Kekenodon onamata , Hector 1881
Creators: name: Corrie, Joshua E
name: Fordyce, R Ewan
Publisher: name: Zenodo
Year: 2022
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Mammalia
Cetacea
Basilosauridae
Kekenodon
Kekenodon onamata

description: KEKENODON ONAMATA HECTOR, 1881
Etymology: Onamata means ‘of long ago’ in Te Reo Māori.
Emended diagnosis of species: Kekenodon onamata is a large-sized (8–9 m estimated body length) heterodont archaeocete that differs from archaeocetes basal to the Basilosauridae (Protocetidae, Remingtonocetidae, Ambulocetidae and Pakicetidae) in possessing a prominent lateral tuberosity located lateral to the mallear fossa and posterior cheek teeth with multiple accessory denticles. Kekenodon onamata differs from Basilosauridae in possessing a subrectangular supraorbital process of the frontal that is transversely wider than anteroposteriorly long, diminutive foramina on the dorsal surface of the frontal, prominent lateral tuberosity, mallear fossa positioned more medial to the lateral tuberosity, a medial edge of the anterior process of the periotic and anterior edge of the pars cochlearis forming an obtuse angle, rounded anteromedial angle of the pars cochlearis, pars cochlearis with a hemispherical ventral profile, reduced superior process of the periotic forming a low-lying ridge with distinct anterior and posterior apices, gently concave suprameatal fossa, and lower molariform teeth with accessory denticles on the anterior surface of the crown in place of an re-entrant groove. Kekenodon onamata differs from other Pelagiceti in possessing a protocone remnant in permanent posterior cheek teeth and upper-posterior cheek teeth with a third lingual root.
Kekenodon onamata retains archaic features differing from Neoceti including: dorsal and ventral vestibular areas of the internal acoustic meatus separated by an indistinct transverse crest and triplerooted cheek teeth.
Kekenodon onamata differs from Odontoceti in lacking evidence of skull telescoping indicative of echolocation including the posterior and posterolateral expansion and inflation of the maxilla and premaxilla dorsally covering the frontal, and in possessing large and strongly heterodont teeth with postcanines having numerous large and triangular accessory denticles on the anterior and posterior margins of the crown (excluding Inticetus vertizi and Squalodontidae).
Kekenodon onamata differs from the taxonomically ambiguous Phococetus vasconum in possessing comparatively larger permanent molariform teeth, crowns of denticulate molariform teeth have a symmetrical profile with a less inclined anterior margin and a more distinct sulcus corresponding to the isthmus joining the anterior and posterior roots.
Kekenodon onamata differs from Mysticeti in possessing a pars cochlearis with a longitudinal ventral ridge; from toothed Mysticeti in possessing a larger total body length [excluding Morawanocetinae gen. et. sp. indet. (AMP 9)], a transverse frontomaxillary suture (excluding Aetiocetus polydentatus Barnes et al., 1995), ascending process of maxilla does not extend posterior to the anterior margin of the frontal, an indistinct superior process that forms a low-lying ridge terminating at anterior and posterior apices (excluding Fucaia buelli Marx, Tsai & Fordyce, 2015) and some upper postcanines that are triple-rooted; from the previously recognized toothed mysticetes Llanocetus denticrenatus Mitchell, 1989, Mystacodon selenesis Lambert et al., 2017 and Mammalodontidae in possessing prominent enamel ornament on both the labial and lingual surfaces of the crown; from Llanocetus denticrenatus in possessing accessory denticles on postcanine teeth that are more closely spaced with a profile that is more triangular and less palmate; from Mammalodon colliveri Pritchard, 1939 in possessing apertures for cochlear aqueduct and fenestra rotunda that are not widely separated; from Janjucetus hunderi Fitzgerald, 2006 and Fucaia in lacking an elongate lateral tuberosity that articulates dorsally with the squamosal; from Aetiocetidae in possessing upper and lower double-rooted posterior cheek teeth with roots joined by a transversely narrow isthmus below the crown base, a transversely broader anterior process of the periotic, dorsoventrally straight anterior keel of the anterior process in the periotic and an ovoid pars cochlearis in medial view; from Aetiocetus Emlong, 1966 in possessing distinctly heterodont teeth and double-rooted postcanine teeth; from the previously recognized toothed mysticete Coronodon havensteini Geisler et al., 2017 in possessing a rounded anteromedial corner of the pars cochlearis, postcanine teeth with a more inclined anterior margin of the crown and a primary denticle that is larger than the accessory denticles; from Mystacodon selenesis in having nasals with a posterior margin positioned further anteriorly; from Chaeomysticeti in possessing an orbitotemporal crest located on the posterior edge of the frontal and functional permanent teeth (possibly excluding the eomysticetid Waharoa ruwhenua Boessenecker & Fordyce, 2015); from Fucaia goedertorum (Barnes et al., 1995) and Chaeomysticeti in possessing an anterior process of the periotic with apex dorsal to the ventral edge of the pars cochlearis; from crown Mysticeti in possessing a straight posterior border of frontal, multiple minute foramina on dorsal surface of the frontals, anterior process of the periotic intermediate in length relative to the pars cochlearis, distinct mallear fossa, distinct fovea epitubaria to accommodate the accessory ossicle of the tympanic bulla, periotic lacking a ventrolateral ridge, ovoid proximal opening of facial canal, relatively wide tympanic bulla, distinct medial and lateral lobes of the tympanic bulla open elliptical foramen of the tympanic bulla, tympanic cavity anteroposteriorly divided by transverse ridge and involucral ridge that is nearly straight; from crown Mysticeti and M. colliveri in lacking a pars cochlearis elongated towards the cranial cavity, tubular fundus of the internal acoustic meatus and protruding lateral wall of internal acoustic meatus from the suprameatal fossa; and from crown Mysticeti and J. hunderi in lacking a fenestra rotunda with a fissure oriented towards the perilymphatic foramen and a caudal tympanic process of the periotic with narrow separation from the facial crest of the periotic with a clear separation between the stapedial muscle fossa and facial sulcus.
Kekenodon onamata shares with Protocetidae, Mammalodontidae and Coronodon havensteini some upper cheek teeth that are triple-rooted with a third lingual root. Kekenodon onamata shares with Basilosauridae heterodont teeth composed of singlerooted and single-cusped conical crowns and doublerooted cheek with transversely compressed and tall triangular crowns with accessory denticles. Kekenodon onamata shares with Llanocetus denticrenatus a large total body length. Kekenodon onamata shares with Coronodon havensteini a pars cochlearis with a longitudinal ventral ridge and an indistinct superior process that forms a low-lying ridge terminating at anterior and posterior apices. Kekenodon onamata shares with Mammalodontidae and some Odontoceti (including Inticetus vertizi) a median furrow forming a continuous anteroposterior grove on the ventral surface of the tympanic bulla. Kekenodon onamata shares with Mammalodontidae lower posterior cheek teeth roots joined below the crown base by a transversely narrow isthmus. Kekenodon onamata shares with Mammalodon colliveri and some Odontoceti (including eurhinodelphinids) an involucrum possessing a transverse groove on its dorsal surface, which divides it into a narrower anterior section and wider posterior section. Kekenodon onamata shares with Eomysticetidae a poorly-developed superior process of the periotic reduced to a low ridge with anterior and posterior apices. Kekenodon onamata shares with Phococetus vasconum longitudinal ridges ornamenting the basal region of the crown of denticulate teeth.
Holotype: NMNZ Ma 306, Museum of New Zealand Te Papa Tongarewa, Wellington, New Zealand; GNS Science, Avalon, New Zealand, preserves the right supraorbital process of frontal, left periotic and tympanic bulla, 15 isolated teeth and atlas. Collected in November 1880 near the junction of the Wharekuri Creek and Waitaki River in the Waitaki Valley, North Otago, South Island, New Zealand, by A. McKay. New Zealand Map Series 260 grid reference I40:010122, GPS coordinates (approximately 44º40’S, 170º21’E). Fossil record number I40/f35 (New Zealand fossil record file, Geosciences Society of New Zealand).
Stratigraphy and age: NMNZ Ma 306 from the upper Oligocene Kekenodon Beds at the approximate junction of the Wharekuri Creek and Waitaki River in the Waitaki Valley, 11 km north-west of Kurow, North Otago, South Island, New Zealand (Fig. 2). The exact type locality and horizon is now submerged below the Waitaki Hydroelectric Lake. McKay (1882a) noted that the source horizon of NMNZ Ma 306 was located on the lower-middle or lower-third of the outcrop of the Kekenodon Beds and was vulnerable to high waters during flooding events of the Waitaki River. The Kekenodon Beds were named by Hector (1882) based on the discovery of Kekenodon onamata and are restricted to the Wharekuri area in the Waitaki Valley. The Kekenodon Beds directly underlie the Otekaike Limestone and unconformably overlie the Wharekuri Greensand at a disconformity (McKay, 1882a, b; Gage, 1957; Marwick, 1959; Fordyce & Watson, 1998). Fordyce & Watson (1998) observed the disconformity between the Kekenodon Beds and theWharekuri Greensand‘ 1–2 m below terrace gravels 100 m upstream (south-west) from the old stone bridge across Wharekuri Creek’. The disconformity separates early Whaingaroan Stage [Globigerina (Subbotina) angiporoides zone] from the upper Whaingaroan and Duntroonian Stages (Globigerina euapertura zone) (Hornibrook et al., 1989: Maxwell in Beu et al., 1990). Near the Wharekuri Creek in the Waitaki Valley, 11 km northwest of Kurow, the Kekenodon Beds are a grey to green, fossiliferous, massively to decimetre-bedded (30–60 m thick), calcareous, glauconitic quartz greensand (Hector, 1881; McKay, 1882a, b; Marwick, 1959). McKay (1882a) noted a higher fossiliferous content toward the central part of the Wharekuri Basin, along the banks of the Waitaki River, with the highest concentration of fossils occurring in the middle to upper portion of the Kekenodon Beds (McKay, 1882b). Hector (1882) and McKay (1882a) previously dated the Kekenodon Beds at Mid-Eocene and ‘Upper Eocene’, respectively. More recently, foraminifera sampling and abundance of the mollusk Serripecten (Janupecten) uttleyi (Marwick, 1924) indicate an Early Oligocene age for the underlying Wharekuri Greensand (Marwick, 1959; Hornibrook et al., 1989). The planktonic Globigerina (S.) angiporoides Hornibrook, 1965 and the benthic Notorotalia stachei (Finlay, 1940) foraminifera, collected 1 m below the unconformity that separates the Wharekuri Greensand from the overlying Kekenodon Beds, support a lower Whaingaroan Stage (Fordyce & Watson, 1998). Early interpretations place the Kekenodon Beds and the Wharekuri Greensand into a single formation (Uttley, 1920; Marwick, 1935). Similarities in lithology indicate the Kekenodon Beds are instead synonymous with the Kokoamu Greensand, a calcareous greensand characterized by medium to coarse glauconite, scattered macrofossils and phosphatized pebbles (Gage, 1957; Field & Browne, 1986; Fordyce & Watson, 1998). Lithology and fossil content of the Kokoamu Greensand indicates a middle-shelf environment, with depths between 50 and 100 m (Field & Browne, 1986; Ayress, 1993). Fossil assemblages of brachiopods (Allan, 1938), ostracods (Ayress, 1993) and planktonic (Jenkins, 1971) and benthic (Hornibrook et al., 1989) foraminifera indicate an upper Whaingaroan Stage (Lower Chattian) for the basal Kokoamu Greensand and the Duntroonian Stage (Upper Chattian) for the upper Kokoamu Greensand (Gage, 1957; Boessenecker & Fordyce, 2015a). Tsai & Fordyce (2015) noted that the lower boundary of the Duntroonian Stage is delineated by a distinct ‘brachiopod- and pectinid-rich shellbed’ located in the lower to middle Kokoamu Greensand. No record exists to indicate this shellbed was present in the Kekenodon Beds and the current known exposure in Wharekuri Creek is partly obscured by slumping. Following the observations of McKay (1882b), the location of the K. onamata -bearing horizon in the area of the Kekenodon Beds with the highest concentration of fossils (middle to upper portion) would suggest a Duntroonian age (27.3–25.2 Mya; Fig. 2).
Anatomical description
Frontal: The right supraorbital is preserved in NMNZ Ma 306 and is cleanly broken at the median interfrontal suture (Fig. 3; Table 1). The dorsal profile of the right supraorbital is subrectangular and is transversely wider than anteroposteriorly long (185.2 mm and 110.3 mm, respectively; Table 1). The supraorbital is dorsoventrally thinnest along the dorsal margin of the orbit. The dorsoventral depth along the preserved medial margin of the supraorbital is greater through the posterior-half than in the anterior-half. Striations at the anteromedial corner of the dorsal surface of the supraorbital represent part of the nasal–frontal suture (medially) and the frontopremaxillary suture (laterally), indicating the nasals and premaxillae overlapped the anterodorsal surface of the frontals. The posterior extension of the striations delineates the posterior termination of the nasals just anterior to the interfrontal suture. The anterior border of the frontal is slightly convex, and its morphology suggests a transversely planar and non-serrate frontomaxillary suture for tight articulation with the maxilla. There is no evidence that an ascending process of maxilla overlapped the dorsal surface of the frontal. The preserved base of the postorbital process of the frontal suggests that the process extended posterolaterally and tapered toward the apex. Several minute foramina are present on the dorsal surface of the frontal. Posteriorly, there is a possible comparatively larger foramen that is oriented toward the postorbital process of the frontal (Fig. 3A), although this may be an artefact of weathering. The preserved orbit has a length of c. 67 mm. In lateral view, the orbit is arched dorsally. The orbitotemporal crest follows the posterodorsal border of the supraorbital from the postorbital process medially toward the parietal. A transversely oriented ridge located anterior to, and running parallel with, the orbitotemporal crest represents the highest point of elevation on the frontals. The posterior ridge extends c. 108 mm mediolaterally across the frontal and may represent an attachment site for anteriorly directed facial muscles or for fascia overlying the m. temporalis. In posterior view, the posterior face of the frontal is concave, forming a fossa probably for the anterior origin of the m. temporalis pars superficialis and m. temporalis pars profunda, agreeing with the interpretation of Carpenter & White (1996) for Zygorhiza kochii (Reichenbach in Carus, 1847) in contrast to Uhen (2004) who argued that the anterior origin of the superficial and deep temporal muscles in Dorudon atrox Andrews, 1906 occurred near the frontoparietal suture and did not extend as far anteriorly as the posterior face of the frontals. Posteromedially, the frontoparietal suture is missing.
The ventral surface of the preserved frontal has suffered heavy weathering. Previously, Fordyce (1978) identified the anteroventral surface of the frontal as being lacrimal, probably due to the appearance of a thin lamella of bone overlapping a second layer. We now interpret this as a laminated pattern representing ossification layers of the frontal. There is no evidence to indicate that the lacrimal articulated with the anterolateral frontal and dorsally overlaid the anterior jugal, forming the anterior wall of the orbit, as seen in basilosaurids (Uhen, 2004). Alternatively, the lacrimal may have been anterior to the preorbital process rather than ventrally, similar to Coronodon havensteini (Geisler et al., 2017). At least four nutrient foramina open on the ventral surface of the frontal. Additionally, the ventral surface of the frontal is highly vascularized. Large sulci are present running mediolaterally to the postorbital process of the frontal. The postorbital ridge has suffered damage but is still slightly noticeable. In the orbital region, the optic infundibulum, along with the orbitosphenoid and presphenoid, are not evident.
Periotic: The left periotic is isolated from the skull and is damaged on the dorsal and ventral surfaces (Figs 1, 4; Table 2). The periotic lacks parts of the anterior process and most of the posterior process. Part of the base of the posterior process is preserved, albeit damaged ventrally. Measurements are, therefore, compromised. The preserved length from the anterior margin of the anterior process to the posterior margin of the posterior process is 57.8 mm but was probably c. 65 mm when complete. The preserved transverse breadth of the periotic is 29.1 mm. In ventral view, the anterior process has a teardrop-shaped profile with a blunted anteroventral margin (Fig. 4A). The anterior process is somewhat long (preserved length = c. 20.6 mm) relative to the length of the pars cochlearis and total preserved length of the periotic (62% and 35%, respectively). Transversely, the anterior process is moderately compressed (width = 17.9 mm), but not to the extent in eomysticetids and aetiocetids, where marked transverse compression produces a bladelike anterior process. However, the dorsal surface of the anterior process is missing, which may have revealed a blade-like morphology. Multiple vascular canals are exposed from the missing dorsal surface (Fig. 4B). Furthermore, the preserved dorsal surface has been polished during previous preparation. The lateral surface of the anterior process is rugose, while the ventrolateral surface is smooth and the medial surface has a smooth-surfaced ventromedial margin with a small section of rugose surface dorsally. The ventral surface of the anterior process is convex, while the medial and lateral surfaces are relatively planar. The anterior keel of the anterior process is
shallowly concave in medial and lateral views. The anteroventral angle is moderately triangular with a rounded apex; the anterodorsal angle is not preserved. The posteromedial border of the anterior process is separated from the pars cochlearis by the anterior incisure (Mead & Fordyce, 2009). The anterior incisure extends posteroventrally and is bordered laterally by the pars cochlearis and medially by the body of the periotic. An additional groove, possibly vascular, is medially parallel with the anterior incisure.
The anterior process is separated from the body by a prominent groove on the ventral surface that represents the posterodorsal margin of the fovea epitubaria (Fig. 4A). The fovea epitubaria accommodates the accessory ossicle of the tympanic bulla (not preserved) and has a triangular profile that forms most of the ventral surface of the anterior process. The posteroventral margin of the fovea epitubaria anteriorly abuts the mallear fossa.The anterior bullar facet is not preserved. A thin lamina of bone separates the fovea epitubaria from the laterally adjacent anteroexternal sulcus. The anteroexternal sulcus extends dorsolaterally from the posteroventral border of the anterior process and runs along the posterolateral face of the anterior process, before terminating at the preserved dorsolateral margin of the anterior process; the anteroexternal sulcus forms most of the posterolateral surface of the anterior process (Fig. 4D). The anteroexternal sulcus is similarly rugose as the majority of the lateral surface of the anterior process. The anteroexternal sulcus widens toward the dorsolateral margin of the anterior process. The mallear fossa is posteriorly adjacent to the fovea epitubaria (Fig. 4A). The mallear fossa is large (maximum diameter = 10.8 mm) and shallow, with a subrounded profile and ventral and slight posteromedial orientation; ridge-like lamellae of bone delineate the margins of the mallear fossa. Minute foramina are present throughout the surface of the mallear fossa, with one relatively larger and more distinct foramen located along the anteromedial margin of the mallear fossa. The fossa incudis is distinct and subrounded and is located posterolateral to the mallear fossa. A small portion of the lateral tuberosity is preserved extending laterally from the lateral margin of the mallear fossa. However, lack of preservation precludes further morphological detail (e.g. if the lateral tuberosity was bulbous or narrow).
Posteromedial to the mallear fossa, the subrounded distal opening of the facial canal opens posterolaterally into the smooth facial sulcus, which bends posteromedially. The facial sulcus terminates at an indistinct crest that separates the sulcus from the more posteromedial stapedial muscle fossa, which is more deeply excavated with a rugose surface. The ovoid fenestra ovalis is located anterior to the stapedial muscle fossa and medial to the facial sulcus and distal opening of the facial canal, and is separated from each by a low-lying ridge. Originally, the stapes was articulated with the fenestra ovalis, but has since been removed and stored separately [image of articulated stapes in Wilson (1973) and Fordyce (1978)].
The pars cochlearis is transversely compressed, with an anteroposterior length of 33 mm and a transverse breadth of 12.3 mm. At the level of the fenestra ovalis, the pars cochlearis has a maximum dorsoventral depth of 16.7 mm; anteriorly, the pars cochlearis becomes shallower. The dorsal face of the pars cochlearis has been slightly damaged, but a rugose and concave surface can be discerned. The ventral surface of the pars cochlearis is convex, while the anteroventral face of the pars cochlearis is steeply convex and ascends anterodorsally. The posterior face of the pars cochlearis is subrectangular and is dominated by the fenestra rotunda, which opens dorsolateral and posterior to the fenestra ovalis and ventral foramen of the facial canal (Fig. 4F). In posterior view, the fenestra rotunda has a subrounded profile and opens posteromedially; a subrounded fossa contains the fenestra rotunda. The dorsal margin of the fenestra rotunda, visible ventrally, shows multiple minute striations. A transversely (dorsoventrally) large and shallow groove is located on the medial surface of the pars cochlearis. Additionally, a minute tubercle projects posteromedially from the dorsomedial margin of the pars cochlearis, and it probably represents an insertion point for endocranial tissue. The groove is probably homologous with the median promontorial groove and runs parallel with the ventral margin of the pars cochlearis for nearly its entire medial length. The anterointernal angle of the pars cochlearis is distinct and angular, giving the pars cochlearis a subrectangular ventral profile (Fig. 4A). A bulbous ridge, less developed than in basilosaurids, extends anteroposteriorly on the ventrolateral surface of the pars cochlearis. The anterior incisure forms a slit-like sulcus on the ventral surface of the periotic that posteriorly extends to the level of the anteromedial margin of the mallear fossa and anteriorly extends to the anterior margin of the pars cochlearis (Fig. 4A). A prominent longitudinal and rounded ventral ridge on the medioventral surface of the pars cochlearis extends nearly the entire length of the pars cochlearis. The transversely compressed caudal tympanic process of the pars cochlearis is posteromedially oriented from the posteroventral margin of the pars cochlearis
The internal acoustic meatus has an ovoid profile in dorsal view and orients posteromedially– anterolaterally (Fig. 4B). A low-lying rim outlines the internal acoustic meatus but has been destroyed along the lateral margin. The medial wall of the internal acoustic meatus ascends from the ventral and dorsal vestibular areas to produce a relatively deep internal acoustic meatus; the elevation of the lateral wall cannot be discerned, but it is similarly elevated as the medial wall in an undescribed putative kekenodontid (OU 22294). The diameter of the internal acoustic meatus increases dorsally, creating a funnel-like appearance. The ventral vestibular area is separated from the dorsal vestibular area by a low-lying transverse crest that is recessed c. 6 mm within the internal acoustic meatus. The opening for the proximal opening of the facial canal within the ventral vestibular area is ovoid and slightly tear-shaped. There is no evidence of a fissure at the anterior edge of the internal acoustic meatus, in addition to no indication of a developed hiatus fallopii. The foramen singulare, central foramen and spiral cribriform tract are located within the dorsal vestibular area. Ichishima et al. (2021) identify the structure commonly labelled the foramen singulare in Cetacea as representing the dorsal vestibular area, with the ‘true’ foramen singulare opening closer to the ventral vestibular area. Here, our identification of the foramen singulare will remain until Micro-CT of the periotic reveals if the pathway of the posterior ampullary nerve, which runs through the foramen singulare, is homologous in K. onamata. The foramen singulare is separated from the spiral cribriform tract by a low-lying transverse crest (identified as the perpendicular crest by Ichishima et al., 2021) that is recessed further relative to the crest separating the ventral and dorsal vestibular areas (c. 8 mm). Relative to the ventral vestibular area, the dorsal vestibular area is larger, but shares a similar ovoid profile. All foramina and openings within the internal acoustic meatus are arranged in a relatively linear fashion; the foramen singulare is positioned anterolateral to the central foramen, and the proximal opening of the facial canal is positioned anterior to the foramen singulare and slightly anterolateral to the central foramen (Fig. 4B). The aperture for the vestibular aqueduct is ovoid and large (maximum diameter of 8 mm) relative to the smaller (maximum diameter of 2.4 mm) and more subrounded aperture for the cochlear aqueduct. A groove located anteromedial to the aperture for the cochlear aqueduct extends between the medial edge of the internal acoustic meatus and the dorsal surface of the pars cochlearis and forms a small, circular fossa at the posteromedial corner of the internal acoustic meatus. A narrow and relatively deep groove on the medial side of the ridge separates the apertures for the vestibular and cochlear aqueducts. A slight dorsal projection from the pars cochlearis indicates the posteromedial border of the aperture for the cochlear aqueduct. The anterior border of the aperture for the cochlear aqueduct is denoted by a triangularshaped dorsal projection posterolateral to the internal acoustic meatus.
The superior process is reduced to a low-lying robust ridge that anteriorly and posteriorly rises to form the anterodorsal angle and posterodorsal angles, respectively (Fig. 4C, D, F). In the holotype, the anterodorsal angle is missing, while the posterodorsal angle is triangular and projects posterodorsally. The preserved posterodorsal section of the anterior process represents the highest point of elevation on the dorsal surface of the periotic. The suprameatal area is shallowly concave. Fordyce (1978) noted three distinct anteroposteriorly oriented sulci on the suprameatal area (Fig. 4B). The medialmost channel is laterally adjacent to the anterior and lateral borders of the internal acoustic meatus. A second channel is located along the approximate anteroposterior midline of the suprameatal area. The path of this channel extends anterodorsally on to a broken section of posterior anterior process, while posteriorly the channel approximates the ventromedial margin of the triangular projection forming the posterodorsal angle. The lateralmost channel seemingly follows the superior process. Anteriorly, the channel terminates lateral to the broken section of posterodorsal anterior, and posteriorly terminates along the dorsolateral and ventrolateral margins of the projection forming the posterodorsal angle. The slit-like and dorsoventrally oriented posteroexternal sulcus is located posterolateral and ventral to the posterodorsal angle.
The posterior process is limited to a small anterior section with few distinct features (Fig. 4C, D, F). A prominent keel of unknown origin is present at the preserved anteromedial margin of the posterior process and may represent the side of the bullar facet or the lateral margin of the stylomastoid fossa.
Stapes: The left stapes was found in articulation with the fenestra ovalis and has since been prepared out, where it is now preserved isolated from the periotic (Fig. 5). The footplate of the stapes broadens posteriorly and is anteriorly and laterally narrow. Additionally, the footplate of the stapes has a convex dorsal (vestibular) surface and a shallowly concave ventral surface. The stapedial foramen for the stapedial artery is subrounded and opens mediolaterally. The anterior crus is broader than the posterior crus and is positioned perpendicular to the footplate, while the posterior crus is at a slight angle to the footplate. The head of the stapes is large and squared off. A depression located on the anteroventral surface of the head of the stapes is for articulation with the crus longum of the incus (Mead & Fordyce, 2009). The insertion for the stapedial muscle is on the medial and dorsal faces of the stapedial head, and partially along the posterior crus.
Tympanic bulla: The incomplete left tympanic bulla is isolated from the skull (Figs 1, 6; Table 3). The element is large and dense, with well-defined medial (inner posterior prominence) and lateral (outer posterior prominence) lobes. The tympanic bulla is damaged and lacks most of the outer lip and associated structures. The anterolateral section of the tympanic bulla has been lost, giving a cordate outline, with a tapering anterior margin and widening posterior margin (Fig. 6A, B). However, more complete tympanic bullae in two undescribed putative kekenodontids (OU 22023 and OU 22394) are rhomboidal-shaped, as in Basilosauridae and Llanocetus denticrenatus, suggesting the complete tympanic bulla of Kekenodon onamata had a similar profile. The medial margin is straight to slightly concave, while the lateral margin is convex (Fig. 6C, D). Ventrally, the involucrum is separated from the outer lip by a distinct median furrow on the otherwise convex ventral surface (Fig. 6D). The median furrow is elongated and transversely narrow and extends for c. 47.3 mm from the posterior limit of the bulla to just posterior to the level of the lateral furrow (Fig. 6B, D, F); the ventralmost lateral furrow is preserved and is level with an ovoid fossa on the dorsal surface of the involucrum (Fig. 6C). The median furrow separates the inner and outer posterior prominences. The interprominential notch is the posterior continuation of the median furrow (Kasuya, 1973) and is marked by a slight concavity in the posterior margin of the tympanic bulla (in dorsal view) just medial to the posteromedial margin of the outer posterior prominence (Fig. 6F). The interprominential ridge is dorsolaterally oriented on the posterior surface of the tympanic bulla and is contiguous with the posterior extension of the involucral ridge from the ventromedial margin of the involucrum; the interprominential ridge terminates at the posterodorsal extension of the median furrow (Fig. 6F). The inner posterior prominence is broader relative to the transversely narrow outer posterior prominence and has greater dorsoventral depth (Fig. 6C, D). The posteromedial margin of the inner posterior prominence is mostly rounded but does present a slight angularity due to the interprominential ridge. However, the angularity of the posteromedial margin of the inner posterior prominence is not as pronounced as in Basilosauridae, but is more angular relative to toothed and edentulous baleen-bearing mysticetes (Boessenecker & Fordyce,
2015b). Although damaged, the posterolateral margin of the outer posterior prominence is sharply rounded due to the prominential ridge (sensu Tsai & Fordyce, 2015), which extends posteroventrally and laterally from the conical process (not preserved). The outer posterior prominence extends further posteriorly, but less ventrally than the inner posterior prominence (Fig. 6C, D).
The involucrum is robust and increases in dorsoventral depth posteriorly; the dorsal and ventral surfaces of the involucrum converge anteriorly (Fig. 6A, C–F). The ventral surface of the involucrum is convex, while the dorsal surface is predominantly convex, with a distinct transverse groove level with the anterior-half of the posterior segment of the tympanic cavity; the groove separates the involucrum into anterior and posterior segments. The posterior segment of the involucrum is more bulbous, transversely broad and anteroposteriorly long relative to the anterior segment. The surface of the posterior segment of the involucrum is convex and smooth. The surface of the anterior segment is convex and seemingly forms a narrow, dorsally projecting ridge. A prominent subrounded fossa is located medial to the tympanic cavity and is possibly confluent with the transverse groove dividing the involucrum (Fig. 6C). Additionally, the fossa may be continuous with a relatively broad and shallow groove running anteriorly into the anterior segment of the tympanic cavity. Subtle striations are present along the dorsal surface of the involucrum (Fig. 6C). Posteriorly, the striations orient mediolaterally, while becoming increasingly anteromedially and ventrally oriented anteriorly. An anteroposteriorly elongate ovoid depression is present on the ventromedial surface of the involucrum. In life, this depression would have closely approximated the medial edge of the basioccipital crest (Boessenecker & Fordyce, 2015b) and possible pharyngeal crest (of Mead & Fordyce, 2009) more anteriorly. The involucral ridge expands the entire anteroposterior length of the medial surface of the involucrum and separates smooth bone (dorsally) from rugose bone (ventrally); the smooth bone dorsal to the involucral ridge (Fig. 6A, E) marks the position of the peribullary sinus (Boessenecker & Fordyce, 2015a). Posteriorly, the involucral ridge extends through the ovoid depression and becomes contiguous with the interprominential ridge on the posterior surface of the inner posterior prominence (Fig. 6F).
Remnants of the inner and outer posterior pedicles of the posterior process are located posteriorly and posterolaterally on the dorsal surface of the tympanic bulla (Fig. 6C). The inner posterior pedicle appears larger than the outer posterior pedicle but is obscured due to weathering. The inner and outer posterior pedicles of the posterior process of the tympanic bulla are visible in lateral view. The inner posterior pedicle is elevated more dorsally than the outer posterior pedicle. In posterior view, a distinct elliptical foramen is present, forming a distinct notch between the inner and outer posterior pedicles. The margins of the elliptical foramen are either missing or damaged, obscuring further morphological detail (Fig. 6F).
Dentition: Kekenodon onamata is heterodont. Six single-rooted teeth, six double-rooted teeth and two triple-rooted teeth are preserved isolated from the rostrum and mandible (Figs 1, 7, 8; Table 4); the inferred left M1 and right p3 were curated at the New Zealand Geological Survey (GS 476 CD 40/2 and GS 476 CD 40/1, respectively), but have since been referred to the holotype specimen (NMNZ Ma 306; Fordyce, 1980). Morphological characteristics (e.g. infilled pulp cavities) indicate that all teeth probably represent permanent dentition. The absence of preserved rostrum and mandibles obscures any evidence of possible dental eruption. Thus, it is uncertain whether K. onamata was monophyodont or diphyodont. Tooth positions and numbers are tentatively identified with reference to previous observations by Hector (1881), Kellogg (1936) and Fordyce (1978), in concert with reference to the crown and root morphology (e.g. profile of crown and number of roots) of Basilosauridae. The absence of the rostrum and mandibles makes all tooth identifications provisional. The dental formula is uncertain, but with comparisons to the well-preserved skull of the provisional kekenodontid OU 22294, it is proposed here that K. onamata retained the basilosaurid count of 3142/3143.
The single-rooted teeth are caniniform, having a subconical profile with a diameter that tapers toward the apex (Figs 1, 7). Two nearly complete crowns of an inferred left C1 (Fig. 7C) and right c1 (Fig. 7D), and a partial crown of an inferred incisor or canine (Fig. 7B), are lingually concave and recurved and lack accessory denticles. Distinct anterior and posterior carinae are present on all caniniform crowns and are not crenulated. The enamel on the lingual surface of the crowns is ornamented with prominent anastomosing longitudinal ridges. The ridges are absent from the labial surface where the enamel is smooth with a slight wrinkled texture. There is no evidence of a developed ecto- or entocingulum. An additional single-rooted tooth preserves a small portion of the basolingual section of the crown with an associated single root (Fig. 7E). The crown was damaged during the early 1980s and lacks morphological detail. However, a more complete version of the crown was figured in Fordyce (1978: 663, figs 148–151), indicating a subconical crown. It is unclear whether p1 was denticulate. Fordyce (1978) provisionally identified the tooth as a left p1. Subhorizontal abrasive (formed by tooth/ food contact) apical wear facets are present on all caniniform crowns. The enamel surrounding the wear facets is smooth, contrasting with the longitudinal ridges that ornament the remainder of the lingual surface of the crown. Elongate longitudinal sections of broken enamel are located on the labial surface of all crowns and the lingual surface of the partial crown with wear exposing windows of underlying dentin.
The associated roots of C1 and c1 are elongate, inflated and have a sigmoidal profile in dorsal and ventral views. All surfaces of the roots are convex, with vascular sulci present throughout. The roots are widest and inflated near midlength. Posteriorly, the diameter of the root markedly decreases. The preserved posterior-half of the roots have an elongate ovoid cross-section, while towards the crown, the cross-section becomes increasingly circular. A large fragment of a single root without an associated crown is similarly elongate and inflated (Fig. 7A). One surface of the root is planar to slightly concave, with a prominent anteroposteriorly oriented groove, perhaps vascular, that extends the entire preserved length. The anteriormost and posteriormost sections of the root are missing, which would have probably revealed a sigmoidal profile in anterior and posterior views. Similar to C1 and c1, the root has an anteroposteriorly elongate ovoid cross-section that is widest near the preserved posterior margin or, if complete, at the approximate midlength of the root. Here, the root is inflated as in C1 and c1. The diameter of the root decreases anteriorly. The morphological similarities of the root fragment relative to C1 and c1, in concert with being proportionally smaller, suggests the single root is associated with a more anterior incisor. The associated root of p1 is proportionally smaller relative to C1 and c1 and the root fragment. The subvertical and slightly anterior orientation of the root indicates a more posterior position in the lower tooth row. The apicalthird of the root is moderately inflated. A narrow and shallow (slit-like) sulcus is anteroposteriorly oriented and bisects the posterior surface of the root into labial and lingual lobes, suggesting p1 is morphologically intermediate between single- and double-rooted teeth.
Subrounded holes around the enamel–cementum junction in C1 and throughout all surfaces of the root in c1 are probably indications of the osteophagous worm Osedax Rouse et al., 2004 (Annelida: Siboglinidae). Small, circular holes on the root in c1 have a diameter between 2.0 and 2.5 mm, while larger pockmarks on the labial surface of the root have a diameter between 3 and 6 mm. The labial surface of the root has a large, subrounded depression (maximum diameter of c. 28 mm) that has been significantly bioeroded. Smaller circular depressions (diameters range between 2 and 12 mm) within the larger depression are possibly the result of collapsed Osedax galleries (Boessenecker & Fordyce, 2015a, d). Previously, Osedax has been found on fossil bones and teeth of Oligocene toothed and baleen-bearing mysticetes, including New Zealand eomysticetids, odontocetes and marine birds (Kiel et al., 2010, 2011, 2013; Higgs et al., 2012; Boessenecker & Fordyce, 2015a, d).
Six preserved double-rooted and two triple-rooted posterior cheek teeth are characterized by robust and moderately inflated crowns (Figs 1, 8). The crowns are elongate and compressed, with a triangular profile in labial and lingual views, similar to Basilosauridae. The archaic toothed mysticete Llanocetus denticrenatus, from the latest Priabonian (latest Eocene) of Antarctica, has posterior cheek teeth with crowns that are similarly compressed, as in K. onamata, but are lower and more elongate, resembling the deciduous teeth of Basilosauridae (Kellogg, 1936; Mitchell, 1989; Uhen, 2004; Fordyce & Marx, 2018). The crowns in K. onamata are subvertically oriented. Presumed upper cheek teeth are differentiated by a prominent lingual concavity that is continuous through the root, forming a crescent-shape; the roots of presumed lower cheek teeth are subvertical and moderately concave lingually. An inferred right P3 (Figs 1, 8B) has the largest crown size in the upper tooth row. The crown of an inferred left P4 (Figs 1, 8D) is badly damaged but is proportionally smaller than P3. The preserved posterior-half of an inferred left M1 (Fig. 8F) indicates that crown size in M1 was subequal to P4. The M1 is broken and longitudinally bisected slightly posterior to the anteroposterior midline, exposing an isthmus of cementum and dentin that extends below to the preserved apical-half of the posterior root. An inferred left P2 was figured by Hector (1881: figs 4–4’), depicting a double-rooted tooth with a large, triangular crown in labial and lingual views (Fig. 1). However, the whereabouts of the tooth are unknown. Based on the figure, crown size appears to be larger in P2 relative to P3, although the exact dimensions of the crown of P2 are unknown. In the basilosaurid Dorudon atrox, the crown of P2 is dimensionally largest in the upper-posterior cheek tooth series (Uhen, 2004). In the lower tooth row of K. onamata, crown size increases posteriorly through the lower premolar series, with p4 having the largest crown size (Figs 1, 8E); p4 is the largest tooth in the lower posterior cheek tooth series in D. atrox (Uhen, 2004). Crown width decreases between an inferred right p2 (Figs 1, 8A) and p3 (Fig. 8C), but is subequal between p3 and an inferred left p4 (Figs 1, 8E). The crown of an inferred right m1 (Fig. 8G) is more elongate and transversely wider than p4 and is slightly less tall, unlike Basilosauridae where m1 is considerably smaller than the lower premolar series. An inferred right m2 (Fig. 8H) is incomplete but is proportionally smaller than m1. The enamel on the labial and lingual surfaces of the posterior cheek teeth is weakly ornamented with a wrinkled texture that is confined to the basal-half to basal-third of the crown. Apically, the enamel is smooth. The ectocingulum and entocingulum are weakly developed thickened bands of smooth enamel on all posterior cheek teeth that preserve the basal crown around the enamel– cementum junction. Three distinct, ovoid, pit-like depressions are present on the labial surface of m2. The enamel is unworn within the depressions, indicating that they are not wear facets but are original structural features. All three depressions are located at different elevations on the crown: the first and most elevated depression is located approximately level to the preserved base of the accessory denticle immediately posteriorly adjacent to the primary denticle and is positioned on the approximate anteroposterior midline of the crown; the second depression is located c. 3 mm below the former and is similarly located on the anteroposterior midline; lastly, the third depression is located c. 5 mm below the second and c. 6 mm anterior to the anteroposterior midline of the crown and is more subrounded than the previous two. All depressions are similarly obliquely oriented relative to the vertical axis of the crown. Two hypotheses are proposed for the formation of the depressions: (1) the depressions are an indication of vertebrate scavenging (e.g. sharks; Deméré & Cerutti, 1982; Boessenecker & Fordyce, 2015d); or (2) the depressions are congenital and represent a malformation of the enamel or ontogenetic damage that occurred early in tooth mineralization, assuming that K. onamata was diphyodont, before the pulp cavity of m2 was infilled. Regarding the former hypothesis, the morphology of the depressions differs from previously interpreted evidence of shark predation or scavenging from fossil whale bones, which are slit-like excavations (Deméré & Cerutti, 1982; Boessenecker & Fordyce, 2015d). Thus, the depressions are probably representative of enamel malformations rather than post-mortem predation by other marine vertebrates.
Kekenodon onamata presents teeth with a vestigial third lingual root identified as the P3 (Figs 1, 8B) and P4 (Figs 1, 8D). In each, the lingual root is fused with the posterior root and is partially fused with the anterior root; a narrow and shallow sulcus separates the lingual and anterior roots and is continuous for the preserved length of P3 and P4. Basally, the lingual root closely approximates the anterior root but is not fused. It is not clear if the lingual and posterior roots become unfused at their distal extremity, similar to the squalodontid odontocete Squalodon catulli (Rothausen, 1968); unfused triple-rooted teeth are absent in Basilosauridae but are known from early Middle Eocene Protocetidae (Bajpai & Thewissen, 1998; Hulbert et al., 1998). The posterolingual surface of the crown of P3 and P4 is inflated, forming a bulge in the cervical region of the crown. A lingual bulge is present in semi-amphibious heterodont archaeocetes and has been interpreted as a protocone remnant (Gingerich & Russell, 1990; Thewissen & Bajpai, 2001; Gingerich, 2010; Gingerich & Cappetta, 2014). In toothed mysticetes, the mammalodontids Mammalodon colliveri and Janjucetus hunderi present postcanine teeth that are lingually inflated, which could represent a vestigial third root, whereas all double-rooted teeth of the toothed mysticete Coronodon havensteini have a small ‘demi-root’ that becomes unfused near its basal extremity and forms a noticeable projection. Elsewhere, Trirhizodon Molin, 1859 and Austrosqualodon trirhizodonta Climo & Baker, 1972, both previously referred to Squalodontidae, have triple-rooted molariform teeth with the basal two-thirds of the lingual root not fused to the anterior and posterior roots (Molin, 1859; Kellogg, 1923; Rothausen, 1968).
All crowns of the posterior cheek teeth are denticulate. The accessory denticles are robust, unlike the comparatively gracile denticles of archaic heterodont Odontoceti (Kellogg, 1923; Fordyce, 1981, 1994, 2002b; Dubrovo & Sanders, 2000; Geisler et al., 2014; Tanaka & Fordyce, 2014, 2015a), in addition to mammalodontid and aetiocetid toothed mysticetes (Emlong, 1966; Fitzgerald, 2006, 2010; Deméré & Berta, 2008; Marx et al., 2015). The accessory denticles have a triangular profile in labial and lingual views, with subvertical axes. A moderately deep V-shaped gap separates all accessory denticles, as in Basilosauridae and Coronodon havensteini (Geisler et al., 2017). In Llanocetus denticrenatus, the accessory denticles are noticeably more palmate and are concave toward the primary denticle, producing less subvertical axes than in Kekenodon onamata (Mitchell, 1989; Fordyce & Marx, 2018). A narrow and shallow sulcus is continuous with the apex of the V and extends basally down the crown on the labial and lingual surfaces, forming a boundary between adjacent denticles. Anterior and posterior carinae are present on all preserved denticles. The primary denticle is distinctly larger than all accessory denticles. In general, approximately one additional denticle is present on the posterior surface of the crown than on the anterior surface, although P3 has an equal number of anterior and posterior accessory denticles (N = 4). Additionally, P4 may have an equal number of anterior and posterior accessory denticles. However, the number of anterior accessory denticles is uncertain. In occlusal view, the accessory and primary denticles of p3 (Fig. 8C) and p4 (Figs 1, 8E) have a sigmoidal alignment anteroposteriorly. Accessory denticles are present on the anterior surface of an inferred m1 (Fig. 8G) and m2 (Fig. 8H), whereas the anterior surface of the lower molars in basilosaurids is occupied by a re-entrant groove to accommodate the posterior edge of the preceding tooth in the upper tooth row (Uhen, 2004); there is no evidence of a re-entrant groove in K. onamata. Within Neoceti, the anterior surface of the m3 crown in the Oligocene odontocete Simocetus rayi Fordyce, 2002 lacks accessory denticles and consists of a planar face with bilateral ridges forming the buccal and lingual margins, possibly representing a re-entrant groove (Fordyce, 2002b). The preserved posterior-half of the left M1 (Fig. 8F) preserves four accessory denticles, in addition to the posterior-third of the primary denticle. The number of accessory denticles on the anterior surface is speculative, while there are four posterior accessory denticles. As in Dorudon atrox (Uhen, 2004), it is possible that the number of anterior and posterior accessory denticles on the M1 of K. onamata were equal.
Subhorizontal abrasional apical wear facets are prevalent on nearly all preserved denticles of the posterior cheek teeth. Moreover, most have been severely worn down to the denticle base. Vertically oriented, teardrop-shaped, attritional wear facets are evident on the lingual surface of provisionally identified upper-posterior cheek teeth (Fig. 8B, D, F). The absence of attritional facets from the labial surface of these teeth probably indicates that the upper teeth were positioned labial to the lower teeth during occlusion, as in basilosaurids (Uhen, 2004). The presence of enamel wear facets of similar profile on the lingual surface of provisionally identified p2 (Fig. 8A), p3 (Fig. 8C) and m1 (Fig. 8G) could represent abrasive wear from hard food particles. Alternatively, the facets may be attritional and indicate a large range of lateral motion. However, this is only speculative given the absence of preserved mandibles and crania. It is also possible that these teeth represent upper teeth, although the vertical orientation of the roots suggests a lower toothrow position. Two relatively large and longitudinal ovoid sections of missing enamel are located along the cervical region on the labial surface of the crown of p3 (Fig. 8C). The sections extend apically on to the crown and on to the root where they terminate at midlength; the enamel and underlying dentin have been removed. The more anterior section on p3 extends slightly further basally relative to the more posterior lesion. Both probably represent attritional wear that marks the extent of emergence from the gingiva. Alternatively, these sections could represent abfraction lesions (from Grippo, 1991) previously associated with crown flexure due to large stress loads (McCoy, 1982; Lee & Eakle, 1984) and stress corrosion (Grippo & Masai, 1991). On the labial surface of the crown of P4 (Fig. 8D), most of the enamel on the preserved anterior-half of the crown has been spalled, creating a large window to expose the underlying dentin. Moreover, nearly all of the enamel on the labial surface of p4 has been spalled; a section of spalled enamel is present on the labiolingual surface of m1 on the cervical region of the crown.
Associated roots of the double-rooted teeth are more vertically oriented than the recurved roots of the single-rooted teeth. The surfaces of the roots are convex, excluding the planar to slightly concave internal surfaces of the anterior and posterior roots. The anterior and posterior roots are fused or closely appressed, similar to mammalodontids, but unlike the widely spaced roots of Basilosauridae and L. denticrenatus, which form an ovoid to teardropshaped cleft that extends apically on to the cervical region of the crown, forming a U-shaped enamel– cementum junction (Mitchell, 1989; Fordyce & Marx, 2018). In p2 (Figs 1, 8A) and p3 (Fig. 8C), the anterior and posterior roots are subvertical, elongate, robust and inflated; the posterior root is proportionally more robust. The anterior and posterior roots have a subrounded cross-section that is greatest in diameter at midlength. From this location, the diameter of the root decreases slightly apically and more prominently basally. Fusion of anterior and posterior roots extends to midlength in p2 and the approximate preserved length in p3. A narrow and moderately deep sulcus forms along the fusion of the anterior and posterior roots on the lingual and labial surfaces; the sulcus is deeper on the labial surface. Basally, the anterior and posterior roots bifurcate, forming a V-shaped gap; the apex of the V extends further apically in p2. The position of cementum bands near the midlength of the anterior and posterior roots, corresponding to the alveolar rim, in all preserved double-rooted posterior cheek teeth indicates that the teeth were moderately emergent. The location of cementum bands becomes increasingly more apical on the anterior and posterior roots of the posteriormost cheek, indicating that the upper and lower posteriormost cheek teeth were comparatively less emergent. Attritional wear facets extend basally to the approximate level of the cementum bands on the labial surface of p2, p3 and m1 (Fig. 8A, C, G), further indicating that the posterior cheek teeth were probably only moderately emergent. The exposed internal surface of the posterior-half of the crown and root of M1 (Fig. 8F) reveals a triangular projection of dentin and pulp cavity, which is difficult to differentiate due to the missing anterior portion of M1 and the high level of mineralization; the projection basally extends c. 33 mm and 28 mm from the enamel– cementum junction on the labial and lingual surfaces, respectively. This basally projecting isthmus probably acted as a buttress for the crown to accommodate large occlusal stress loads.
Kellogg (1936) and Abel (1914) noted similarities between the p3 of K. onamata (Fig. 8C) and the taxonomically ambiguous Phococetus vasconum from the Burdigalian of France (Delfortrie, 1873; Kellogg, 1936; Uhen, 2008b), including the size and shape of the crown, lack of prominent cingulum in the cervical region of the crown, enamel ornamented with longitudinal ridges around the base of the crown on the labial and lingual surfaces, smooth enamel on the accessory denticles, the number and arrangement of the accessory denticles, the decrease of accessory denticle size towards the base of the crown, the lowlying and transversely compressed dimensions of the crown, the teeth are double-rooted and the anterior and posterior roots are joined by an isthmus. Despite these similarities, several differences are evident: the p3 crown and at least the apicalmost roots of K. onamata are larger than in P. vasconum; the crown and accessory denticles of P. vasconum are more asymmetrical, with the anterior edge of the crown being more inclined relative to the anterior and posterior edges of the p3 of K. onamata; the labial surface of the crown in P. vasconum lacks longitudinally elongate and ovoid abrasional occlusal wear facets; and the sulcus corresponding to the isthmus joining the anterior and posterior roots is more prominent in K. onamata and is difficult to discern in P. vasconum. Moreover, P. vasconum shares phenetic similarities with archaic toothed mysticetes from the Oligocene of South Carolina (Geisler et al., 2017) and even more so with Inticetus vertizi, an enigmatic and contemporaneous large inticetid odontocete from the Burdigalian of Peru (Lambert et al., 2018). In agreement with Boessenecker (2019), several isolated teeth identified as cf. Phococetus sp. from the Pungo River Formation (Burdigalian) of Lee Creek Mine, North Carolina also resemble I. vertizi, further suggesting that Phococetus has affinities within the Odontoceti. Thus, Phococetus vasconum is provisionally recognized as an Inticetus -grade odontocete until taxonomic affinities can be confirmed by the discovery of more complete specimens.
Atlas: The atlas represents the single preserved postcranial element in the holotype of K. onamata and is not fused to the axis (Fig. 9; Table 5). The atlas is ovoid in anterior view, anteroposteriorly thick and transversely broad. In lateral view, the anteroposterior length of the atlas increases dorsally and narrows ventrally. A break along the ventral arch bisects the atlas into subequal left and right parts, which have been rejoined by an adhesive. The eroded reniform anterior articular surfaces, or condyloid fossae, for the occipital condyles are dorsoventrally tall, shallowly concave and oriented dorsolaterally. The condyloid fossae are mediolaterally broadest at a level just ventral to the base of the transverse processes. In life, the margins of the condyloid fossae were probably marked by crests, now eroded. The neural canal is proportionally large and pyriform, with a transverse diameter that ventrally tapers and dorsally broadens; weathering may have altered the dimensions of the neural canal. The condyloid fossae are too damaged to judge whether they were angled or parallel to the posterior articular surfaces in order to interpret head orientation relative to the body axis. The preserved bases of the left and right neural pedicles project dorsomedially and anteriorly from the dorsal surface of the vertebral body. Additionally, the bases of the left and right transverse processes are preserved, and project dorsolaterally and posteriorly from the dorsolateral surface of the vertebral body; the transverse processes are located ventral and slightly posterior to the neural arch. The medialmost section of the transverse processes has a dorsoventrally oriented ovoid cross-section in lateral view. There is no indication of a vertebrarterial canal. In left-lateral view, a U-shaped depression on the anterodorsal surface of the vertebral body denotes the position of the transversely oriented left transverse foramen (for the first spinal nerve); the right foramen is not preserved. The ventral margin of the transverse foramen is level with the preserved dorsal margin of the base of the left transverse process, with the foramen opening mediolaterally.
The posterior articulating surfaces for the axis are reniform with a dorsoventral long axis and are planar to slightly convex. The mediolateral broadest point of the posterior articulating surfaces (45.6 mm) is located just ventral to the ventral margin of the base of the transverse processes (measurement taken from the left posterior articulating facet). The odontoid fossa is located medial to the posterior articular surfaces and forms the posteroventral margin of the neural canal. The odontoid fossa is concave and rises dorsolaterally toward the posterolateral margins of the neural canal. There is no indication of developed tubercles for the transverse ligament, which would separate the odontoid region (ventrally) from the neural canal (dorsally). The preserved hypapophysis is relatively indistinct and projects posteroventrally from the posteroventral margin of the ventral arch.
description: Published as part of Corrie, Joshua E & Fordyce, R Ewan, 2022, A redescription and re-evaluation of Kekenodon onamata (Mammalia: Cetacea), a late-surviving archaeocete from the Late Oligocene of New Zealand, pp. 1637-1670 in Zoological Journal of the Linnean Society 196 (4) on pages 1640-1656, DOI: 10.1093/zoolinnean/zlac019, http://zenodo.org/record/7381112