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1 July 2016 A New Extinct Inioid (Cetacea, Odontoceti) from the Upper Miocene Senhata Formation, Chiba, Central Japan: The First Record of Inioidea from the North Pacific Ocean
Mizuki Murakami
Author Affiliations +
Abstract

Inioidea, which consists of Iniidae and Pontoporiidae, includes only four extant species, all of which occur in South America. Inioids were, however, more diversified and widely distributed in the past, starting from the late middle Miocene, as recorded from the eastern Pacific, North and South Atlantic, North Sea and the riverine systems of South America. In this paper, the author describes a new extinct inioid, Awadelphis hirayamai gen. et sp. nov. (WU SILS G 408), from the uppermost Miocene Senhata Formation (6.3–5.7 Ma) of Chiba, central Japan. Morphological cladistic analysis supports placement of this new taxon within the Inioidea. The new species is characterized by a right premaxillary eminence with a pronounced overhang on the right maxilla and a short zygomatic process of the squamosal. This is the first fossil record of an inioid not only from Japan but also from the North Pacific. The discovery of this new taxon considerably extends the paleobiogeographic range of inioids, and indicates that inioids co-occurred with other small odontocetes of the western North Pacific, where delphinoids dominated. The new species is also the first valid occurrence of a small odontocete from tropical—subtropical climates of the late Miocene in the western North Pacific.

Introduction

Inioidea, which consists of Iniidae and Pontoporiidae, includes only two genera and four extant species, all of which occur in South America (Best and da Silva, 1993; Costa-Urrutia et al., 2012; Hrbek et al., 2014). Inia spp. are distributed in the Amazon, Orinoco, and Araguaia-Tocantins rivers (Hrbek et al., 2014), and Pontoporia is distributed along the South Atlantic coastal waters of Brazil, Uruguay, and Argentina (Jefferson et al., 2011). Inioids were, however, more diversified and more widely distributed in the past, beginning in the late middle Miocene, as recorded from the eastern Pacific, North and South Atlantic, North sea, and the riverine systems of South America. Most of our knowledge about inioids outside South America has arisen in the last decade (Lambert and Post, 2005; Ahrens, 2005; Pyenson and Hoch, 2007; Gibson and Geisler, 2009; Bianucci et al., 2011; Geisler et al., 2012; Pyenson et al., 2015), although inioid fossils were first discovered in the nineteenth century within the riverine systems of South America and along its South Atlantic coast (Burmeister, 1871, 1885; Ameghino, 1891). These records indicate that inioids were a significant component of the small odontocete fauna from the late Miocene to the early Pliocene, after archaic odontocetes (e.g. kentriodontids, eurhinodelphinids, and most platanistoids) became extinct or declined steeply in taxonomic diversity (e.g. Fordyce and Barnes, 1994). However, the reliable fossil record of Inioidea in the Pacific Ocean is restricted to Peru and Chile (Muizon, 1983, 1984, 1988b; Gutstein et al., 2009; Lambert and Muizon, 2013). Here, however, the author describes the first fossil record of an inioid (WU SILS G 408) from the uppermost Miocene Senhata Formation (6.3–5.7 Ma) of Chiba, central Japan. This discovery considerably extends the paleobiogeographic range of inioids and indicates that they were a constituent of the small odontocete fauna of the western North Pacific, which was dominated by Delphinoidea.

Institutional Abbreviations.—CMM, Calvert Marine Museum, Solomons, Maryland, USA; MACN, Museo Argentino de Ciencias Naturales, Buenos Aires, Argentina; MNHN, Muséum National d'Histoire Naturelle, Paris, France; NMNS, National Museum of Nature and Science, Tokyo, Japan; WU SILS, Waseda University School of International Liberal Studies, Tokyo, Japan.

Anatomical terms.—The anatomical terminology of the skull and ear bones follows that of Mead and Fordyce (2009).

Geological setting

The middle Miocene to upper Pliocene Miura Group is well exposed on the Boso and Miura peninsulas of central Japan where its maximum thickness is over 4000 m (e.g. Suzuki et al., 1995; Figure 1A–D). These deposits are interpreted as forearc basin deposits that accumulated in response to a northward to northwestward subduction of the Philippine Sea Plate beneath the Eurasia Plate at the Sagami Trough (Ito and Masuda, 1988). The Miura Group is a shallowing-upward succession comprised of turbiditic sandstones, hemipelagic mudstones, and coarse-grained shallow marine deposits (e.g. Nakajima et al., 1981; Ito et al., 2002).

The Senhata Formation occupies and gains exposure within the mid-western Boso Peninsula (Figure 1C, D). The formation overlies the Amatsu Formation (unconformably in western exposures and conformably in eastern exposures) and is overlain by the Inakozawa Formation (Yabe and Hirayama, 1998). The Senhata Formation is 1.5–130 m thick, and consists of massive fine-medium-grained sandstone with tuff, calcareous sandstone, pebbly sandstone, and conglomerate (Yabe and Hirayama, 1998). The lithofacies comprise three sedimentary facies: shoreface, lower shoreface to inner shelf, and shelf to upper slope (Ito et al., 2002). The molluscan fauna of the formation consists of a mixed assemblage of intertidal to bathyal species (e.g. O'Hara and Ito, 1980; Tomida, 1989). The shallow marine assemblages are allochthonous and the deeper assemblages are autochthonous (O'Hara and Ito, 1980; Tomida, 1989). The selachian fauna of the formation is also composed of neritic, epipelagic, and bathypelagic species. Consequently, the sedimentary environments inferred from the facies and both the invertebrate and vertebrate fauna are mutually consistent.

The Miura Group of the Boso Peninsula contains about 300 key tephra beds. The Senhata Formation is located between the key tephra beds Am 68 and Am 69 (Nakajima and Watanabe, 2005). However, the ages of these key beds are unknown. Am 61 and Am 78 are the nearest key tephra beds to the Senhata Formation whose ages are known, and their fission track ages are 6.3 ± 0.5 Ma and 5.7 ± 0.4 Ma, respectively (Tokuhashi et al., 2000). Consequently, the maximum age of the formation is 6.3–5.7 Ma.

The Senhata Formation yields abundant marine bivalves, gastropods (e.g. Tomida, 1983), argonautid cephalopods (Tomida, 1989), crustaceans (e.g. Kato, 2002), cetaceans (Takahashi, 1954), a pinniped (Kohno, 1992), and selachians (Uyeno et al., 1990; Yabe and Hirayama, 1998). The marine invertebrate fauna indicates that the paleoclimate was tropical to subtropical (e.g. Tomida, 1983, 1989).

Systematic paleontology

Order Cetacea Brisson, 1762
Suborder Odontoceti Flower, 1867
Superfamily Inioidea Gray, 1846
Or Inioidea incertae fam.
Awadelphis gen. nov.

  • Type and only known species.—Awadelphis hirayamai sp. nov.

  • Diagnosis.—Inioidea with the following synapomorphies in the current cladistic analysis: U-shaped external bony nares; premaxillary eminence. Differing from all inioids by having a pronounced laterally overhanging premaxillary eminence; the anteroposteriorly long cleft not widely separating the posteromedial splint and the posterolateral plates; ascending process of maxilla standing perpendicular towards supraorbital process of maxilla; prominent, dorsoventrally thick nasal; strongly narrowing maxilla, posterior to the level of the postorbital process; transversely narrow temporal fossa; and very short zygomatic process of the squamosal.

  • Etymology.—From the Japanese, Awa (ancient name of southern Chiba Prefecture, the region where the holotype was collected); and the Greek delphis, the dolphin.

  • Awadelphis hirayamai sp. nov.
    Figures 27

  • Holotype.—WU SILS G 408; a partial skull, two isolated teeth, and a rib fragment.

  • Diagnosis.—As for the genus.

  • Etymology.—The species name honors Ren Hirayama, who collected the holotype and contributed to the study of vertebrate paleontology in Japan for most of his career.

  • Locality.—The holotype was collected by Ren Hirayama at a quarry owned by Towa Stone Limited, Kyonan, Chiba, central Japan (35°09′40″ N, 139°51′51″ E; Figure 1). The specimen was discovered in a light-gray medium grained tuffaceous sandstone derived from the upper part of the uppermost Miocene Senhata Formation.

  • Figure 1.

    The type locality (A–C) of Awadelphis hirayamai gen. et sp. nov., WU SILS G 408, age of the Senhata Formation (D), and the columnar section of the type locality (E). The geological map and stratigraphy were modified from Suzuki et al. (1995). Fission track ages of key tephra beds on Tokuhashi et al. (2000). The columnar section was modified from Tomida (1989).

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    Description

    Skull

    The posterior part of the skull is distorted leftwards and compressed mediolaterally, representing the effects of postmortem deformation. The precise length of the neurocranium is uncertain because the occipital condyle is missing, as is the area near the antorbital notch, and the skull is distorted. However, the length of the neurocranium would not have been more than 200 mm. The vertex is low and skewed leftward.

    Premaxilla.—The right premaxilla anterior to the premaxillary foramen (including rostrum) has weathered away (Figure 2). The medial part of the premaxillary eminence is convex and low. The premaxillary eminence is extremely wide; the ratio of the greatest width of the premaxillae to the greatest width of the maxillae at the level of the postorbital processes is 0.58. The lateral edge of the premaxilla is elevated above the maxilla by 22.2 mm, and is 6.6 mm lateral to the premaxilla-maxilla suture. The posterolateral sulcus is absent in the preserved region of the premaxillary eminence, as in Protophocaena minima Abel, 1905 and Auroracetus bakerae Gibson and Geisler, 2009. This may suggest that the posterolateral sulcus is very short or hidden by the laterally extended premaxillary eminence (as in all extinct phocoenids and the pontoporiid Auroracetus bakerae).

    Figure 2.

    Dorsal view of the skull of Awadelphis hirayamai gen. et sp. nov., WU SILS G 408. A, photograph whitened with ammonium chloride; B, corresponding illustration of A.

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    The posterior part of the premaxilla adjacent to the external bony nares is concave. This region may correspond to the fossa of the inferior vestibule lateral to the premaxilla as in Delphinoidea (but their homology is uncertain). The premaxilla extends distinctly posterior to the external bony nares. The premaxilla contacts the nasal (contact length, 21.3 mm). The premaxilla extends further posteriorly (9.5 mm) from the posterior extremity of the contact point with the nasal. The premaxillary cleft (length, 18.2 mm) is well developed at the posterior edge of the premaxilla; the cleft divides at the posterior edge of the premaxilla into the posterolateral plate of the premaxilla and the posteromedial splint of the premaxilla. The premaxillary crest is absent.

    Maxilla.—The supraorbital process of the maxilla, posterior to the antorbital process and to the level of the anterior part of the temporal fossa, is flat and the posterior portion is inclined gently upward (Figure 3). However, the medial part of the maxilla is steeply inclined laterally, and the maxilla is almost perpendicular to the horizontal line near the suture line with the premaxilla (Figures 2, 3). The posterior dorsal infraorbital foramen is located at the base of this perpendicular wall. The posterior dorsal infraorbital foramen near the posterior edge of the premaxilla, pneumatic maxillary crest, and supracranial basin are absent. It is uncertain whether the maxillary ridge was present or absent in the original skull because the anterior part of the maxilla is missing.

    In dorsal view, the maxilla becomes very narrow posterior to the level of the postorbital process (Figure 2). However, this dimension may have been affected by postmortem deformation. Gaps (max. 8 mm) exist between the posterior end of the ascending process of the maxilla and the frontal, or parietal. This dimension was also affected by postmortem distortion.

    Lacrimal.—The dorsolateral edge of the internal opening of the infraorbital foramen is formed by the maxilla and lacrimal (Figure 5). The fossa for the preorbital lobe of the pterygoid sinus is widely developed on the lacrimal.

    Vomer.—The mesorostral groove widens anterolaterally anterior to the external bony nares (Figures 2, 4). Both the premaxilla and maxilla are partly weathered, and are inferred to converge by at least the posterior end of the mesorostral groove based on the morphology of the vomer. The external bony nares are U-shaped. The vomer appears between the nasal spines of the palatines in ventral view.

    Nasal.—In dorsal view, the right nasal is triangular and elongate anteroposteriorly (Figure 2A, B). The ratio of the transverse width of the nasal to its maximum length is 0.48. The anterior edge of the nasal is located at the level of the postorbital process of the frontal and extends almost straight transversely. There is a smoothsurfaced fossa on the anterolateral surface of the nasal (Figures 2, 4A). The lateral edge of the nasal does not overhang the premaxilla. The medial part of the nasal is decidedly convex dorsally and higher than the frontal in lateral and anterior views (Figures 3, 4). The nasal protuberance is absent.

    Frontal.—On the vertex, the anterior edge of the frontal wedges between the posteromedial edge of the nasals (Figure 2). The vertex portion of the frontal posterior to the nasal is transversely narrower than the nasal, but diverges posteriorly. The frontal boss is absent. The postorbital process of the frontal and dorsal half of the orbit are incomplete (Figure 3). The origin of the temporal muscle is situated on the posteroventral face of the postorbital process and is directed roughly ventrally.

    In ventral view, the infratemporal crest is prominently developed; it inclines steeply anteriorly and posteriorly (Figures 5, 6). The dorsal development of the fossa for the preorbital lobe of the pterygoid sinus toward the fronto-maxillary suture is absent. The fossa for the postorbital lobe of the pterygoid sinus is well developed posterior to the infratemporal crest, and extends to the dorsoanterior part of the temporal fossa (Figures 3, 5, 6). The fossa for the postorbital lobe of the pterygoid sinus is extended very widely anteroposteriorly (64.6 mm) and dorsoventrally (52.6 mm).

    Palatine.—The lateral lamina of the palatine is incomplete but extends at least to the anterior edge of the line of the orbitosphenoid. The fossa for the hamular process is also anteroposteriorly long (Figures 5, 6), although it is partially covered by broken bone. The palatine forms the posterior part of the nasal cavity.

    Pterygoid.—The possible hamular process of the pterygoid is developed and extends posteroventrally onto the palate (Figure 5). The mid-lateral part of it is convex, and regions anterior and posterior to the area are flat. The lateral lamina of the pterygoid is not preserved. However, the possible articulated surface of the alisphenoid with the lateral lamina of the pterygoid is preserved (Figures 5, 6).

    Supraoccipital.—In dorsal view, the supraoccipital is strongly convex anteriorly (Figure 2). The supraoccipital is completely fused with the interparietal, and the latter is not distinguishable from the former. The nuchal crest is not developed.

    Figure 3.

    Right lateral view of the skull of Awadelphis hirayamai gen. et sp. nov., WU SILS G 408. A, photograph whitened with ammonium chloride; B, corresponding illustration of A.

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    Figure 4.

    The holotype of the skull of Awadelphis hirayamai gen. et sp. nov., WU SILS G 408 in antero-dorsal (A, C) and posterior views (B, D). A, B, photograph whitened with ammonium chloride; C, D, corresponding illustration of A, B.

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    Figure 5.

    Ventral view of the skull of Awadelphis hirayamai gen. et sp. nov., WU SILS G 408. A, photograph whitened with ammonium chloride; B, corresponding illustration of A.

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    Figure 6.

    Ventrolateral view of the skull of Awadelphis hirayamai gen. et sp. nov., WU SILS G 408. A, photograph whitened with ammonium chloride; B, corresponding illustration of A.

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    Parietal.—In dorsal view, the parietal is visible only as a triangular area between the frontal and the supraoccipital (Figure 2). The parietal is not fused with the frontal or the supraoccipital. Consequently, this individual might have died young. The lateral surface of the parietal is not convex because of postmortem deformation (Figure 4B). The temporal fossa is anteroposteriorly long relative to its height (Figure 3, Table 1). The suture between the parietal and the squamosal is irregular and nearly horizontal.

    Exoccipital.—The jugular notch between the paroccipital process of the exoccipital and the basioccipital crest is wide in posterior view (Figure 4B). In ventral view, the anterior surface of the paroccipital concavity for the posterolateral sinus is weakly concave (Figure 6). The posterior edge of the paroccipital process is located well anterior to the posterior edge of the occipital condyle (not preserved).

    Orbitosphenoid.—The frontal groove is wide anteroposteriorly (width, 17 mm) and is very short (18.6 mm, Figures 5, 6). The orbitosphenoid does not contact the lacrimal.

    Alisphenoid.—The narrow strip-like area of the alisphenoid is visible on the ventral edge of the temporal fossa in lateral view (Figure 3). The alisphenoid probably articulated with the lateral lamina of the palatine, as inferred from the articular process on the alisphenoid just posterior to the inferior and superior orbital fissures (Figures 5, 6). The pterygoid sinus fossa is well developed on the alisphenoid (Figures 5, 6). The groove for the mandibular branch of the trigeminal nerve is directed laterally and is located entirely posterior to the pterygoid sinus fossa. The cranial hiatus is widely developed posterior to the alisphenoid. This condition also suggests that this individual was young as in Delphinidae (Dawbin et al., 1970). A transverse ridge is absent.

    Squamosal.—The zygomatic process of the squamosal is short anteroposteriorly (Figure 3). The length of the zygomatic process of the squamosal, as a percent of the greatest width of the maxilla at the postorbital process, is less than 30%; Therefore, only a small area of the process is visible in dorsal view. The process is directed anteroposteriorly. The tip of the process is narrow dorsoventrally and directed anterodorsally in lateral view. A large gap (length, 8 mm) is present between the zygomatic process and the postorbital process of the frontal. The dorsal surface of the zygomatic process is concealed in lateral view by the well developed supramastoid crest. The ventral surface of the zygomatic process of the squamosal is concave. The fossa between the supramastoid crest and the parietal margin of the squamosal is deep (Figure 3). The tip of the retroarticular process of the squamosal is rectangular. The ventral projection of the retroarticular process and the post-tympanic process are at the same level. The external acoustic meatus is narrow. The sternomastoid and mastohumeral fossae are deep.

    Table 1.

    Measurements (mm) of the skull of Awadelphis hirayamai gen. et sp. nov., WU SILS G 408. *indicates estimated transverse measurements that are half-skull measurements multiplied by 2. + indicates incomplete to some extent.

    t01_207.gif

    In ventral view, the tympanosquamosal recess is deep and bifurcate (Figure 5). The medial part of the recess extends almost to the anterior tip of the squamosal. The mandibular fossa is shallow, narrow, and long. The falciform process is triangular and short (9.7 mm). The medial surface of the process is tightly articulated with the alisphenoid, but not with the lateral lamina of the pterygoid (Figures 5, 6).

    Basioccipital.—The left basioccipital is incomplete, whereas the right basioccipital is complete but distorted ventrally. The basioccipital crest widens gradually. The muscular tubercle is indistinct.

    Tooth.—An isolated tooth is preserved (Figure 7A). The crown is conical and the tip of the crown probably curves labially, and the distal tip of the root is twisted proximally. The crown is smooth in the dorsal part, but short striations are developed in its ventral part. The entire height of the tooth is 13.9 mm, and the crown height is 5.5 mm. The greatest width of the crown is 2.9 mm proximodistally and 2.8 mm labiolingually. The root of the tooth is slightly bulbous (3.4 mm proximodistally and 3.2 mm labiolingually).

    Postcranial skeleton

    Rib.—The first right rib, which is missing the neck and most of the shaft, is preserved (Figure 7B). The medial edge of the rib from the neck to the proximal part of the shaft is strongly curved. The neck and the tubercle are at nearly an acute angle, whereas the tubercle and the shaft form an angle of 120°.

    Figure 7.

    Photographs of an isolated tooth (A) and the left first rib whitened with ammonium chloride (B) of Awadelphis hirayamai gen. et sp. nov., WU SILS G 408. A, proximal or distal view; B, anterior view.

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    Cladistic analysis

    Inioidea is generally divided into two families: Iniidae and Pontoporiidae (e.g. Muizon, 1988a; Geisler et al., 2012). Muizon (1988a) considered Brachydelphis as a member of Brachydelphinae (sic) in Pontoporiidae. On the other hand, Cozzuol (2010) elevated Brachydelphinae (sic) as Brachydelphidae (sic), a more basal clade than Iniidae + Pontoporiidae. The studies by Muizon (1988a) and Cozzuol (2010) were not based on computer-assisted cladistic analyses. Furthermore, Geisler et al. (2012) found both Pontoporiinae and Brachydelphininae to be paraphyletic, and the extinct iniid Ischyrorhynchus vanbenedeni Ameghino, 1891 fell in tentatively with Platanistidae. Pontoporiidae is also paraphyletic in Pyenson et al. (2015). Thus, phylogenetic relationships within Inioidea are unresolved and controversial.

    Materials and methods.—A cladistic analysis was performed to determine the phylogenetic position of Awadelphis hirayamai gen. et sp. nov. For this purpose, A. hirayamai and Meherrinia isoni were added (Appendix 1) as OTUs (operational taxonomic units) to the matrix of Murakami et al. (2014). I considered a total of 85 ingroup taxa, and the stem cetacean Georgiacetus was used as the outgroup taxon in the present cladistic analysis. 282 characters from Murakami et al. (2014) were used in this analysis. However, the status of many characters was revised by new observations of the specimens or a reevaluation of a character's status. The character data were entered into a character-taxon matrix using Mesquite 2.74 (Maddison and Maddison, 2010). The cladistic analysis was performed with TNT 1.1 (Goloboff et al., 2008). All characters were treated as unweighted and unordered. The heuristic searches was performed with the Sectional Search and Tree Fusing option with 1000 replicates. The decay index (Bremer, 1994) for the strict consensus tree was used to measure node stability.

    Results and discussion.—The phylogenetic analysis found three most parsimonious trees with a length of 1888 steps. Awadelphis hirayamai is included in Delphinida. Then, Awadelphis belongs to Inioidea and forms a clade with Brachydelphis (Figure 8). Pliopontos is not the sister clade of Pontoporia. The later taxon is located as the sister taxon of Inia. Meherrinia is the most basal taxon of Inioidea in this cladistic analysis. Therefore, the monophyly of Iniidae and Pontoporiidae is not supported in the present analysis, as in Cozzuol (2010) or Geisler et al. (2012).

    Delphinida is supported by 12 synapomorphies in the present cladistic analysis. Except for six unscored characters, Awadelphis shares five of the six synapomorphies with other Delphinida: joined premaxillae (or maxillae) closing at least at the posterior part of the mesorostral groove [character 66, state 1; Lambert, 2005]—hereafter, character number and its apomorphic state in the present analysis are indicated by text in square brackets; smoothsurfaced fossa on the anterior to anterolateral surface of the nasal [89-1]; absence of the synvertex [96-1]; reduced or absent subtemporal crest [127-1]; and presence of a large deep fossa in a posterior portion of the periotic fossa [151-3]. One of the synapomorphies is absent in Awadelphis; absence of the emargination of the posterior edge of the zygomatic process by the sternomastoid muscle fossa [110-0]. The six unscored synapomorphies in Awadelphis are as follows: combined anteroposterior length of the lacrimal and jugal exposure that is posterior to the antorbital notch forms >69% of the anteroposterior distance from the antorbital notch to the postorbital ridge [42-3]; anterior level of the pterygoid sinus fossa extends beyond the level of the antorbital notch [131-1]; lateral tuberosity without a marked transverse groove in lateral view, continuous with the anterior process of the periotic [169-1; Lambert, 2005]; nearly flat dorsal surface of the periotic in lateral view [176-2]; in dorsal view, the aperture for the vestibular aqueduct is located more lateral than the tractus spiralis foraminosus [186-1]; and the transverse processes of the lumbar vertebrae are triangular [224-1; Muizon, 1984]. Awadelphis has the lateral lamina of the palatine and a well developed tympanosquamosal recess. Furthermore, the nasal process of the premaxilla in Awadelphis extends posteriorly beyond the anterior edge of the nasal. These characters are treated as synapomorphies of Delphinida in past cladistic analyses (e.g. Muizon, 1988a; Fordyce, 1994; Lambert, 2005). Therefore, these characters also suggest that Awadelphis be assigned to the Delphinida.

    Figure 8.

    Morphological cladistic analysis of Inioidea and phylogenetic position of Awadelphis hirayamai gen. et sp. nov. The heuristic searches was performed with the Sectional Search and Tree Fusing option with 1000 replicates. Decay indices are indicated above the nodes.

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    Awadelphis falls within Inioidea because it share the following synapomorphies of that clade: presence of Ushaped bony nares [67-1; Muizon, 1988a] presence of the premaxillary eminence [74-1 and 2; Muizon, 1988a]. The former character was suggested by Muizon (1988a) to be a synapomorphy of Inioidea, although Gutstein et al. (2009) argued that the status of this character is polymorphic in Brachydelphis mazeassi (the present cladistic analysis also treated it as a polymorphic state in Brachydelphis). The later character is regarded as a synapomorphy of Inioidea in Muizon (1988a). Inioidea is supported by 13 unequivocal synapomorphies in Geisler et al. (2012), although they noted only eight synapomorphies. Awadelphis has two of the eight synapomorphies (narrow nasal bones and premaxillary eminence). Awadelphis does not possess one synapomorphy (posterior end of the premaxilla separated from the margin of the external bony nares), and five synapomorphies cannot be scored. However, the posterior end of the premaxilla is also not separated from the margin of the external bony nares in two pontoporiids (Brachydelphis and Pontistes rectifrons Bravard, 1858). Awadelphis and two pontoporiids, Pontistes and Pliopontos littoralis Muizon, 1983, have a posterolateral plate and a posteromedial splint of the premaxillae and premaxillary cleft. Some specimens of Pontoporia and Inia also possess the concavity on the posterior end of the premaxillae. No delphinoids have a posterolateral plate or a posteromedial splint of the premaxillae and premaxillary cleft, although several delphinoids have the premaxillary eminence (Geisler et al., 2012). Several Oligocene to early Miocene archaic odontocetes such as Agorophius, Patriocetus, Waipatia, and Squalodon also have a premaxillary cleft, as in Awadelphis. However, as mentioned above, several inioids have the same or similar characters. The premaxillary cleft in Awadelphis differs from archaic odontocetes by the anteroposteriorly long cleft not widely separating the posteromedial splint and the posterolateral plates. Furthermore, Awadelphis is also clearly distinguishable from those archaic odontocetes in having a derived air sinus system (preorbital and postorbital sinuses, and a well developed tympanosquamosal recess), conical teeth (not transversely flattened), the supraorbital process of the maxilla covering most of the zygomatic process in dorsal view, a rectangular skull in posterior view, and the lateral lamina of the palatine, and other synapomorphies with Delphinida and Inioidea. Furthermore, Awadelphis does not have the primitive or specialized characters of archaic odontocetes: parietal exposed between the vertex and the supraoccipital; spiny process which is an articulated surface between the skull and the periotic. Therefore, the premaxillary cleft in Awadelphis is considered to have evolved convergently with that of the archaic odontocetes.

    Awadelphis has one of the three synapomorphies among Inioidea excluding Meherrinia: the ratio of the greatest width of the premaxillae to the greatest width of the maxillae at the level of the postorbital processes is ≥⃒ 0.50 [74-0]. The two other synapomorphies of the clade cannot be scored: the apex of the postorbital process of the frontal projects posterolaterally and slightly ventrally [46-0]; and the longitudinal maxillary ridges on the supraorbital process of the maxilla is present, except at the lateral edge of the antorbital process [64-1]. Awadelphis and Brachydelphis share two synapomorphies: the length of the zygomatic process of the squamosal, as a percentage of the greatest width of the maxilla at the postorbital process, is ≤⃒ 30% [152-1]; and the angle formed by the basioccipital crests in ventral view is ca. 15°–40° [157-1].

    Comparisons with other inioids

    Awadelphis (Figure 9A) differs from all inioids in having a pronounced laterally overhanging premaxillary eminence; an anteroposteriorly long cleft not widely separating the posteromedial splint and the posterolateral plates; perpendicularly ascending process of the maxilla towards the supraorbital process of the maxilla; prominent dorsoventrally thick nasal; strongly narrowing maxilla posterior to the level of the postorbital process (more than in Inia); transversely narrow temporal fossa; very short zygomatic process of the squamosal; and teeth with short striations. The premaxillary eminence of Awadelphis is lower than that of Inia, Ischyrorhynchus, Pontoporia (Figure 9E), Auroracetus bakerae (Figure 9C), and Meherrinia isoni Geisler et al., 2012. The premaxillary eminences of Auroracetus overhang laterally the facial portion of the maxilla, but less so than in Awadelphis. Awadelphis differs from all inioids, except Brachydelphis (Figure 9B) and Pontistes (Figure 9G), by having the premaxilla contact the nasal, while the anterior edge of the nasal is located at the level of the postorbital process. The suture between the premaxilla and the nasal in Awadelphis is longer than those of Brachydelphis and Pontistes. Awadelphis is similar to Auroracetus and Protophocaena minima in lacking or having only a weakly developed posterolateral sulcus on the premaxilla.

    The anteroposteriorly long nasal in Awadelphis is similar to those of all pontoporiids: i.e., Auroracetus, Brachydelphis spp., Pliopontos, Pontoporia, Pontistes, Protophocaena minima Abel, 1905, and Stenasodelphis. In this point, Awadelphis is more similar to Pontoporiidae than to Iniidae except Isthminia panamensis Pyenson et al., 2015, although both families are paraphyletic in the present cladistic analysis. The neurocranium is slightly smaller in Awadelphis than in Pontistes, but clearly larger than in Auroracetus, Stenasodelphis russellae Godfrey and Barnes, 2008, Protophocaena, Pontoporia, Pliopontos, or Brachydelphis. Furthermore, the robust and conical tooth in Awadelphis differs from those in Pontoporia, Pliopontos (Figure 9F), and Brachydelphis, yet it is similar to those preserved in Pontistes. Awadelphis is different from Pontoporia and Pliopontos in having a more left-skewed skull. The postorbital lobe of the pterygoid sinus of Awadelphis is similar to that of Pliopontos, extending widely in the anterodorsal part of the temporal fossa. As in Pontoporia, the tympanosquamosal recess of Awadelphis is more developed than in Pliopontos and Brachydelphis. The recess extends nearly to the anterior tip of the zygomatic process of the squamosal. Awadelphis is similar to Auroracetus and Protophocaena in an absent or only weakly developed posterolateral sulcus on the premaxilla. The position of the postorbital process of the frontal in Awadelphis is similar to those in Pontistes and Brachydelphis, but more posterior than those in Pliopontos and Pontoporia.

    Figure 9.

    Comparisons of Awadelphis hirayamai gen. et sp. nov. with other inioids having low vertex. Illustrations of skulls in dorsal view (A–G). A, Awadelphis hirayamai gen. et sp. nov., WU SILS G 408 (reconstruction); B, Brachydelphis mazeasi MNHN PPI 266; C, Auroracetus bakerae USNM 534002; D, Stenasodelphis russellae CMM-V-2234; E, Pontoporia blainvillei NMNS-M 24982; F, Pliopontos littoralis MNHN SAS 193; G, Pontistes rectifrons MACN 3190.

    f09_207.jpg

    Awadelphis is similar to Inia in the wide expression of the postorbital lobe of the pterygoid sinus in the anterodorsal part of the temporal fossa (Fraser and Purves, 1960). However, Awadelphis is clearly distinguishable from Inia and Ischyrorhynchus by not having an elevated vertex, frontal boss, or a posterior extension of the maxillary crest on the temporal fossa. Awadelphis is different from Inia, Ischyrorhynchus, Isthminia, Goniodelphis hudsoni Allen, 1941, Saurocetes argentines Burmeister, 1871, and Saurocetes gigas Cozzuol, 1988 in smaller teeth and alveoli. The only preserved tooth of Awadelphis has many short striations, which differ from the reticulate striations in Inia, Ischyrorhynchus, Isthminia, and Saurocetes. The length of the neurocranium in Ischyrorhynchus and Goniodelphis is about 1.5 times larger than that of Awadelphis. The facial region anterior to the external bony nares of Goniodelphis is unusually long and differs from all other inioids, including Awadelphis. The skull of Awadelphis is more left-skewed than that of Meherrinia. The postorbital process of the frontal in Awadelphis is more posteriorly located than in Inia. Thus, Awadelphis is more similar to pontoporiids than some inioids, especially to Pontistes and Brachydelphis.

    Paleobiogeography of Inioidea

    Most of the reliable fossil records of Inioidea were restricted to South America as is, with few exceptions, their recent distribution (Allen, 1941; Morgan, 1994). However, discoveries in the last decade have expanded our knowledge of their morphological and paleobiogeographic diversity (Lambert and Post, 2005; Ahrens, 2005; Pyenson and Hoch, 2007; Godfrey and Barnes, 2008; Gutstein et al., 2009; Gibson and Geisler, 2009; Bianucci et al., 2011; Geisler et al., 2012; Pyenson et al., 2015). These discoveries indicate that inioids were significant members of the small odontocete fauna in the late Miocene to early Pliocene, especially in the North Atlantic, the North Sea, and the Mediterranean. The late Miocene corresponds to the period when archaic odontocetes such as Eurhinodelphinidae and Kentriodontidae were in steep decline, or had already became extinct (e.g. Fordyce and Barnes, 1994). Inioidea may have flourished in the region not only because of the decline of archaic odontocetes, but also because of the absence of Delphinidae, Monodontidae, and Phocoenidae. Bianucci et al. (2011) reported a fossil periotic of a possible phocoenid from Malta in the late Miocene (Tortonian). However, they cautioned that the periotic may rather belong to a kendriodontid pithanodelphinine. The fossil record of Inioidea in the Pacific region is restricted to Peru and Chile (Muizon, 1983, 1984, 1988b; Gutstein et al., 2009), where it extends from the late middle Miocene to the early Pliocene. Parapontoporia Barnes, 1985, from the North Pacific region (e.g. Miller, 1980; Barnes, 1985; Oishi and Hasegawa, 1994), was previously assigned to the Inioidea, but most authors now regard Parapontoporia as a lipotid rather than an inioid. An occurrence of Pontoporia sp. from Mexico (Barnes, 1998) has not been described. Hesperocetus californicus True, 1912, representing incomplete mandibles and teeth, is difficult to classify at the family level (Uhen et al., 2008). Therefore, the discovery of Awadelphis is the first record of a fossil inioid from the North Pacific.

    Late Miocene small odontocetes in Japan

    All the previously diagnosed late Miocene small odontocete fossils in the western North Pacific have come from Hokkaido, northern Japan (Horikawa, 1977; Tomida and Kohno, 1992; Murakami et al., 2012a, b, 2014). They are assigned to Delphinoidea (i.e., Delphinidae, Phocoenidae, and Albireonidae) but most belong to the Phocoenidae. Interestingly, these delphinoids are endemic to Hokkaido. No other delphinidans or archaic odontocetes have been reported in the late Miocene. It is uncertain whether this late Miocene endemism is real or the result of sampling, preservational, or latitudinal bias. However, co-occurring fossil organisms help. The late Miocene delphinoid odontocetes from Hokkaido cooccur with cool to temperate molluscan faunas: the cool to temperate Lower Togeshita Fauna (ca. 14–10 Ma; Amano, 1986), the temperate Upper Togeshita Fauna (ca. 10–6 Ma; Amano, 1986) or the cool-temperate to subarctic Takikawa-Honbetsu fauna (ca. 6–2 Ma; Fujie and Uozumi, 1957; Uozumi, 1962). Furthermore, cold-current diatoms co-occur with the late Miocene delphinoid odontocetes in Hokkaido (Shimada et al., 1998). Therefore, Murakami and Koda (2013) suggested that the delphinoid assemblages of the late Miocene of Hokkaido probably represents a subarctic to cool-temperate faunas, and that some of the species may have been eurythermal. Contrary to this, the small odontocete faunas of the late Miocene from the eastern North Pacific (Baja California, Mexico) and eastern South Pacific (Peru) are assumed to have been subtropical to temperate (Muizon and Devries, 1985; Barnes, 2008). Unfortunately, there are no data on the differences of paleo-sea surface temperatures between Hokkaido and other areas of the Pacific. However, if we can extrapolate the average annual water temperatures in the modern world, the paleo-sea temperatures around Hokkaido (north of 44°N) would have been colder than those of other areas (Chilean coast near the Bahia Ingles Formation [27°S], Peruvian coast near the Pisco Formation [between 13°N and 16°N], and Mexican coast near the Almejas Formation [28°N]): According to the Japanese Meteorological Agency, the average annual water temperature around Hokkaido presently is 8–12°C colder than those of the other aforementioned areas. This explains why there are no small odontocete species in common between the western North Pacific and other areas in the Pacific Ocean (Murakami and Koda, 2013). The molluscan fauna from the Senhata Formation correlates to the Zushi Fauna, which is considered to be a tropical to subtropical one (Ozawa and Tomida, 1992). Other marine invertebrate assemblages also indicate that the paleoclimate was tropical to subtropical (e.g. Tomida, 1983, 1989). Consequently, Awadelphis is not only the first described inioid species from the western North Pacific but also the first small odontocete species from a tropical to subtropical climate in the late Miocene in the western North Pacific. Many cetacean fossils (e.g. Takahashi, 1954) from the Senhata Formation have yet to be described. To describe and evaluate them will allow us to compare the small odontocete fauna in Japan with the fauna of other regions of the Pacific.

    Conclusions

    Awadelphis hirayamai (WU SILS G 408) from the uppermost Miocene Senhata Formation (6.3–5.7 Ma) of Chiba, central Japan is described. This is the first fossil inioid from Japan and the North Pacific. The discovery of this new taxon considerably extends the paleobiogeographic range of inioids and indicates that Inioidea were members of the small odontocete fauna of the western North Pacific dominated by Delphinoidea. This new species is also the first small odontocete from the tropical to subtropical climate in the late Miocene of the western North Pacific.

    Acknowledgments

    I am grateful to R. Hirayama for providing the opportunity to study the holotype specimen, to Towa Stone Limited for their support during fieldwork, and to M. Fumibe, K. Innomiya, Y. Shimizu, and A. Yamamoto for help in collecting and preparing the specimen. I thank G. Bianucci for providing unpublished photographs of Ischyrorhynchus vanbenedeni, J. H. Geisler for providing his Inioidea data matrix, and C. Argot, L. G. Barnes, D. J. Bohaska, T. A. Deméré, J. P. Dine, S. J. Godfrey, D. S. Janiger, O. Lambert, J. G. Mead, C. Mejia, C. de Muizon, J. J. Ososky, C. W. Potter, N. Pyenson, G. T. Takeuchi, Y. Tajima, Y. Tulu, G. B. Vai, E. Westwig, and T. K. Yamada for access to the specimens under their care and their valuable suggestions, and extend thanks to the late D. E. Hurlbert and B. Watanabe for residential support. I also thank the editor and two reviewers (S. J. Godfrey and C. S. Gutstein) for their comments, which greatly improved the manuscript.

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    Appendices

    Appendix 1. Character state scoring of Awadelphis hirayamai and Meherrinia isoni, for the matrix of Murakami et al. (2014). Abbreviations used in the matrix; a = 0 or 1; g = 1 or 2.

    eA01_207.gif

    Appendix 2. Revised character status from Murakami et al. (2014).

    • Georgiacetus: Character 5, changed from ? to 0.

    • Agorophius: Character 84, changed from 1 or 2 to 1.

    • Squalodon calvertensis: Character 84, changed from 2 to 1.

    • Tasmacetus: Character 84, changed from 0 to 1.

    • Berardius bairdii: Character 84, changed from 0 to 1; Character 218, changed from ? to 0; Character 219, changed from ? to 2

    • Ziphius: Character 84, changed from 0 to 1.

    • Mesoplodon ginkgodens: Character 84, changed from 0 to 1.

    • Lipotes: Character 163, changed from ? to 1; Character 173, changed from 1 to 0; Character 195, changed from 0 to 1.

    • Parapontoporia sternbergi: Characters 162, 163, 165, 166, 167, 170, 173, 176, 177, 179, 180, 181, 183, 185, 187, 190, 196, 197, 199, 200, 201, 203, 204, 211, and 214, changed from ? to 0; Characters 164, 168, 169, 171, 174, 175, 182, 186, 188, 189, 191, 193, 195, 198, 205, 208, 209, 210, and 213, changed from ? to 1; Character 184, changed from ? to 0 or 1; Character 192, 206, and 207, changed from ? to 2.

    • Inia geoffrensis: Character 84, changed from 0 to 0 or 1; Character 195, changed from 0 to 1.

    • Pontoporia: 84, changed from 0 to 0 or 1; Character ; Character 191, changed from 0 or 1 to 0.

    • Pliopontos: Character 3, changed from 0 to ?; Characters 17, 84, 91,123, and 195, changed from 0 to 1; Characters 82 and 126, changed from ? to 1; Character 97, changed from 1 to 0; Character 125, changed from ? to 3.

    • Brachydelphis mazeasi: Characters 62 and 195, changed from 0 to 1; Character 191, changed from 1 to 0.

    • Hadrodelphis: Character 168, changed from ? to 2; Characters 174, 179, 191, 198, 212, and 225, changed from ? to 1; Characters 175, 177, 183, 185, 187, 202, 211, 213, 214, 223, and 226, changed from ? to 0.

    • Odobenocetops peruvianus: Character 56, changed from 1 to ?

    • Monodon: Character 223, changed from ? to 0.

    • Haborophocoena toyoshimai: Character 17, changed from? to 1.

    • Haborophocoena minutus: Characters 17, 40, and 161, changed from? to 1; Characters 43 and 44, changed from ? to 2; Characters 101 and 137, changed from ? to 0; Character 114, changed from 1 to 0; Character 154, changed from 0 to 1.

    • Miophocaena nishinoi: Character 170, changed from ? to 0.

    • Piscolithax boreios: Character 172, changed from ? to 3.

    • Salumiphocaena: Character 20, changed from 1 to 2; Characters 24, 42, 46, 153, 174, 221, 230, 238, 239, and 240, changed from ? to 1; Characters 30, 40, 91, 112, 117, 148, 160, 171, 175, 176, 177, 178, 179, 187, 189, 204, 226, and, 237, changed from ? to 0; Characters 217, 219, 222, changed from ? to 2.

    • Hemisyntrachelus cortesii: Characters 164, 166, 167, 178, 179, 183, 191, and 199, changed from ? to 0; characters 169, 173, 174, 177, changed from ? to 1; Character 176, changed from 2 to 1 or 2.

    • Etruridelphis: Character 20, changed from 0 to 1.

    • Tursiops aduncus: Characters 223 224, 226, and 246, changed from ? to 0; Characters 225, 230, 231, 243, 244, and 245, changed from ? to 1.

    © by the Palaeontological Society of Japan
    Mizuki Murakami "A New Extinct Inioid (Cetacea, Odontoceti) from the Upper Miocene Senhata Formation, Chiba, Central Japan: The First Record of Inioidea from the North Pacific Ocean," Paleontological Research 20(3), 207-225, (1 July 2016). https://doi.org/10.2517/2015PR031
    Received: 13 March 2014; Accepted: 1 October 2015; Published: 1 July 2016
    KEYWORDS
    Cetacea
    Inioidea
    Late Miocene
    Paleobiogeography
    small odontocete fauna
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