not available for study at present, and has possibly been lost.

Geographic and stratigraphic range.—Type locality and horizon only.