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Genus Mystacodon Lambert, Martínez-Cáceres, Bianucci, Di Celma, Salas- Gismondi, Steurbaut, Urbina & Muizon, 2017 TYPE AND ONLY SPECIES INCLUDED. — Mystacodon selenensis. ETYMOLOGY. — From ancient Greek mystacos (“moustache”) in reference to the suborder Mysticeti and odontos (“tooth”), “mysticete with teeth”. DIAGNOSIS. — The same as for the type species, Mystacodon selenensis., Published as part of Muizon, Christian de, Bianucci, Giovanni, Martínez-Cáceres, Manuel & Lambert, Olivier, 2019, Mystacodon selenensis, the earliest known toothed mysticete (Cetacea, Mammalia) from the late Eocene of Peru: anatomy, phylogeny, and feeding adaptations, pp. 401-499 in Geodiversitas 41 (11) on pages 405-406, DOI: 10.5252/geodiversitas2019v41a11, http://zenodo.org/record/3699733, {"references":["LAMBERT O., MARTINEZ- CACERES M., BIANUCCI G., DI CELMA C., SALAS- GISMONDI R., STEURBAUT E., URBINA M. & MUIZON C. DE 2017 a. - Earliest mysticete from the Late Eocene of Peru sheds new light on the origin of baleen whales. Current Biology 27 (10): 1535 - 1541. e 2. https: // doi. org / 10.1016 / j. cub. 2017.04.026"]}

Mystacodon selenensis Lambert, Martínez-Cáceres, Bianucci, Di Celma, Salas- Gismondi, Steurbaut, Urbina & Muizon, 2017 HOLOTYPE. — MUSM 1917, a partial skeleton. Associated elements include the skull, missing most of the basicranium and the right squamosal and jugal; a fragmentary left tympanic; both dentaries; thyrohyal; cervical and thoracic vertebrae; ribs; sternum (four elements, including manubrium and xiphisternum); partial right and left forelimbs; and left innominate. ETYMOLOGY. — From Selene, the Greek goddess of the moon, in reference to the Playa Media Luna, the type locality. TYPE LOCALITY. — The type specimen comes from the Playa Media Luna (14°36’7.2”S, 75°54’48W), close to the Pacific coast in the southern part of the Pisco Basin, at about the level of the km 400 of the South Panamerican Highway (Fig. 1). TYPE HORIZON AND AGE. — The holotype of Mystacodon selenensis MUSM 1917 was discovered in the middle part of the Yumaque Member of the Paracas Formation, 77 m above the contact with the Los Choros Member (Figs 2; 3) (see DeVries 2017 for a revised interpretation of the Eocene depositional sequence in the area). Layers of the Yumaque Member comprise finely laminated to massive, diatomacetous siltstones, containing pelagic microfossils, thin-shelled pectinid bivalves, and numerous fish scales; they represent deposition in distal (outer shelf), low-energy marine settings (Lambert et al. 2017a; DeVries 2017). The calcareous nannofossil investigation of 37 sediment samples from the Yumaque Member and the overlying lower part of the Otuma Formation, taken along the stratigraphic section in the type locality of Playa Media Luna, allowed positioning the type horizon in the lower part of calcareous nannofossil zone NP19/20 of Martini (1971); based on age estimations by Agnini et al. (2014), an age of 36.4 Ma (early Priabonian, early late Eocene) has been proposed (see Lambert et al. 2017a for more details). EMENDED DIAGNOSIS. — Mystacodon selenensis is identified as a Neoceti based on the following derived characters, absent in basilosaurid archaeocetes: partly open mesorostral groove; anteroposteriorly elongated rostral portion of maxilla; presence of an antorbital process of the maxilla; supraoccipital shield anterodorsally inclined; and distal epiphysis of the humerus divided in two angled radial and ulnar facets. Mystacodon selenensis is referred to the Mysticeti due to the following combination of derived characters: dorsoventrally thin posterolateral region of maxilla on rostrum; antorbital process of maxilla separated from lacrimal; presence of a maxillary infraorbital plate; apex of zygomatic process of squamosal closely apposed to postorbital process of frontal or situated ventral to the latter; external occipital crest restricted to anterodorsal half of supraoccipital shield; and triangular supraoccipital shield. Mystacodon selenensis differs from all other toothed mysticetes in bearing the following combination of features: 1) elongated nasal, the nasal being longer than the length of the frontal + parietal, at midline; 2) anteriorly located antorbital foramina, at about the level of the diastema between P2 and P3; 3) absence of real antorbital notch; 4) long jugo-squamosal suture, with an anterior prolongation of the zygomatic process of the squamosal, extending as far as the level of the postorbital process of the frontal anteriorly; 5) short anterior process of the frontal separating the posterior apices of the nasals; 6) sutured mandibular symphysis, the posterior edge of which is at the level of the i3-p1 diastema; and 7) ventral border of the mandibular ramus being slightly concave ventrally. Mystacodon selenensis also bears four additional derived characters: upturned (i.e., dorsally concave) anterior region of the rostrum; posteriormost upper tooth anterior to level of antorbital process of maxilla; broad-based rostrum (ratio between width of skull at rostrum base and width at postorbital process> 0.8); and strong tuberosity on anterior edge of radius (the latter condition is unknown in the other toothed mysticetes). Furthermore, Mystacodon selenensis bears a series of plesiomophic features: supraoccipital shield not extending anterior to anterior level of squamosal fossa; only two dorsal infraorbital foramina; a basilosaurid-like dental formula 3.1.4.2/3.1.4.3; no wide diastemata between posterior cheek teeth; sutured mandibular symphysis; and well-defined acetabulum on innominate. Mystacodon selenensis differs from Llanocetus denticrenatus in the following characters: 1) much smaller size (less than half the estimated body size); 2) rostrum narrower with concave edges while the preserved posterior part of the rostrum of Llanocetus is much wider and laterally expanded; 3) alveolar border of the maxilla on posterior region of the rostrum rounded dorsolaterally in cross-section with a subvertical lateral wall, while it is flat and crest-like in Llanocetus; 4) preserved parts of the palate lack deep lateral sulci, while they are distinctly present and abundant in Llanocetus; 5) premaxillae abruptly depressed just anterior to the nasals, while in Llanocetus the transition to the post narial part of the premaxillae is smooth; 6) upper molars closely approximated, while they are separated by large diastemata in Llanocetus; 7) upper molars much larger; 8) short diastema between P3 and P4 (less than one tooth mesiodistal length), while in Llanocetus the diastema is two to three times a tooth length; 9) much longer orbit; 10) nearly straight lateral edge of the supraorbital process, while it is deeply concave in Llanocetus; 11) posterior edge of the supraorbital process roughly straight, while it is concave in Llanocetus; 12) orbitotemporal crests extend posteriorly until anteroposterior mid-point of the parietals, while in Llanocetus they end anteriorly, at the posteromedial angle of the supraorbital process of the frontal; 13) dorsal edge of the parietals is subhorizontal in lateral view, while it ascends steeply toward the vertex in Llanocetus; 14) intertemporal bridge much narrower than long transversely, while it is as wide as long in Llanocetus; 15) anterior angle of nuchal crest posterior to anterior margin of squamosal fossa, while it approximately reaches the level of the middle of the fossae in Llanocetus; 16) lateral edges of the supraoccipital in dorsal view straight, while they are sigmoid in Llanocetus; 17) vertex and dorsal aspect of the braincase almost in the same plane as the posterior region of the rostrum, while in Llanocetus they slope anteriorly and distinctly overhang the rostrum; 18) distal extremity of anterior ribs expanded and pestle-like, while this condition is absent in Llanocetus. Mystacodon selenensis differs from Coronodon havensteini in having a longer rostrum and longer diastemata between the premolars; much longer nasals associated to more anterior bony nares, located in the anterior half of the rostrum (in the posterior third in Coronodon); temporal fossae more elongated anteroposteriorly; a long and thin (dorsoventrally) zygomatic process of the squamosal (short and stout in Coronodon); a sharp external occipital crest; a tightly articulated mandibular symphysis; and an sub-horizontal or gently sloping tooth wear on all preserved teeth with strong edges at the apical surface. In addition to its autapomorphies, Mystacodon selenensis differs from mammalodontids in having: a rostrum at least proportionally twice longer; much more anteriorly placed bony nares; a sharply triangular occipital shield; a well-developed external occipital crest (low, when present, in mammalodontids); a strong palatine sulcus on the medial edge of the premaxilla and maxilla anteriorly; much longer diastemata between the premolars; and absence of the markedly V-shaped fronto-parietal suture (as observed in mammalodontids). As compared with aetiocetids, Mystacodon selenensis has a basilosaurid-like dental formula; an almost horizontal apical tooth wear; much larger postcanine teeth; and a long and slender zygomatic process of the squamosal, which is lowest at its anteriormost portion. M. selenensis lacks all the aetiocetid characteristic features, such as the extreme dorsoventral compression of the rostrum; the deeply concave lateral edge of the supraorbital process of the frontal in dorsal view; the anteroposteriorly short zygomatic process of the squamosal (as compared to that of basilosaurids); and the very thin jugal. Otuma Formation Yumaque Member of the Paracas Formation Los Choros Member of the Paracas Formation Finally, Mystacodon selenensis lacks cranial synapomorphies of Odontoceti: facial concavity; presence of premaxillary foramen and premaxillary sac fossa; and wide posterior expansion of maxilla overlapping the supraorbital region.

Genus Africanacetus Bianucci, Lambert & Post, 2007 TYPE SPECIES. ��� Africanacetus ceratopsis Bianucci, Lambert & Post, 2007 by original designation. OTHER REFERRED SPECIES. ��� Africanacetus gracilis Ichishima, Augustin, Toyofuku & Kitazato, 2017., Published as part of Lambert, Olivier, Muizon, Christian de, Duhamel, Guy & Plicht, Johannes Van Der, 2018, Neogene and Quaternary fossil remains of beaked whales (Cetacea, Odontoceti, Ziphiidae) from deep-sea deposits off Crozet and Kerguelen islands, Southern Ocean, pp. 135-160 in Geodiversitas 40 (6) on page 143, DOI: 10.5252/geodiversitas2018v40a6, http://zenodo.org/record/5745694, {"references":["BIANUCCI G., LAMBERT O. & POST K. 2007. - A high diversity in fossil beaked whales (Odontoceti, Ziphiidae) recovered by trawling from the sea floor off South Africa. Geodiversitas 29: 5 - 62.","ICHISHIMA H., AUGUSTIN A. H., TOYOFUKU T. & KITAZATO H. 2017. - A new species of Africanacetus (Odontoceti: Ziphiidae) found on the deep ocean floor off the coast of Brazil. Deep Sea Research Part II. https: // doi. org / 10.1016 / j. dsr 2.2016.12.002"]}

Genus Izikoziphius Bianucci, Lambert & Post, 2007 TYPE SPECIES. ��� Izikoziphius rossi Bianucci, Lambert & Post, 2007 by original designation. OTHER REFERRED SPECIES. ��� Izikoziphius angustus Bianucci, Lambert & Post, 2007., Published as part of Lambert, Olivier, Muizon, Christian de, Duhamel, Guy & Plicht, Johannes Van Der, 2018, Neogene and Quaternary fossil remains of beaked whales (Cetacea, Odontoceti, Ziphiidae) from deep-sea deposits off Crozet and Kerguelen islands, Southern Ocean, pp. 135-160 in Geodiversitas 40 (6) on page 149, DOI: 10.5252/geodiversitas2018v40a6, http://zenodo.org/record/5745694, {"references":["BIANUCCI G., LAMBERT O. & POST K. 2007. - A high diversity in fossil beaked whales (Odontoceti, Ziphiidae) recovered by trawling from the sea floor off South Africa. Geodiversitas 29: 5 - 62."]}

Genus Khoikhoicetus Bianucci, Lambert & Post, 2007 TYPE SPECIES. ��� Khoikhoicetus agulhasis Bianucci, Lambert & Post, 2007 by original designation. OTHER SPECIES INCLUDED. ��� Khoikhoicetus kergueleni n. sp., Published as part of Lambert, Olivier, Muizon, Christian de, Duhamel, Guy & Plicht, Johannes Van Der, 2018, Neogene and Quaternary fossil remains of beaked whales (Cetacea, Odontoceti, Ziphiidae) from deep-sea deposits off Crozet and Kerguelen islands, Southern Ocean, pp. 135-160 in Geodiversitas 40 (6) on page 138, DOI: 10.5252/geodiversitas2018v40a6, http://zenodo.org/record/5745694, {"references":["BIANUCCI G., LAMBERT O. & POST K. 2007. - A high diversity in fossil beaked whales (Odontoceti, Ziphiidae) recovered by trawling from the sea floor off South Africa. Geodiversitas 29: 5 - 62."]}

Genus Xhosacetus Bianucci, Lambert & Post, 2007 TYPE AND ONLY REFERRED SPECIES. ��� Xhosacetus hendeysi Bianucci, Lambert & Post, 2007 by original designation., Published as part of Lambert, Olivier, Muizon, Christian de, Duhamel, Guy & Plicht, Johannes Van Der, 2018, Neogene and Quaternary fossil remains of beaked whales (Cetacea, Odontoceti, Ziphiidae) from deep-sea deposits off Crozet and Kerguelen islands, Southern Ocean, pp. 135-160 in Geodiversitas 40 (6) on page 153, DOI: 10.5252/geodiversitas2018v40a6, http://zenodo.org/record/5745694, {"references":["BIANUCCI G., LAMBERT O. & POST K. 2007. - A high diversity in fossil beaked whales (Odontoceti, Ziphiidae) recovered by trawling from the sea floor off South Africa. Geodiversitas 29: 5 - 62."]}

Pontogeneus priscus Leidy, 1852 HOLOTYPE. — ANSP 13668, isolated middle cervical centrum. TYPE LOCALITY AND HORIZON. — The original description indicates that the holotype is "from the Eocene formation of Ouachita, Louisiana " (Leidy 1852: 52). Recently, Uhen (2005) pointed out the possibility that it comes from the vicinity of the Caldwell Parish, Louisiana, where the upper Eocene Jackson Group (Eargle 1959) is exposed. DISCUSSION Leidy (1852) created the taxon Pontogeneus priscus based on an isolated cervical vertebra (Fig. 4), figured by Kellogg (1936) and interpreted as a fifth cervical. Uhen (2005) recognized it as a more anterior cervical (C3 or C4) and considered both genus and species as nomina nuda because the original description was not diagnostic and the vertebra could belong to a small specimen of Basilosaurus (indeed, the centrum is quite similar in size to that of UM-WH74, a small Basilosaurus isis). However, it can be evaluated that the vertebarterial foramina in the holotype of P. priscus are proportionally larger than in Basilosaurus (Fig. 4). Such a feature is diagnostic for the genus Cynthiacetus (Martínez-Cáceres & Muizon 2011). Because of this character Pontogeneus priscus could have closer affinities with Cynthiacetus than with Basilosaurus (as proposed by Gingerich 2015). However, such a holotype (the centrum of an isolated cervical vertebra) is not regarded as diagnostic enough to characterize a basilosaurid taxon and it is suggested here that the binome Pontogeneus priscus should only refer to the cervical vertebra described by Leidy (1852). Therefore, the genus name and the species name should be regarded as a Basilosauridae or a Pelagiceti incertae sedis and restricted to the type specimen of P. priscus., Published as part of Martínez-Cáceres, Manuel, Lambert, Olivier & Muizon, Christian de, 2017, The anatomy and phylogenetic affinities of Cynthiacetus peruvianus, a large Dorudon-like basilosaurid (Cetacea, Mammalia) from the late Eocene of Peru, pp. 7-163 in Geodiversitas 39 (1) on page 13, DOI: 10.5252/g2017n1a1, http://zenodo.org/record/4535622, {"references":["LEIDY J. 1852. - Description of Pontogeneus priscus. Proceedings of the Academy of Natural Sciences of Philadelphia 6: 52.","UHEN M. D. 2005. - A new genus and species of archaeocete whale from Mississippi. Southeastern Geology 43 (3): 157 - 172.","KELLOGG R. 1936. - A review of the Archaeoceti. Carnegie Institution of Washington Publications 482: 1 - 366.","GINGERICH P. D. 2015. - New partial skeleton and relative brain size in the Late Eocene archaeocete Zygorhiza kochii (Mammalia, Cetacea) from the Pachuta Marl of Alabama, with a note on contemporaneous Pontogeneus brachyspondylus. Contributions from the Museum of Paleontology, University of Michigan 32: 161 - 188. http: // hdl. handle. net / 2027.42 / 113064"]}

Cynthiacetus maxwelli Uhen, 2005 Zeuglodon cf. brachyspondylus – Müller 1849: 26-28. — Kellogg 1936: 248 (in part). HOLOTYPE. — MMNS VP 445. It includes a partially preserved skull, dentaries, upper and lower dentitions, petro-tympanic complex, humerus, radius, ulna, cervical, thoracic and lumbar vertebrae. TYPE LOCALITY. — Cynthia Clay Pit, Hinds County, Mississippi, USA. TYPE HORIZON. — Yazoo Clay Formation, Priabonian, late Eocene. EMENDED DIAGNOSIS. — Cynthiacetus maxwelli differs mainly from C. peruvianus in having one more cusp on both mesial and distal edges on p3 and p4 (5 and 6 denticles respectively in p3 and p4). It also possesses a longer and lower dome-shaped neural arch on the atlas and shorter and more massive humerus, radius and ulna (cf. below)., Published as part of Martínez-Cáceres, Manuel, Lambert, Olivier & Muizon, Christian de, 2017, The anatomy and phylogenetic affinities of Cynthiacetus peruvianus, a large Dorudon-like basilosaurid (Cetacea, Mammalia) from the late Eocene of Peru, pp. 7-163 in Geodiversitas 39 (1) on page 16, DOI: 10.5252/g2017n1a1, http://zenodo.org/record/4535622, {"references":["UHEN M. D. 2005. - A new genus and species of archaeocete whale from Mississippi. Southeastern Geology 43 (3): 157 - 172.","MULLER J. 1849. - Uber die fossilen Reste der Zeuglodonten von Nordamerica, mit Rucksicht auf die europaischen Reste aus dieser Familie. G. Reimer, Berlin, 38 p.","KELLOGG R. 1936. - A review of the Archaeoceti. Carnegie Institution of Washington Publications 482: 1 - 366."]}

Genus Masracetus Gingerich, 2007 TYPE SPECIES. — Masracetus markgrafi Gingerich, 2007 by original designation., Published as part of Martínez-Cáceres, Manuel, Lambert, Olivier & Muizon, Christian de, 2017, The anatomy and phylogenetic affinities of Cynthiacetus peruvianus, a large Dorudon-like basilosaurid (Cetacea, Mammalia) from the late Eocene of Peru, pp. 7-163 in Geodiversitas 39 (1) on page 15, DOI: 10.5252/g2017n1a1, http://zenodo.org/record/4535622, {"references":["GINGERICH P. D. 2007. - Stromerius nidensis, new archaeocete (Mammalia, Cetacea) from the upper Eocene Qasr El-Sagha Formation, Fayum, Egypt. Contributions from the Museum of Paleontology, University of Michigan 31 (13): 363 - 378. http: // hdl. handle. net / 2027.42 / 57499"]}

Martínez-Cáceres, Manuel, Lambert, Olivier, Muizon, Christian de (2017): The anatomy and phylogenetic affinities of Cynthiacetus peruvianus, a large Dorudon-like basilosaurid (Cetacea, Mammalia) from the late Eocene of Peru. Geodiversitas 39 (1): 7-163, DOI: 10.5252/g2017n1a1, URL: http://dx.doi.org/10.5252/g2017n1a1

Genus Pontogeneus Leidy, 1852 TYPE SPECIES. — Pontogeneus priscus Leidy, 1852., Published as part of Martínez-Cáceres, Manuel, Lambert, Olivier & Muizon, Christian de, 2017, The anatomy and phylogenetic affinities of Cynthiacetus peruvianus, a large Dorudon-like basilosaurid (Cetacea, Mammalia) from the late Eocene of Peru, pp. 7-163 in Geodiversitas 39 (1) on page 13, DOI: 10.5252/g2017n1a1, http://zenodo.org/record/4535622, {"references":["LEIDY J. 1852. - Description of Pontogeneus priscus. Proceedings of the Academy of Natural Sciences of Philadelphia 6: 52."]}

Masracetus markgrafi Gingerich, 2007 Zeuglodon isis Stromer, 1908: 128 (in part). Zeuglodon cf. brachyspondylus Müller, 1849:26-28. — Stromer 1908: 136. — Kellogg 1936: 262. — Slijper 1936: 319. Cynthiacetus maxwelli Uhen, 2005: 160 (in part). HOLOTYPE. — SMNS 11414, vertebral column, part of specimen 'St. 8' in Stromer (1908: 129). The associated skull is catalogued as SMNS 11413. TYPE LOCALITY AND HORIZON. — Uhen (2005) pointed out that SMNS 11414 and SMNS 11413 could come from the Gehannam Formation (Bartonian) of the Fayum Province, Egypt. However, Gingerich (2007) follows Kellogg (1936) and indicates that the holotype of M. markgrafi comes from the Birket Qarun Formation, in the vicinity of Dimeh, also in the Fayum Province. This formation is regarded as early Priabonian in age by Gingerich (1992). DIAGNOSIS. — Large basilosaurid lacking the vertebral elongation observed in Basilosaurus. Masracetus is slightly larger than Cynthiacetus with a lower lumbar length-to-width ratio Llum / Wlum (Gingerich 2007). DISCUSSION The holotype of Masracetus markgrafi was initially referred to Z. brachyspondylus (Stromer 1908) and more recently to C. maxwelli (Uhen 2005). Gingerich (2007) pointed out that Masracetus differs from other basilosaurids in the size and proportions of its lumbar vertebrae. Based on this feature, Gingerich (2007) hypothesizes that Pontogeneus brachyspondylus could be referred to Masracetus. Because a comparison of lumbar ratios in a large sampling of basilosaurid specimens (Table 2) does not clearly distinguish Cynthiacetus from Masracetus, the differences between the two genera could very well represent intra-specific variation (when compared to the variation observed in D. atrox and Z. kochii). Until study of the most complete specimen of M. markgrafi (collected in 2006: Gingerich 2007) clarifies the validity of both Masracetus and M. markgrafi, we follow Gingerich (2007) and regard M. markragi as a different species from Cynthiacetus spp., Published as part of Martínez-Cáceres, Manuel, Lambert, Olivier & Muizon, Christian de, 2017, The anatomy and phylogenetic affinities of Cynthiacetus peruvianus, a large Dorudon-like basilosaurid (Cetacea, Mammalia) from the late Eocene of Peru, pp. 7-163 in Geodiversitas 39 (1) on page 15, DOI: 10.5252/g2017n1a1, http://zenodo.org/record/4535622, {"references":["GINGERICH P. D. 2007. - Stromerius nidensis, new archaeocete (Mammalia, Cetacea) from the upper Eocene Qasr El-Sagha Formation, Fayum, Egypt. Contributions from the Museum of Paleontology, University of Michigan 31 (13): 363 - 378. http: // hdl. handle. net / 2027.42 / 57499","STROMER E. 1908. - Die Archaeoceti des agyptischen Eozans. Beitrage zur Palaontolgie und Geologie Ostrerreich- Ungarns und des Orients 21: 106 - 178.","MULLER J. 1849. - Uber die fossilen Reste der Zeuglodonten von Nordamerica, mit Rucksicht auf die europaischen Reste aus dieser Familie. G. Reimer, Berlin, 38 p.","KELLOGG R. 1936. - A review of the Archaeoceti. Carnegie Institution of Washington Publications 482: 1 - 366.","SLIJPER E. J. 1936. - Die Cetaceen. Capita Zoologica 7: 1 - 590.","UHEN M. D. 2005. - A new genus and species of archaeocete whale from Mississippi. Southeastern Geology 43 (3): 157 - 172.","GINGERICH P. D. 1992. - Marine mammals (Cetacea and Sirenia) from the Eocene of Gebel Mokattam and Fayum, Egypt: stratigraphy, age, and paleoenvironments. University of Michigan Papers on Paleontology 30: 1 - 84. http: // hdl. handle. net / 2027.42 / 48630"]}

FAMILY ODOBENOCETOPSIDAE MUIZON, 1993 Type and only included genus: Odobenocetops Muizon, 1993. Diagnosis of the family: As for the type genus., Published as part of Muizon, Christian de & Domning, Daryl P., 2002, The anatomy of Odobenocetops (Delphinoidea, Mammalia), the walrus-like dolphin from the Pliocene of Peru and its palaeobiological implications, pp. 423-452 in Zoological Journal of the Linnean Society 134 (4) on page 424, DOI: 10.1046/j.1096-3642.2002.00015.x, http://zenodo.org/record/4687167

ODOBENOCETOPS LEPTODON MUIZON, DOMNING & PARRISH, 1999 (FIGS 1���12, TABLES 1 AND 2) Holotype: SMNK PAL 2492, an incomplete skull (lacking most of the dorsal and right sides of the braincase, right orbit, and right side of the right alveolar sheath), with the right ear bones and the atlas. Referred specimen: MNHN SAO 202, partial skeleton with partial skull lacking the right tusk and with the alveolar sheaths very damaged. Most of the braincase has been weathered away but the left periotic and a partial tympanic are preserved in situ. The postcranial skeleton is very poorly preserved, comprising only a few hyoid fragments, 26 partial vertebrae, several rib fragments, the distal half of the left humerus, most of the left radius, a proximal portion of the ulna lacking the olecranon, and fragments of carpals and metacarpals. MNHN PPI 249, a right periotic is referred to O. cf. leptodon . Etymology of the species name: from Greek: leptos (thin) and odon (tooth), in reference to the needle-like morphology of the large right tusk. Diagnosis: Odobenocetops leptodon differs from O. peruvianus in the morphology of the snout, which is more rounded and wider in dorsal view; in the lack of large premaxillary foramina; in the presence of small depressions for premaxillary sacs in the premaxillae, anterior to the bony nares; in the presence of supplementary rostral bones at the anterodorsal apex of the snout; in the broader, deeper, and longer palate; in the anterior border of the palate, which is more U-shaped, while it is more V-shaped in O. peruvianus; and in the straight (or slightly concave) anterodorsal border of the orbit, which is deeply concave in O. peruvianus. Locality, Horizon and Age: the holotype and MNHN SAO 202 are from the SAO level of the Pisco Formation in Peru (at approximately km 540 of the Panamerican Highway south of Lima). The SAO horizon has been defined by de Muizon & Bellon (1980) but see also de Muizon (1981), de Muizon & DeVries (1985), and de Muizon & Bellon (1986). Its age spans approximately from 4 to 3 Ma; therefore, the specimens are early Pliocene in age. The SAO level of the Pisco Formation is slightly younger than the SAS level (5���4 Ma), which yielded the holotype of O. peruvianus and the third specimen referred to here as a female of O. peruvianus. MNHN PPI 249, an isolated periotic, is from beds of the Pisco Formation located at c. 3 km east of Yauca (these beds may be slightly younger than those of the SAO horizon and may belong to the late Pliocene). Skull Basicondylar length: 415e Bizygomatic width: 150 �� 2 = 300e Minimum width of the skull posterior to the postorbital processes: 70 �� 2 = 140e Maximum width between the postorbital processes: 130 �� 2 = 260 Maximum width of both nares: 59e Maximum length of the left naris: 53.5e Width of the choanae: 104 Periotic Maximum width of the skull at the level of the exoccipitals: 123.5 �� 2 = 257e Anteroposterior length: 77.5e Maximum width (measured ventrally, approximately perpendicular to anteroposterior length: 42e Maximum width between the pterygoid crests: 142 Width between the apices of the hamular processes: 112 Length of pars cochlearis from contact with anterior process anteriorly to medial edge of aqueductus cochleae (measured dorsally): 29.5 Tympanic Length of the lateral lobe of the left tympanic: 54.2 Atlas Maximum width of the pars cochlearis in medial view: 30.8 Maximum width of the atlas: 174 Length of internal auditory meatus: 14.3 Maximum height of the atlas at sagittal plane: 106 Maximum vertical diameter of neural canal: 64.5 Maximum horizontal diameter of neural canal: 56.5 Limb Tympanic Length of the involucrum (measured medially): 58.8 Malleus Maximum length in posteromedial view (cf. Fig. 10A): 11.4 Maximum length of the radius: 117 Anteroposterior length of distal end of humerus: 60.1 Mediolateral width of distal end of humerus: 43 Maximum width in posteromedial view (cf. Fig. 10A): 6.05 DESCRIPTION The following description focuses on the differences between the two species of Odobenocetops. We refer to de Muizon et al. (2002) for the aspects of the cranial anatomy common to the two species. GENERAL FEATURES The general morphology of the skull of Odobenocetops leptodon is similar to that of O. peruvianus in its basic pattern (Figs 1���4). However, in dorsal view, the former differs from the latter in the morphology of the snout, which is much wider and more blunt. In anterior view the snout was more massive. Furthermore, if a melon was actually present, as hypothesized below, it is likely that the head of O. leptodon was more rounded than that of O. peruvianus. The skull of O. leptodon has the same bilobate outline as is observed in O. peruvianus, which is unique in a Neogene odontocete (de Muizon et al., 2002). This condition, where the frontals and maxillae do not overhang the temporal fossa, is found in primitive odontocetes such as Xenorophus, Agorophius and Archaeodelphis and in archaeocetes because of the lesser telescoping of the skull (Kellogg, 1936; Fordyce, 1981). In Odobenocetops it is regarded as a reversal of this typically cetacean telescoping. In general, the skull of O. leptodon is very heavily built and the bone is much denser, thicker, and more solid than in other delphinoids, being almost osteosclerotic in its posteroventrolateral angle. PREMAXILLA The premaxillae of O. leptodon are built on the same pattern as those of O. peruvianus (de Muizon et al., 2002). They both present a large alveolar process housing one tusk, the right tusk (of the presumed male, which is the only sex represented) being much larger than the left. Since the left alveolar process is not preserved on the holotype of O. peruvianus (the only specimen known when this species was first described) and because there are no other mammals which exhibit pronounced cranial asymmetry, it was hypothesized that the processes sheathing the tusks were both of the same size and that, consequently, the left tusk (which was judged to be shorter than the right sheath) was not erupted. The holotype of O. leptodon, however, indicates that the left sheath was clearly smaller than the right and that the left tusk was actually erupted. It is therefore probable that the condition of O. leptodon was also present in O. peruvianus. In the holotype of O. leptodon, the left sheath is transversely flattened and lies almost in a parasagittal plane (it diverges only slightly laterally). The right sheath is thicker because of the larger size of the tusk and diverges more markedly laterally. The region of the sheaths anterior to the tusks is strongly flattened and forms a posteriorly bent U-shaped lamina which borders the palate anteriorly. The anteroventral edge of this lamina is 0.5���1 cm thick, and presents longitudinal irregular ridges of bone to which was firmly attached the strong upper lip that was very probably present in O. leptodon as in O. peruvianus. The anterior and anterolateral side of each premaxillary sheath is a broad convex surface bearing several large grooves and foramina. These structures indicate an extensive vascularization of the anterior region of the skull, which is regarded here as related to the presence of a thick, muscular upper lip. In anterior view, in the sagittal plane, the snout is more elevated than in O. peruvianus and the surface of attachment of the upper lip is larger. Furthermore, the snout is wider in anterior view, and the anterior edge of the palate is grossly U-shaped rather than more Vshaped as in O. peruvianus. Therefore it is clear that the upper lip of O. leptodon was much larger than in O. peruvianus, indicating a larger vibrissal array and/or better suction ability in the former than in the latter. Dorsally, the morphology of the premaxillae of O. leptodon presents the major characters which differentiate this species from O. peruvianus. These bones are much wider than in O. peruvianus, and the right premaxilla is more expanded laterally because of the greater size of the tusk (Figs 2A and 3A). There are no enlarged premaxillary foramina as are observed in the other odontocetes, including O. peruvianus. This absence is regarded as a loss in O. leptodon which, in this respect, is more derived than O. peruvianus. The premaxillary foramina transmit branches of the internal maxillary artery and maxillary nerve from the infraorbital foramen (internal to the orbit) to parts of the nasofacial muscles anteriorly and posteriorly on the rostrum (see e.g. Schenkkan, 1973: fig. 5, on Mesoplodon bidens). In Odobenocetops these vessels and nerves are probably related to the irrigation and innervation of the strong upper lips and associated musculature as well as the possible vibrissae. In O. leptodon the canals conducting the nerves and vessels do not open dorsally but are clearly present. This is observed on the referred skull MNHN SAO 202 (Fig. 3A), in which the apex of the snout is eroded above the pulp cavity of the tusks. The canals passed above the proximal extremities of the tusks and apparently (clearly observable on the right side) divided into several smaller canals perforating the premaxillae to reach the upper lip anteriorly. Ramifications of the premaxillary canals are also observable on the lateral side of the premaxilla at the level of the antorbital notch, where several large foramina are present. It is likely that irrigation and innervation for the pulp cavities of the tusks were also transported by the premaxillary canals. The premaxillae of O. leptodon bear a small concave area, anterior to the bony nares, where lay the premaxillary sacs of the air sac system (Mead, 1975). These fossae for the premaxillary sacs are limited medially by the medial border of the premaxilla which overhangs the vomerian gutter; they are limited anteromedially by the suture with the rostral supplementary bone (see below); and anterolaterally they are medial to the posterior edge of each tusk alveolus (Figs 2A, 3A and 5). Fossae for premaxillary sacs are absent in O. peruvianus, where the space between the large premaxillary foramina and the anterior edge of the nares is so reduced that it was supposed that the premaxillary sacs were either very reduced or absent in this species (de Muizon et al., 2002). On the dorsal face of the skull the premaxillae of O. leptodon extend further posteriorly than in O. peruvianus. The posterior extremity of the left premaxilla is damaged on MNHN SAO 202, but it is clear that it lay slightly posterior to the posterior border of the nares. The posterior extremity of the right premaxilla lies about 5 cm posterior to the nares. On the holotype of O. peruvianus, the posterior extremity of the left premaxilla is located half-way between the anterior and posterior limits of the naris and on the right side (as well as on both sides of SMNK PAL 2491) it reaches the posterior edge of the naris. The Pmx-Mx suture is external to the nares, the border of which is formed by the maxillae only, in contrast to the condition of O. peruvianus in which the Pmx-Mx suture is internal to the nares in the anterior two-thirds of their length. In their dorsal portions the premaxillae of O. leptodon are relatively flat and not as salient as is observed in O. peruvianus. This feature is clearly observed on the lateral edges of the nares and at the level of the antorbital notch, where the premaxillae markedly overhang the maxillae in O. peruvianus, whereas the relief of these regions is much smoother in O. leptodon. The anterior sides of the premaxillae of O. leptodon (SMNK PAL 2492) bear strong foramina and grooves, which indicates an abundant vascularization related to the development of the upper lip. The premaxillae are much broader and much more elevated than in O. peruvianus, again indicating a larger and stronger upper lip (Figs 1B and 3C). The premaxillary sheaths, which house the tusks, are partly preserved on SMNK PAL 2492. The right is complete and the left is broken, apparently no more than 2 or 3 cm from the apex. Contrary to what was hypothesized in the case of O. peruvianus, the sheaths are not identical in size; the left one is clearly smaller and shorter than the right and reflects the size difference between the tusks. In lateral view, the right alveolar process forms an angle of c. 60�� with the dorsal plane of the skull (from the vertex to the anterodorsal tip of the rostrum). The anteroventral edges of the sheaths (which include the anterior edge of the palate) are thickened and corrugated, as is observed in O. peruvianus, which indicates attachments for strong, possibly horny, connective tissue for the attachment of the upper lip. This narrow edge also bears (in both species) a series of foramina, which could have been related to a fringe of vibrissae. In the style of feeding we have hypothesized, these could have allowed the animal to detect when the front edges of the sheaths were in contact with the sea floor. The anteroventral crest of the premaxillae (Figs 1B and 3C) has a very wide U-shape whereas it is more V-shaped in O. peruvianus. This morphology is the result of the widening of the snout and is possibly related to the presence of the supplementary bones described below. The U-shaped anterior border of the palate is not perpendicular to the sagittal plane. In other words, in anterior view, if the sagittal plane is positioned vertically (based on pterygoids, basioccipital and vomerian groove), the plane of the anterior crest of the palate dips towards the right side of the skull. This is because the anteroventral edge of the smaller and less laterally divergent left sheath is dorsal to (more elevated than) that of the right sheath. This condition implies that when the animal was feeding (i.e. with the anterior border of the palate parallel to the sea floor; see de Muizon, 1993b and de Muizon et al., 2002), the sagittal plane was inclined toward the left side of the animal (i.e. dipping toward the right side). Therefore, because of the asymmetry of the premaxillae, O. leptodon was feeding slightly inclined on left side. The premaxillae form the anterior region of the palate as is observed in O. peruvianus. Although the palate of O. leptodon is much broader than in O. peruvianus, the area occupied by the premaxillae in this part of the skull is only slightly larger than in O. peruvianus (see below). ROSTRAL BONES One of the most striking differentiating characters of O. leptodon is the presence of two supplementary bones (one on each side) anterior to the premaxillae at the anterodorsal apex of the snout. These bones are apparently appressed against the medial edges of the premaxillae and lie partly on the dorsal part of the skull and partly on the anterior slope of the snout. The dorsal portion is narrower than long but broadens anteriorly. It is limited posteriorly by the fossa for the premaxillary sac of the premaxilla, while medially it has a small contact with the medial portion of the maxilla on the dorsal face of the rostrum (Figs 1B, 2A, 3A and 5). Within the vomerian groove, the rostral supplementary bone is bordered by the medial portion of the maxilla, but the edge of the groove is formed by the rostral bone itself. The lateral suture is directed anteroposteriorly and is located just medial to the tusk alveoli. The anterior portion is not observable on MNHN SAO 202 but it is in relatively good condition on the left side of SMNK PAL 2492. It is also relatively narrow and forms a strip of bone overlying the premaxilla, tapering ventrally, and reaching (or almost reaching) the ventromedial border of the premaxilla at the anterior edge of the palate. Its medial suture is with the vomer. These supplementary bones are in fact wedged (as far as the dorsal portion is concerned) between the tusks laterally and the vomer and maxilla medially. They are partly responsible for the breadth of the snout in O. leptodon and tend to separate the proximal extremities of the tusks and alveoli (i.e. the premaxillae). As a probable consequence of their presence, the tusks are less divergent distally than in O. peruvianus and the anterior border of the palate is Ushaped in O. leptodon rather than more V-shaped as in O. peruvianus. Apparently, as is observed on the left premaxilla of SMNK PAL 2492, the rostral bones were anterodorsally protruding, forming an elevated prominence at the apex of the snout. This condition is not observable on MNHN SAO 202 since the bones have been partly eroded in their apical portions. These supplementary bones could be ossa prenasalia, cartilage bones which appear late in development (mainly in postnatal life) and which are not uncommon in mammals. They are paired or unpaired bones which articulate with the nasals or premaxillae and which are always related to the presence of a rhinarium (Rhynchocyon, Sus, Talpa, Chiroptera) or a proboscis (Starck, 1967). In the case of O. leptodon the supplementary bones present at the anterodorsal apex of the snout could be related to the thick and powerful upper lip used by Odobenocetops to forage on the bottom in search of food (de Muizon, 1993a;b; de Muizon et al., 2002). They could also indicate of the presence of a very sensitive tactile organ, with or without vibrissae (see de Muizon, 1993a), much more developed in O. leptodon than in O. peruvianus. It is likely that strong muscles for the upper lip of O. leptodon were attaching on the (?)prenasalia. However, it is noteworthy that the prenasalia of other mammals are generally not tightly articulated with the premaxillae and/or the nasals, and not distinctly paired. If these supplementary bones are not prenasalia, another alternative which could explain the condition of O. leptodon is the presence of septomaxillae. These bones are present in crossopterygians, lissamphibians, squamates, in all nonmammalian therapsids, and in all the nontherian mammals except multituberculates. The septomaxilla is absent in marsupials and placentals except in Xenarthra (Wible, 1990; Zeller et al., 1993). However, we are reluctant to interpret the supplementary bones of O. leptodon as septomaxillae, since the study of the development of the nasal capsule of odontocetes (Klima & Van Bree, 1985; Klima et al., 1986; Klima, 1987) does not show any indication that such bones are present at any developmental stage in this group. A third alternative is to regard the supplementary bones of O. leptodon as neomorphic elements different from prenasalia and septomaxillae, possibly related to the muscle attachment of the lip, which was probably much stronger than in O. peruvianus. TUSKS The tusks of O. leptodon are similar to those of O. peruvianus (Figs 1A,B). They are much better preserved in SMNH PAL 2492 than in the ho, Published as part of Muizon, Christian de & Domning, Daryl P., 2002, The anatomy of Odobenocetops (Delphinoidea, Mammalia), the walrus-like dolphin from the Pliocene of Peru and its palaeobiological implications, pp. 423-452 in Zoological Journal of the Linnean Society 134 (4) on pages 425-432, DOI: 10.1046/j.1096-3642.2002.00015.x, http://zenodo.org/record/4687167, {"references":["de Muizon C, Domning DP, Parrish M. 1999. Dimorphic tusks and adaptive strategies in a new species of walrus-like dolphin (Odobenocetopsidae) from the Pliocene of Peru. Comptes-rendus de l'Academie des Sciences, Paris, Sciences de la Terre et des Planetes 329: 449 - 455.","de Muizon C, Bellon H. 1980. L'age mio-pliocene de la Formation Pisco (Perou). Comptes Rendus de l'Academie des Sciences, ser. D 290: 1063 - 1066.","de Muizon C. 1981. Les vertebres fossiles de la formation Pisco (Perou). Premiere partie: Deux nouveaux Monachinae (Phocidae, Mammalia) du Pliocene de Sud-Sacaco. Travaux de l'Institut Francais d'Etudes Andines 22: 1 - 160.","de Muizon C, DeVries TJ. 1985. Geology and paleontology of late Cenozoic marine deposits in the Sacaco area (Peru). Geologisches Rundschau 74: 547 - 563.","de Muizon C, Bellon H. 1986. Nouvelles donnees sur l'age de la Formation Pisco (Perou). Comptes Rendus des Seances de l'Academie des Sciences, Ser. II (303): 1401 - 1404.","de Muizon C, Domning DP, Ketten DR. 2002. Odobenocetops peruvianus, the walrus-convergent delphinoid (Cetacea, Mammalia) from the lower Pliocene of Peru. Smithsonian Contribution to Paleobiology 93: in press.","Kellogg, AR. 1936. A review of the Archaeoceti. Carnegie Institute of Washington Publication 482: 1 - 366.","Fordyce RE. 1981. Systematics of the odontocete Agorophius pygmaeus and the family Agorophiidae (Mammalia: Cetacea). Journal of Paleontology 55: 1028 - 1045.","Schenkkan EJ. 1973. On the comparative anatomy and function of the nasal tracts in odontocetes (Mammalia, Cetacea). Bijdragen tot de Dierkunde 43: 127 - 159.","Mead JG. 1975. Anatomy of the external nasal passages and facial complex in the Delphinidae (Mammalia, Cetacea). Smithsonian Contributions to Zoology 207: 1 - 72.","de Muizon C. 1993 b. Odobenocetops peruvianus, una remarcable convergencia de adaptacion alimentaria. Bulletin de l'Institut Francais d'Etudes Andines 22: 671 - 683.","Starck D. 1967. Le crane des mammiferes. In: Grasse, PP, ed. Traite de Zoologie. Paris: Masson, 405 - 549.","de Muizon C. 1993 a. Walrus-like feeding adaptation in a new cetacean from the Pliocene of Peru. Nature 365: 745 - 748.","Wible JR. 1990. Petrosals of Late Cretaceous marsupials from North America, and a cladistic analysis of the petrosal in therian mammals. Journal of Vertebrate Paleontology 10: 183 - 205.","Zeller U, Wible JR, Elsner M. 1993. New ontogenetic evidence on the septomaxilla of Tamandua and Choloepus (Mammalia, Xenarthra), with a re-evaluation of the homology of the mammalian septomaxilla. Journal of Mammalian Evolution 1: 31 - 46.","Klima M, van Bree PJ. 1985. Uberzahlige Skeletelemente im Nasenschadel von Phocoena phocoena und die Entwicklung der Nasenregion bei den Zahnwalen. Gegenbaurs Morphologisches Jahrbuch 131: 131 - 178.","Klima M, Seel M, Deimer P. 1986. Die Entwicklung des hochspezialisierten Nasenschadels beim Pottwal (Physeter macrocephalus). Gegenbaurs Morphologisches Jahrbuch 132: 245 - 284.","Klima M. 1987. Morphogenesis of the nasal structures of the skull in toothed whales (Odontoceti). In: Kuhn H-J, Zeller U, eds. Morphogenesis of the Mammalian Skull. Mammalia Depicta 13: 105 - 121.","Hay KA, Mansfield AW. 1989. Narwhal, Monodon monoceros Linnaeus, 1758. In: Ridgway, SH, Harrison R, eds. Handbook of Marine Mammals, Vol. 4: River Dolphins and the Larger Toothed Whales. London: Academic Press, 145 - 176.","Porsild MP. 1922. Scattered observations on narwhals. Journal of Mammalogy 3: 8 - 13.","Miller ME, Christensen GC, Evans HE. 1964. Anatomy of the dog. Philadelphia: W. B. Saunders."]}

Muizon, Christian de, Domning, Daryl P. (2002): The anatomy of Odobenocetops (Delphinoidea, Mammalia), the walrus-like dolphin from the Pliocene of Peru and its palaeobiological implications. Zoological Journal of the Linnean Society 134 (4): 423-452, DOI: 10.1046/j.1096-3642.2002.00015.x, URL: https://academic.oup.com/zoolinnean/article-lookup/doi/10.1046/j.1096-3642.2002.00015.x

ODOBENOCETOPS PERUVIANUS MUIZON 1993 (FIGURES 13���15, TABLE 3) A second skull of Odobenocetops peruvianus has been discovered in the earliest Pliocene beds of the Pisco Formation at Sud-Sacaco. This specimen is of special interest because it is more complete than the holotype and especially because it bears two small tusks similar in size to the small tusk of the holotype (USNM 488252) and has therefore been regarded as a female of O. peruvianus (de Muizon et al., 1999). In the following discussion, we do not exhaustively describe the new skull, but simply describe the new features O. peruvianus provided by the new specimen. Holotype: USNM 488252, an incomplete skull lacking much of the left side and all of the auditory bones. Referred specimens: SMNK PAL 2491, a relatively complete skull which lacks only the posterodorsal roof of the braincase, and which still retains the ear bones in situ; MNHN SAS 1613, a left periotic and partial tympanic; and MNHN SAS 1614, a left periotic. Emended diagnosis: Odobenocetops peruvianus differs from O. leptodon in its snout, which is narrower at its apex and tapers dorsally (although the apex is still rounded); in the presence of large premaxillary foramina, anterior to the nares; in the absence of depressions for premaxillary sacs; in the shorter and shallower palate (especially in the anterior region); in the anterior border of the palate which is more Vshaped whereas it is more U-shaped in O. leptodon; and in the deeply notched anterodorsal edge of the orbit, which is slightly concave in O. leptodon. DESCRIPTION TUSKS AND PREMAXILLAE The major differences between the new specimen and the holotype are in the size of the tusks and morphology of the premaxillae. Both tusks are small and the left tusk is slightly smaller in diameter than the right. Neither of the tusks is complete. The left tusk is broken inside the sheath; however, the right tusk is only lacking its apex and, as preserved, is 23 cm long and is probably missing a few cm only. Therefore, the right tusk was very close to the length as well as the diameter of the small left tusk of O. leptodon. Although the tusk is not complete, given the length of the tusk (as preserved) and the sheath, it is certain that it was erupted. The premaxillary sheaths are small, and, in agreement with the sizes of the tusks, the left one is smaller than the right. Because of this slight asymmetry and although the apices of the left sheath and tusk are not preserved, it is likely that the left tusk was also erupted. Furthermore, the walls of the left sheath at its apex are very thin, which suggests that very little of the bone is missing. The apex of the right sheath is not completely preserved either, but given the thickness of the bone at the broken extremity, it is probable that no more than a few centimeters are lacking. Basicondylar length: 412 Bizygomatic width: 158 �� 2 = 316e Minimum width of the skull posterior to the postorbital processes: 116 The dorsal aspect of the premaxillae is very similar to that of the holotype although slightly more symmetrical. The posterior extension of the right prem axilla behind the nares is only slightly greater than that of the left, contrary to the condition of the holotype, in which the posterolateral process of the left premaxilla reaches the middle of the lateral edge of the left naris, whereas the right process almost reaches the posterior edge of the right naris. Since the premaxillae of SMNK PAL 2491 project posteriorly slightly behind the bony naris, this feature shows some variation within the species. The asymmetry of the tusks and alveolar processes may thus be reflected in the asymmetry of the posterior processes of the premaxillae. However, this inference must be made cautiously since most odontocetes (including monodontids) present an asymmetry of the dorsal bones of the skull, the right premaxilla and maxilla extending farther posteriorly than the left and slightly displacing the vertex on the left side of the skull. Nevertheless, the asymmetry of the male narwhal does not reflect the tusk asymmetry since the right maxilla is slightly more expanded posteriorly than the left, which carries the large tusk. Minimum width between the dorsal edges of the orbits: 123 Maximum width between the postorbital processes: 226 Width of the snout between the antorbital notches: 87 Maximum width of both nares: 60 Maximum length of the right naris: 51.5 Width of the choanae: 96 Maximum width of the skull at the level of the exoccipitals: 250 Bicondylar width: 143 Width between the apices of the hamular processes: 128 Length of the lateral lobe of the left tympanic: 48.5 In SMNK PAL 2491 the snout is very complete and the structure of the bone is perfectly preserved. Therefore, muscle scars and foramina are clearly observable and provide important data. Anterior to the large premaxillary foramina is a rough, triangular surface, distinct on each bone, which reaches the apex of the snout. This area corresponds to the attachment scar for the nasal plug muscle in the other odontocetes. These muscle scars are so well marked that it is unlikely that there was no nasal plug muscle in O. peruvianus. It is therefore very probable that O. peruvianus had nasal plugs, which is to be expected in an odontocete, but was not clear on the holotype (de Muizon et al., 2002). Posterior to the premaxillary foramina are two short saddle-shaped grooves, which form the anterior edges of the bony nares. The morphology of the new specimen confirms the conclusions of de Muizon et al. (2002) that, if premaxillary sacs (which are located between the premaxillary foramina and the anterior edge of the nares) were present in Odobenocetops peruvianus, they were very reduced, probably vestigial. Therefore, and considering the reduced space on the dorsal face of the rostrum, it is highly probable that O. peruvianus had no (or only a vestigial) melon. Although the nasal plugs, premaxillary sacs and melon are tightly related organs, the well-developed muscle scars for the insertion of the nasal plug muscle force us to conclude that O. peruvianus exhibited a strong reduction of the two latter organs but little reduction of the former. On the anterior face of the snout the premaxillae bear several large foramina indicative of an extensive vascularization of the snout related to the inferred presence of a large and strong upper lip. The irregular surface of the bone in this region corresponds to the attachment of a very robust musculature of the lip. The morphology of the much better preserved female specimen of O. peruvianus (SMNK PAL 2491) confirms the hypotheses based on the holotype of the species. MAXILLAE AND FRONTALS Posterior to the nares, between the mesethmoid and the maxillae, are two small triangular medial portions of the frontals, which have the same condition as the maxillae in relation to the premaxillae at the anterior region of the nares. In other words, the maxillae, which lay on the frontals, have withdrawn posterior to the nares, uncovering the frontals. Posteriorly, the medial portions of the frontals become thin and progressively disappear after the maxillae become joined. Further posteriorly, the medial edges of the maxillae separate again and uncover a thin strip of the frontals which widens posteriorly. On the holotype it is probable that the lower portion of the mesethmoid posterior to (and overhung by) the Y-shaped portion which separates the nares (see de Muizon et al., 2002) is in fact the medial portion of the frontal observed on the female skull fused to the mesethmoid. A medial portion of the frontal is clearly present in O. leptodon (MNHN SAO 202). The extensions of the maxillae posterior to the nares narrow posteriorly but are, like the premaxillae and tusks, relatively symmetrical, whereas in the male the right maxilla is wider and extends further posteriorly than the left. The preorbital process of the frontal of the female skull is anteriorly elongated and blade-like, as in the holotype. However, in this specimen it is clear that both the maxilla medially and the frontal laterally are components of the process. Therefore, although highly modified, the preorbital process of O. peruvianus resembles that of the other odontocetes in the combined participation of the maxilla and frontal. The postorbital process is similar in shape to that of the holotype but is larger and longer, indicating some individual variation of this character. A clear asymmetry of the frontal is present, as is observed in the other odontocetes, in which the right supraorbital process of the frontal is distinctly larger than the left. PALATINE, PTERYGOID AND VOMER The vomer is very wide, the maxillae do not participate in the construction of the palate, and the premaxillae and the palatines have a large V-shaped suture on both sides as is observed in the holotype. The posterior region of the palate is perfectly preserved and adds to the knowledge of the species. In the posterior region of the palate, the medial edges of the palatines are in contact, hiding the vomer for a few centimetres of its length. Further posteriorly the palatines separate again, uncovering the vomer ventrally. The median ridge of the vomer projects about 7 cm posterior to the posterior edges of the palatines. The sutures with the pterygoids are widely separated. The apices of the pterygoid hamuli are sharply pointed and extend toward the anterior angle of the basioccipital crests formed by the medial laminae of the pterygoids. They are almost in contact on the right side and the anterior end of the left medial lamina is apparently broken. Because of this projection and the long nonpalatal ridge of the vomer, the palate of O. peruvianus is clearly shorter than in O. leptodon and its posterior border is strongly concave. The pterygoid hamulus is transversely compressed and the lateral lamina of the pterygoid almost contacts the medial lamina. Therefore, the fossa for the pterygoid sinus is extremely reduced (as observed by de Muizon et al., 2002 on the holotype) and hardly present in O. peruvianus. It is clearly more reduced than in O. leptodon, in which the fossa is very small but conspicuous. As in O. leptodon, there is a short but continuous lateral lamina of the pterygoid, which contacts the alisphenoid, contrary to the condition in Recent delphinoids. However, in Recent monodontids, a continuous lateral lamina of the pterygoid, although almost absent from the pterygoid hamulus, is sometimes present and contacts the squamosal dorsally. ALISPHENOID The alisphenoid is similar to that of O. leptodon. It is very thick and bears an enormous foramen ovale not confluent with the cranial hiatus. de Muizon et al. (2002) stated that in the holotype of O. peruvianus the foramen ovale was confluent with the cranial hiatus. Because this is not the case in the three other skulls referred to Odobenocetops, it is possible that the condition observed in the holotype is due to a postmortem breakage of the alisphenoid. As in O. leptodon the external opening of the foramen ovale is formed anteriorly by the pterygoid and posteriorly by the alisphenoid. Internally, the foramen ovale is fully enclosed in the alisphenoid. It is noteworthy, however, that this condition is not easy to observe because of the partial fusion of the bones. SQUAMOSAL AND EAR BONES The squamosal of SMNK PAL 2491 is basically similar to that of the holotype. The tympanic and the periotic are preserved in situ on both sides of the specimen (Fig. 15). Since the bones could not be removed from the skull it is not possible to describe them thoroughly. However, it is clear that they are extremely similar to those referred to the species by de Muizon et al. (2002). The lateral face of the tympanic bears a large sigmoid process which is grossly perpendicular to the anteroposterior axis of the bone and not shifted posteriorly as in the classical (i.e. non-odobenocetopsid) delphinoids. The apex of the process is straight, rounded, and not strongly recurved anteriorly as is observed in the classical delphinoids. Posterior to it is a low conical process and a relatively large anterior border of the posterior process. These three elements surround the external auditory meatus and, as a consequence of their morphology, the meatus is much larger than in the classical delphinoids. It is slightly smaller than in O. leptodon (SMNK PAL 2492). This condition would indicate a more efficient (effective) tympanic membrane in the Odobenocetopsidae and therefore possibly better sound perception via the middle ear than in the other odontocetes. The lateral wall of the tympanic is high, as is observed in O. leptodon. This condition indicates that the middle ear cavity was larger than in the classical delphinoids. It resembles the condition of most early diverging odontocetes (Squalodontidae, Squalodelphinidae, Platanistidae, Eurhinodelphinidae), and could represent a plesiomorphic condition or, more likely, a reversal corresponding to a need for enlargement of the middle ear cavity. The anterior extremity of the tympanic is blunt and rounded, as is observed in the other delphinoids. The ventral lobes are well separated by a deep median groove which is well-marked on the posterior two-thirds of the bone. Their posterior extremities are rounded and wide and the medial lobe is slightly posterior to the lateral. This condition is similar to that of O. leptodon but clearly differs from that of the Recent monodontids, in which the posterior extremity of the medial lobe is narrower and more anterior than that of the lateral lobe. This feature is especially obvious in Monodon. It is also present to a lesser extent in the delphinids and phocoenids. Again, the odobenocetopsid condition is found in the early diverging odontocetes mentioned above, but is regarded here as a reversal (rather than a plesiomorphy). Not much can be said about the periotic, which is preserved in situ, except that it has the massive morphology observed on the referred specimens described by de Muizon et al. (2002). Contrary to what is observed in the classical delphinoids, the lateral portion of the bone is closely appressed against the squamosal, in this respect resembling the condition observed in the platanistoids (sensu de Muizon, 1987; 1991; 1994; Fordyce & Barnes, 1994; Fordyce et al., 1994; Fordyce, 1994; not sensu McKenna & Bell, 1997). Furthermore, the posterior process of the periotic is deeply wedged between the squamosal and the exoccipital, which is in agreement with the long posterior process observed on MNHN SAS 1614 (an isolated periotic from the same horizon as the holotype of O. peruvianus and referred to this species). A similar condition is present in Delphinapterus (Kasuya, 1973) and Monodon. The delphinids and phocoenids are characterized by a petrotympanic completely isolated from the skull and maintained in situ by ligaments only (Fraser & Purves, 1960). Early derived odontocetes such as platanistoids have a tympanic and a periotic still articulated with the squamosal, which is regarded as a plesiomophic character state (de Muizon, 1987; 1991; 1994). Therefore, the condition of Odobenocetops and the Monodontidae could be regarded as plesiomorphic. However, because they have synapomorphies of the Delphinoidea absent in the Platanistoidea, we rather consider the condition of Odobenocetops as a reversal due to its hyperspecialization. The fact that a condition approaching this one is also found in monodontids could represent a synapomorphy of the two families, possibly related to sound transmission. In general, the periotic and tympanic of Odobenocetops have relatively tight and extensive contacts with the surrounding bones: basioccipital and squamosal for the tympanic, and squamosal and exoccipital for the periotic. The lateral tuberosity and rim of the anterior process of the periotic are closely appressed against the squamosal in a similar pattern to that observed in the platanistoids. The squamosal in this region is greatly thickened and forms the medial border of the trough-like glenoid cavity. On the anterior region of the anterior process of the periotic are several independent bony nodules which are appressed against the anterior process and apparently part of it. Such a condition has been observed in O. leptodon and is also present sometimes in Platanista. BASIOCCIPITAL The basioccipital is extremely wide and basically similar to that of O. leptodon although slightly wider. The basioccipital crests are large and undergird the tympanics, which are partially imbedded in their lateral face. The posteromedial faces of the basioccipital crests bear very large and deep muscular scars for insertion of the scalenus, at least as strong as in O. leptodon. EXOCCIPITAL The paroccipital process is very large and stout. It differs from that of the holotype in being orientated ventrally and not ventrolaterally. Its ventromedial angle contacts the posteroventral angle of the basioccipital crest and encloses the jugular notch to form a relatively large window, as is observed in O. leptodon. The exoccipital border of the window is very thick and forms an elevated crest. This indicates a strong scalenus dorsalis, which is inserted on the posteroventral face of the paroccipital process. This part of the occipital indicates an exceptionally strong musculature of the neck as was noted in regard to the holotype of Odobenocetops peruvianus (de Muizon et al., 2002). The anterior face of the paroccipital process does not present any fossa for the posterior sinus such as is observed in many other delphinoids. Fraser & Purves (1960) have stated that the absence of a fossa is not equivalent to absence of a posterior sinus. As a matter of fact, in the Recent monodontids the anterior face of the paroccipital process is not excavated by a fossa and a small posterior sinus is present. However, the presence of a large sinus is related to a deep fossa. In SMNK PAL 2491 the anterior face of the paroccipital process has a reduced area and is not indicative of a large posterior sinus. This statement contrasts with the observations made on the holotype of O. peruvianus (de Muizon et al., 2002). The occipital condyles are more convex and salient than in any recent delphinoid and confirm the great mobility of the neck which was noted with regard to the holotype. In fact, the occipital condyles form a quarter of a sphere (Figs 13B,D and 14) and are proportionally larger and more convex than in any other cetacean (Recent or fossil) or in the walrus. The morphology of the condyles of SMNK PAL 2491 therefore indicates an exceptional mobility of the neck in Odobenocetops; certainly much greater than that of any other odontocete., Published as part of Muizon, Christian de & Domning, Daryl P., 2002, The anatomy of Odobenocetops (Delphinoidea, Mammalia), the walrus-like dolphin from the Pliocene of Peru and its palaeobiological implications, pp. 423-452 in Zoological Journal of the Linnean Society 134 (4) on pages 439-444, DOI: 10.1046/j.1096-3642.2002.00015.x, http://zenodo.org/record/4687167, {"references":["de Muizon C, Domning DP, Parrish M. 1999. Dimorphic tusks and adaptive strategies in a new species of walrus-like dolphin (Odobenocetopsidae) from the Pliocene of Peru. Comptes-rendus de l'Academie des Sciences, Paris, Sciences de la Terre et des Planetes 329: 449 - 455.","de Muizon C, Domning DP, Ketten DR. 2002. Odobenocetops peruvianus, the walrus-convergent delphinoid (Cetacea, Mammalia) from the lower Pliocene of Peru. Smithsonian Contribution to Paleobiology 93: in press.","de Muizon C. 1987. The affinities of Notocetus vanbenedeni, an early Miocene platanistoid (Cetacea, Mammalia) from Patagonia, southern Argentina. American Museum Novitates 2904: 1 - 27.","de Muizon C. 1991. A new ziphiid (Odontoceti, Mammalia) from the Early Miocene of Washington state (USA) and a phylogenetical analysis of the major groups of odontocetes. Bulletin du Museum national d'Histoire naturelle, 4 e ser., Sect. C 12: 279 - 326.","de Muizon C. 1994. Are the squalodonts related to the platanistoids? In: Berta A, Demere TA, eds. Contributions in Marine Mammalogy Honoring Frank C. Whitmore Jr. Proceedings of the San Diego Society of Natural History 29: 135 - 146.","McKenna MC, Bell SK. 1997. Classification of Mammals Above the Species Level. New York: Columbia University Press.","Kasuya T. 1973. Systematic consideration of Recent toothed whales based on the morphology of the tympano-periotic bone. Scientific Reports of the Whales Research Institute 25: 1 - 103.","Fraser FC, Purves PE. 1960. Hearing in cetaceans: evolution of the accessory air sacs and the structure and function of the outer and middle ear in recent cetaceans. Bulletin of the British Museum of Natural History (Zoology) 7: 1 - 140."]}

Bouetel, Virginie, Muizon, Christian de (2006): The anatomy and relationships of Piscobalaena nana (Cetacea, Mysticeti), a Cetotheriidae s.s. from the early Pliocene of Peru. Geodiversitas 28 (2): 319-395, DOI: http://doi.org/10.5281/zenodo.5376036

Bouetel, Virginie, Muizon, Christian de (2006): The anatomy and relationships of Piscobalaena nana (Cetacea, Mysticeti), a Cetotheriidae s.s. from the early Pliocene of Peru. Geodiversitas 28 (2): 319-395, DOI: http://doi.org/10.5281/zenodo.5376036, {"references":["BARNES L. G. & MCLEOD S. A. 1984. - The fossil record 1984, 1988; Barnes 1985a, b). According to this, and phyletic relationships of gray whales, in JONES it seems that the Cetotheriidae s.s. have a relatively M. L., SWARTZ S. L. & LEATHERWOOD S. (eds), The wide paleogeographic distribution, although mainly Gray Whale: Eschrichtius robustus. Academic Press, located in the northern hemisphere. Orlando: 3-32.","H. 1994. - Classification and distribution of the","Oligocene Aetiocetidae (Mammalia; Cetacea; Mys- Acknowledgements ticeti) from western North America and Japan. The Special thanks are due to R. E. Fordyce (University Island Arc 3: 392-431. of Otago, Dunedin, New Zealand) and anony- BENHAM W. B. 1937. - Fossil Cetacea of New Zealand. mous reviewer for their very careful work, which II. On Lophocephalus, a new genus of zeuglodont","Cetacea. Transactions of the Royal Society of New greatly helped in improving the initial version of Zealand 67: 1-7. this work. Specimens were collected by CM with BENHAM W. B. 1939. - Mauicetus: a fossil whale. funds of the Institut francais d'Etudes andines Nature 143: 765. (IFEA) and of the Museum national d'Histoire BENHAM W. B. 1942. - Fossil Cetacea of New Zeanaturelle (MNHN). The \"Societe des Amis du land. V. Mauicetus, a generic name substituted for","Lophocetus Benham. Transactions of the Royal Society Museum national d'Histoire naturelle et du Jar- of New Zealand 71: 260-270. din des Plantes\" provided financial support to BOUETEL V. 2005. - Phylogenetic implications of VB for two short term visits to the USNM and skull structure and feeding behavior in balaenopto the LACM. E. Frey (Staatlische Museum fur terids (Cetacea, Mysticeti). Journal of Mammalogy Naturkunde, Karlsruhe, Germany) gave CM access 86 (1): 139-146.","BRANDT J. F. 1843. - De Cetotherio, novo balaenarum to the holotype of Piscobalaena nana and provided, familiae genere, in Rossia meridionali anti aliquot anas usual, an extremely helpful and kind attention. nos effoso. Bulletin de l'Academie imperiale des Sciences M. Elstrup kindly provided original photographs de St Petersbourg (2) 1: 145-148. of the holotypes of Cetotherium rathkei and Metopo- BRANDT J. F. 1872. - Bericht uber den bereits vollencetus durinasus. Thanks are due also to L. G. Barnes deten, druckfertigen Theil seiner Untersuchungen uber die fossilen und subfossilen Cetaceen Europas. (LA county Museum), D. Bohaska (Department Compte rendu de l'Academie imperiale des Sciences de of Paleobiology, USNM), J. G. Mead and C. W. St Petersbourg tome 17: 407-408. Potter (Marine Mammal Program, USNM), who BRANDT J. F. 1873. - Untersuchungen uber die fosprovided VB access to the specimens of fossil and silen und subfossilen Cetaceen Europas. Memoires de Recent mysticetes under their care. Photographs l'Academie de St Petersbourg 20 (1), ser. 7: 1-371.","CABRERA A. 1926. - Cetaceos fosiles del Museo de la are by P. Loubry and D. Serrette (MNHN); line Plata. Revista del Museo de la Plata 29: 363-411. drawings are by VB except Figure 10, which is by CAPELLINI G. 1901. - Balenottera miocenica del monte F. Pilard. titano, Republica di S. Marino. Memorie della reale","COPE E. D. 1896. - Sixth contribution to the knowledge GEISLER J. H. & SANDERS A. E. 2003. - Morphologiof the Miocene fauna of North Carolina. Proceed- cal evidence for the phylogeny of Cetacea. Journal of ings of the American Philosophical Society 35 (151): Mammalian Evolution 10: 23-129.","139-146. HASEGAWA Y., NOKARIYA H., SATO J. & OISHI M. CUMMINGS W. C. 1985. - The right whales Eubalae- 1985. - Part III. Fossil whale, the 1st specimen na glacialis (Muller 1776) and Eubalaena australis from Maesawa-cho, Iwate Prefecture, Japan,in OISHI (Desmoulins, 1822), in RIDGWAY S. H. & SIR HAR- M., ONO K., KAWAKAMY T., SATO J., NOKARIYA RISON R. (eds), Handbook of Marine Mammals. Aca- H. & HASHEGAWA Y. (eds), Pliocene baleen whales demic Press, New York: 275-304. and bony-toothed bird from Iwate Prefecture, Japan DOOLEY A. C. JR., FRASER N. C. & LUO Z. 2004. - The (Parts I-VI). Bulletin of the Iwate Prefectural Museum earliest known member of the rorqual-Gray whale 3: 148-150.","clade (Mammalia, Cetacea). Journal of Vertebrate HATAI K., HAYASAKA S. & MASUDA K. 1963. - Some Paleontology 24 (2): 453-463. fossil tympanics from the Mizuno Period of northern EMLONG D. R. 1966. - A new archaic cetacean from Japan. Saito Ho-on Kai Museum of Natural History, the Oligocene of Northwest Oregon. Bulletin of the Research Bulletin 32: 5-17.","Museum of Natural History, University of Oregon ICHISCHIMA H. 1997. - Systematics of the Latest Oligocene 3: 1-51. to the Earliest Miocene Mysticeti from New Zealand. FORDYCE R. E. 1980. - Whale evolution and Oligocene Dissertation, University of Otago, Dunedin, New Southern Ocean environments. Palaeogeography Pal- Zealand, xx + 352 p.","aeoclimatology Palaeoecology 55: 319-336. KELLOGG R. 1924. - Description of a new genus and FORDYCE R. E. 1989. - Origins and evolution of Ant- species of whalebone whale from the Calvert Cliffs arctic marine mammals. Special Publication of the Maryland. Proceedings of the United States National Geological Society of London 47: 269-281. Museum 63: 1-14.","FORDYCE R. E. 1992. - Mauicetus and late Oligocene KELLOGG A. R. 1928. - History of whales - their adapbaleen-bearing primitive Mysticeti from the South- tation to life in the water. Quarterly Review of Biology west Pacific, in Abstracts, 29th International Geological 3: 29-76, 174-208.","Congress, Kyoto, Japan 2: 349. KELLOGG R. 1929. - A new cetothere from south- FORDYCE R. E. 1994. - Waipatia maerewhenua, new ern California. University of California Publications, genus and new species (Waipatiidae, new family), an Bulletin of the Department of Geological Sciences 18: archaic late Oligocene dolphin (Cetacea: Odontoceti: 449-457.","Platanistoidea) from New Zealand. Proceedings of the KELLOGG R. 1931. - Pelagic mammals from the Tem- San Diego Museum of Natural History 29: 147-176. blor formation of the Kern River region, California. FORDYCE R. E. & MUIZON C.DE 2001. - Evolutionary Proceedings of the Californian Academy of Science series history of cetaceans: a review, in MAZIN J. M. & BUF- 4, 19: 217-297.","FRENIL V. DE (eds), Secondary Adaptation of Tetrapods KELLOGG R. 1934. - The patagonian fossil whalebone to Life in Water. Pfeil Verlag, Munich: 169-233. whale Cetotherium moreni (Lydekker). Carnegie Insti- FORDYCE R. E. & WATSON A. G. 1998. - Vertebral tution of Washington Publication 447: 64-81."]}