AMNH, American Museum of Natural History, New York, USA; IRSNB, Institut Royal des Sciences Naturelles de Belgique, Brussels, Belgium; MGP, Museo di Geologia e Paleontologia, Padova, Italy; MLP, Museo de La Plata, La Plata, Argentina; MNHN, Muséum National d’Histoire Naturelle, Paris, France; MSNUP, Museo di Storia Naturale, Università di Pisa; MUSM, Museo de Historia Natural, Universidad Nacional Mayor de San Marco, Lima, Peru; RMNH, Naturalis, Leiden, The Netherlands; USNM, United States National Museum of Natural History, Smithsonian Institution, Washington D.C., USA.

Order CETACEA Brisson, 1762

Suborder ODONTOCETI Flower, 1867

Superfamily PLATANISTOIDEA Gray, 1863

Family SQUALODELPHINIDAE Dal Piaz, 1917

Type genus: Squalodelphis Dal Piaz, 1917

Other genera included: Huaridelphis, Medocinia, Notocetus, Phocageneus.

Genus Notocetus Moreno, 1892

Type and only included species: N. vanbenedeni Moreno, 1892.

Emended diagnosis: Same as for only included species, N. vanbenedeni.

Remark: A specimen presumably from the Earliest Miocene of New Zealand was tentatively referred to the genus Notocetus (Fordyce, 1994), but a redescription and a more detailed comparison with other known squalodelphinids are needed.

Notocetus vanbenedeni Moreno, 1892

Emended diagnosis: N. vanbenedeni differs from Huaridelphis, Phocageneus, and Squalodelphis in the retention of accessory denticles in the distal keels of the posterior teeth (teeth unknown in Medocinia). It further differs from Huaridelphis in: its larger size; the less abrupt anterior tapering of the rostrum in dorsal view; the lower tooth count (21–24 teeth per row contra 28–30 in Huaridelphis); the thicker antorbital process of the frontal, distinctly thicker than the antorbital process of the maxilla in lateral view; the more elongated and robust postorbital process of the frontal; the flat dorsal surface of the vertex parallel to the horizontal plane, not anteroventrally sloping; the absence of a dorsoventral compression of the periotic at the level of the pars cochlearis and superior process (sensu Kasuya, 1973); and the less ventrally bent posterior process of the periotic. It further differs from Medocinia and Squalodelphis in: the dorsal opening of the mesorostral groove being narrower than the premaxilla in the base of the rostrum; and the wider dorsal exposure of the maxilla in the base of the rostrum (the premaxilla nearly reaches the lateral margin of the rostrum in Medocinia and Squalodelphis). It further differs from Phocageneus and Squalodelphis in the shorter anterior spine of the tympanic bulla. It further differs from Squalodelphis in: the higher tooth count (15 per row in Squalodelphis); the transversally wider nuchal crest (approximately as wide as the greatest width of the premaxillae in Squalodelphis and considerably wider than the greatest width of the premaxillae in Notocetus); and the stronger manubrium of the malleus with a rounded rather than pointed apex.

Holotype: MLP 5-5, skull, lacking the ear bones, and associated mandible (Lydekker, 1893 and Moreno, 1892), Chubut province, Argentina.

Previously referred specimens from Argentina: AMNH 9485, skull, including the right tympanic bulla, mandible and some vertebrae and ribs, Santa Cruz province (True, 1910); AMNH 29026, fragment of skull, right periotic, tympanic bulla and malleus, several teeth, scapula and fragments of vertebrae and ribs, Chubut province (de Muizon, 1987).

Referred specimen: MUSM 1395, skull lacking the left squamosal and the teeth, with associated periotics (the left one in situ) and one cervical vertebra.

Locality: Ullujaya, the type locality of H. raimondii. Pisco Basin, 58 km SSE of Ica, 1.5 km north from Cerro Las Tres Piramides (see Alván De la Cruz, 2008, fig. 1). Approximate geographic coordinates: S14°34′30″-W75°38′40″ (Fig. 1).

Horizon: Chilcatay Formation, latest Oligocene to Early Miocene (DeVries, 1998, DeVries, 2001 and Dunbar et al., 1990). A section of the Chilcatay Formation at Cerro Buque, 1.65 km NWW to the Ullujaya locality has been published (Alván De la Cruz, 2008). Based on the analysis of the stratigraphical sequence and shark teeth (presence of Isurus desori and Carcharocles chubutensis, absence of C. megalodon), the beds of the Chilcatay Formation in this section were dated to the Early Miocene (Alván De la Cruz, 2008). In the same locality and approximately in the same stratigraphical horizon of MUSM 1395, we found a rich fossil vertebrate assemblage represented by other odontocetes (e.g., H. raimondii, kentriodontids, a stem physeteroid and a new archaic homodont odontocete), a mysticete, sea turtles, sharks, and teleostean fishes (Fig. 1b).

Description and comparison: skull ( Fig. 2 ). For most of the skull measurements available (see True, 1910), MUSM 1395 falls in the size interval between the holotype (MLP 5-5) of N. vanbenedeni and the somewhat larger referred specimen AMNH 9485 (Table 1). No major morphometric differences are noted, and all the minor dissimilarities can be explained with intraspecific variation and/or slight post-mortem deformation of the specimens. The condylobasal length (CBL) of MUSM 1395 is 600 mm, close to MLP 5-5 (CBL = 582 mm), feebly smaller than AMNH 9485 (CBL = 634) and the holotype of Squalodelphis fabianii (CBL = 640), but distinctly larger than the holotype of H. raimondii (CBL = 494 mm).

As in other specimens of N. vanbenedeni the rostrum of MUSM 1395 is more robust and, in dorsal view, less abruptly tapering in its proximal third than in Huaridelphis. In lateral view, the rostrum displays an apex distinctly curved upward, a feature seemingly absent in the holotype and AMNH 9485. A similar variation is observed in H. raimondii, with an apex of the rostrum horizontal in the holotype and curved upward in MUSM 1403. We observed a wide intrapecific variation of this character in the extant delphinids, particularly in some species with a proportionally elongated and thin rostrum (e.g., Steno bredanensis). Contrary to platanistids, the lateral maxilla-premaxilla suture is not located in a deep groove along the rostrum. The mesorostral canal is dorsally closed by the joined premaxillae along the distal third of the rostrum, whereas a narrow gap is observed in the holotype and AMNH 9485. Toward the base of the rostrum, the dorsal opening of the mesorostral canal feebly and progressively broadens. A similar condition is observed in AMNH 9485, whereas in the holotype a narrow separation between the premaxillae is observed along the whole length of the rostrum. The maxillary alveoli are larger than in Huaridelphis, and the original upper tooth count was likely close to the count in the holotype of N. vanbenedeni and AMNH 9485 (21–23), lower than in Huaridelphis (28–30), but higher than in Squalodelphis (15).

The widening of the premaxilla towards the base of the rostrum is more developed on the right side, as in the holotype and AMNH 9485. The ratio between the width of the rostrum and the transverse width of the premaxillae at the level of the antorbital notches is 0.61, a value intermediate between AMNH 9485 (0.56) and the holotype (0.68). A similar widening of the premaxillae is observed in Huaridelphis (ratio 0.60–0.61), whereas in Medocinia and Squalodelphis the premaxilla nearly reaches the lateral margin of the rostrum (ratio 0.82). As in all squalodelphinids, allodelphinids, and pomatodelphinines the dorsal surface of the premaxillae at the base of the rostrum shows a deep medial depression. Both premaxillary foramina are slightly posterior to the level of the antorbital notches, whereas they are anterior, with longer premaxillary sac fossae, in the holotype and AMNH 9485. The left and right premaxillary sac fossae are distinctly separated medially, leaving the mesethmoid uncovered. The latter is ossified on a short length anterior to the bony nares. A similar condition is observed in AMNH 9485, whereas in the holotype the premaxillae are almost in contact medially. The right antorbital notch is shallower than the left; the anterior margin of the right antorbital process is roughly rectilinear and transversely directed, a condition differing from the narrower V-shaped notch and anteriorly pointed antorbital process on the left side. Asymmetrical antorbital notches are also observed in the holotype, AMNH 9485, and, less accentuated, in Huaridelphis and Squalodelphis. In lateral view, the antorbital process is robust, with a thick frontal covered by a proportionally thinner sheet of the maxilla, as in AMNH 9485, whereas the limit between maxilla and frontal is more difficult to distinguish in the holotype. Together with the massive postorbital process, this condition leads to a proportionally smaller orbit than in Huaridelphis. As in several other squalodelphinids, the left antorbital region is more elevated than the right, a condition best seen in anterior view.

The posterior apex of the premaxilla is pointed and narrower than in pomatodelphinine platanistids. As in many other platanistoids, the mesethmoid of MUSM 1395 is pierced by a pair of crescent-like fenestrae on the posterior wall of the bony nares. Also observed in Huaridelphis, these fenestrae much likely represent reduced olfactory openings (Godfrey, 2013, Hoch, 2000 and Lambert et al., 2014).

The lateral shift of the vertex on the left side is similar to the other known specimens of N. vanbenedeni for which this area is preserved. The nasal-frontal suture is anteromedially pointed, as in AMNH 9485 (obscured in the holotype), and the dorsal surface of the frontals and nasals on the vertex is subhorizontal, a clear difference between Notocetus and other squalodelphinids, the latter displaying a marked anterior slope of that surface.

As in the two other known skulls of Notocetus, the nuchal crest is transversely wider than in Squalodelphis. In the latter, the nuchal crest is approximately as wide as the greatest width of the premaxillae, whereas in Notocetus it is considerably wider.

The pterygoid reaches 50–60 mm beyond the level of the antorbital notch. As in other squalodelphinids and pomatodelphinines, the palatine is probably only exposed on the ventral surface of the base of the rostrum, as a thin plate lateral to the pterygoid and slightly anteriorly longer than the antorbital notch. The lateral lamina of the pterygoid forms a continuous plate until its contact with the long and wide falciform process of the squamosal, a feature also seen in AMNH 9485.

On the roof of the orbit, the frontal is excavated by a large fossa anterior to the postorbital ridge, a character also present in the holotype; its presence in other squalodelphinids and pomatodelphinines has been interpreted as the indication of an extension of the pterygoid sinus in the orbit region (Lambert et al., 2014).

In lateral view, the swollen zygomatic process of the squamosal is similar to that of the holotype and AMNH 9485, with a moderately convex posterodorsal margin and a slightly convex anteroventral margin. As in AMNH 9485, the postglenoid process is slender. Somewhat higher than in the holotype, the temporal fossa has a general outline similar to AMNH 9485 (in the latter, the fossa is likely somewhat dorsoventrally crushed).

In general, the ventral view of the squamosal resembles those of the fragmentary skull AMNH 29026 of N. vanbenedeni (de Muizon, 1987). A small but deep fossa dorsal to the spiny process of the squamosal likely corresponds to the subcircular fossa mentioned by de Muizon (1987) in Notocetus, Squalodelphis, and platanistids, a feature considered as homologous with the anterior part of the periotic fossa by Fordyce (1994). The tympanosquamosal recess is only deep on the medial flank of the postglenoid process. The anterior wall of the paroccipital process of the exoccipital is excavated by the fossa for the posterior sinus, as in AMNH 29026 (see de Muizon, 1987, fig. 3).

Periotic (Fig. 3). The periotic of N. vanbenedeni AMNH 29026 has been described in detail by de Muizon (1987). The periotic of MUSM 1395 is similar to AMNH 29026 in having a square-shaped pars cochlearis in ventral view, an elongated anterior process, an anteroexternal sulcus on the anterior process, an articular rim, a subrectangular posterior bullar facet, and approximately the same size and position for the openings in the pars cochlearis.

Cervical vertebra (Fig. 4). No cervical vertebra other than the atlas has been described for Notocetus (True, 1910: pl 5, figs. 1 and 2). The only preserved cervical of MUSM 1395 shares similarities with the third cervical of Phocageneus USNM 21039 (Kellogg, 1957). In MUSM 1395, the transverse foramen is more rounded and located lower relative to the centrum than in USNM 21039, being more similar in this regard to a cervical vertebra of Huaridelphis (MUSM 1403) that is presumably from a roughly similar position. However, the ventral lamina is more ventrally directed in MUSM 1395 and USNM 21039 than in Huaridelphis, and the centrum is more dorsoventrally flattened in the latter. As in USNM 21039, the neural canal is proportionally higher than in Platanista.

The new record of N. vanbenedeni in the same locality and approximately same horizon of H. raimondii, suggests the sympatric coexistence of two squalodelphinids during the Early Miocene along the Pacific coast of South America. This joined record contrasts with the globally scarce record of squalodelphinids (Fig. 5). Indeed, outside of Peru, diagnostic material of squalodelphinids has only been reported from a few Late Oligocene-Early Miocene localities of Argentina, Europe and possibly New Zealand (Cozzuol, 1996, Dal Piaz, 1917, de Muizon, 1988 and Fordyce, 1994). The published Argentinean records, from marine beds outcropping in Patagonia, consist of three significant specimens, all referred to N. vanbenedeni (de Muizon, 1987, Moreno, 1892 and True, 1910). Considering the new record described here, N. vanbenedeni lived both along the Atlantic and Pacific coasts of South America. During the Late Oligocene and Early Miocene, contacts between these two distant marine areas were possible through the widely open Panama Seaway and via the southern tip of South America (Smith et al., 2004). Unfortunately, the only other South American records for the family are fragmentary undiagnostic specimens from the Early Miocene of Venezuela (Sánchez-Villagra et al., 2001). More complete specimens from the North of the continent and the Caribbean region will be necessary to better apprehend the South American distribution and migration patterns of N. vanbenedeni. To our knowledge, this is the only fossil marine mammal species currently recognized from both South Pacific and South Atlantic.

The European record of squalodelphinids includes the holotype (skull with associated mandible) and referred material of Squalodelphis fabianii from the molasses of Belluno (northeastern Italy) and the fragmentary skull referred to Medocinia tetragorhina (Delfortrie, 1875) from the Atlantic coast of southern France (de Muizon, 1988). Squalodelphinids are also reported from the Early Miocene of the East Coast of the USA: an isolated tooth from Virginia described as the holotype of Phocageneus venustus Leidy, 1869, a partial skeleton lacking the skull (USNM 21039) from Maryland referred to the same species (Kellogg, 1957), and a damaged skull (USNM 475596) and isolated ear bones and teeth from the Lee Creek Mine, North Carolina (Kellogg, 1957 and Whitmore and Kaltenbach, 2008).

The presumable squalodelphinid record of New Zealand includes the holotype of ‘Prosqualodon’ marplesi Dickson, 1964, an incomplete skull with associated ear bones, teeth, parts of the mandible, vertebrae, ribs, one scapula, and one ulna (Fordyce, 1994), and another unnamed Notocetus-like specimen (Tanaka and Fordyce, 2012).

In conclusion, based on the new Peruvian specimen referred to N. vanbenedeni and other published accounts, Squalodelphinidae represent a poorly diversified Late Oligocene-Early Miocene group of marine platanistoids, with a scarce fossil record but a wide palaeogeographic distribution, including northern and southern Atlantic realms and the southern Pacific.