Tokarahia kauaeroa, Boessenecker & Fordyce, 2015

Boessenecker, Robert W. & Fordyce, R. Ewan, 2015, A new genus and species of eomysticetid (Cetacea: Mysticeti) and a reinterpretation of ‘ Mauicetus’ lophocephalus Marples, 1956: Transitional baleen whales from the upper Oligocene of New Zealand, Zoological Journal of the Linnean Society (Zool. J. Linn. Soc.) 175 (3), pp. 607-660 : 613-631

TOKARAHIA KAUAEROA GEN. ET SP. NOV.

Etymology

Kauaeroa, meaning long jaw (referring to the elongate, delicate mandibles and rostrum of the holotype), from the Ma¯ ori ‘kauae’ (jaw) and ‘roa’ (long). Pronunciation: Kau-ae-roa, with au as in English ‘hoe’, ae as in ‘I’, o as in ‘toe’, and a as in ‘far’.

Diagnosis of species

A species of large eomysticetid differing from all other eomysticetids except Eomysticetus whitmorei in possessing a deeply incised median furrow of the tympanic bulla in dorsal view, and from all eomysticetids in exhibiting a pars cochlearis that is anterodorsally excavated and deeper posterodorsally.

Holotype

OU 22235 , partial skeleton, including partial cranium, left and right tympanoperiotics, mandibles, cervical and thoracic vertebrae, ribs, sternum, scapula, humeri, radii, and ulnae. Specimen recovered from field ventral-up, and remaining in field jacket, with dorsal surface completely prepared, with part of postcranial skeleton removed and prepared in three dimensions. Cast also deposited in the Museum of New Zealand Te Papa Tongarewa.

Type locality and stratigraphic context

OU 22235 was collected by R. E. Fordyce, A. Grebneff, B.V.N. Black, G.B. McMurtrie, G. Curline, and C.M. Jenkins, 10 January–9 February 1994, from massive glauconitic limestone facies (grainstone according to the Dunham classification scheme for carbonate rocks) of the lower Maerewhenua Member of the Otekaike Limestone , from a north-facing hilltop at Island Cliff , Awamoko Valley , near Tokarahi , 12 km south to south-west of Duntroon , North Otago, New Zealand ( Figs 1 , 2 ). New Zealand Map Series 260 grid reference I41 (1984) 256811, near 44°58′S, 170° 59′E. Fossil record number I41/f0183 ( New Zealand fossil record file, Geological Society of New Zealand). An upper Duntroonian age is likely for OU 22235 (approximately 26.0–25.2 Mya; see Geological background) .

Tentatively referred specimen

OU 21975, isolated right periotic, identified as Tokarahia sp. , cf. T. kauaeroa gen. et sp. nov. OU 21975 was collected by R.E. Fordyce on 12 August 1987, from the diffuse brachiopod– Lentipecten shell bed in a fallen block of upper Kokoamu Greensand, along the eastern end of Kokoamu Cliffs, a few metres north of the cliff face, 4.5 km south-east of Duntroon, North Otago, New Zealand ( Fig. 1 ). New Zealand Map Series 260 grid reference J40 (1984) 309(5) 901(5), near 44°52′S, 170°44′E. Fossil record number J40/f0229 ( New Zealand fossil record file, Geological Society of New Zealand). The brachiopod– Lentipecten shell bed at the type Duntroonian locality at Landon Creek marks the base of the Duntroonian Stage (27.3–25.0 Mya). Therefore, this specimen is lower Duntroonian (approximately 27.3–26.0 Mya).

Description

Premaxilla

The left premaxilla is nearly complete ( Fig. 4 ; Table 2). The anterior one-third of the premaxilla is dorsally flat- tened and slightly transversely wider than the posterior two-thirds. In the region of the narial fossa, the premaxilla becomes transversely constricted where it is medially excavated by the fossa. Adjacent to the narial fossa, the premaxilla is raised and forms a transversely rounded crest; posteriorly it widens and becomes evenly transversely convex. The premaxilla–maxilla suture is open and unfused along its entire length; the sharp lateral edge of the premaxilla articulates within a longitudinal groove on the dorsomedial surface of the maxilla. The elongate, gradually tapering posterior end of the premaxilla articulates with and is underlain by an anteroposteriorly elongate anteromedial prong of the frontal. The fronto-premaxillary suture is closed; where the posterior end of the left premaxilla is missing, parallel longitudinal sutural ridges and grooves are present on the frontal. The posterior end of the Measurements to nearest millimetre. *Incomplete measurement (owing to breakage or incomplete preparation). †Estimated measurements.

premaxilla terminates 5 cm anterior to the posterior end of the nasals. Together with the nasals, the premaxillae share a posteriorly directed V-shaped suture with the frontals.

Maxilla

The maxillae are incomplete ( Fig. 4 , Table 2), and the right maxilla is almost completely missing, whereas much of the anterior part of the left maxilla is present. The shape of the lateral margin of the maxilla is approximated by the dorsal edge of the left mandible, to which it nearly comes into contact; the mandible suggests a nearly straight but faintly laterally convex profile of the rostral margin, as in the more completely preserved eomysticetid Yamatocetus canaliculatus . The visible portion of the maxilla is dorsally flat and smooth; medially it rises towards a laterally sloping surface adjacent to a longitudinal medial ridge. Medial to this ridge is the unfused premaxilla–maxilla suture. Anteriorly, the maxilla and premaxilla are splayed apart slightly, suggesting a greater degree of flexibility than the comparably tight (and ankylosed) premaxilla– frontal suture. Posteriorly, no obvious articular surface for the maxilla on the frontal is present. Ventral details are not exposed.

Nasal

The nasal is very long (90% of postorbital width) and rectangular in dorsal view ( Fig. 4 ; Table 2); the anterior tip is damaged, but dorsally flat. Posteromedially opening foramina with longitudinal sulci are present on the posterior half of the nasal. The posterior third of the nasal is transversely arched, forming the middle portion of a transverse arching of the median rostral elements (nasal and premaxilla). Each nasal terminates along a posteriorly directed V-shaped suture, and extends approximately 7 cm behind the posterior tips of the premaxillae. The nasofrontal suture is similar to the fronto-premaxillary suture, a horizontal planar and ankylosed suture with parallel, longitudinal ridges and grooves exposed on the frontal where the nasal is missing.

Frontal

The frontals are incompletely preserved, and the anterior margins of the supraorbital processes are incomplete ( Fig. 4 ; Table 2). Medially, the supraorbital process of the frontal is anteroposteriorly narrower than the lateral part; it widens laterally towards the orbital margin. The posterior margin of the supraorbital process is concave. An anteroposteriorly elongate anteromedial prong of the frontal is present, and bears many longitudinal ridges and grooves for the articulation of the nasal and premaxilla. Medially, the dorsal surface of the frontal bears many anteriorly to anterolaterally directed radially arranged foramina, up to 3 mm in diameter; foramina close to the midline are vertical and lack sulci. Unlike Tohoraata raekohao (OU 22178), OU 22235 exhibits these foramina posterior to the orbitotemporal crest. The orbitotemporal crest is low, relatively straight, and transversely oriented; laterally, the crest diverges from the posterolaterally direct- ed posterior margin of the frontal. The orbital margin of the supraorbital process is shallowly dorsally arched in the anteroposterior plane.

The median frontal suture is partially open as a longitudinal median groove; two bilaterally symmetrical and anteriorly diverging fissures occur on either side of the median frontal suture and extend posteriorly into the frontoparietal suture. The frontal slopes gradually laterally from the midline; posterior to the orbitotemporal crest, the frontal is more acutely arched transversely, grading smoothly into the low sagittal crest of the parietal. The frontoparietal suture is V-shaped, with the suture originating at the anteromedial edge of the temporal fossa and converging posteriorly.

Parietal

The parietal is exposed in the posterior interorbital region and the anterolateral wall of the braincase ( Fig. 4 ; Table 2). The sagittal crest is low but sharp, and bisected by an unfused median parietal suture; it rises posteriorly to meet the apex of the occipital shield and is dorsally raised above the frontals, giving the dorsal margin of the intertemporal region a concave profile. The lateral surface of the parietal is obscured in dorsal view by the laterally overhanging nuchal crest. The anterior part of the occipital is broken away, exposing the longitudinal ridges and troughs of the unfused occipital–parietal suture. An interparietal is not evident.

Occipital

The occipital shield is triangular in dorsal view, with a slightly anterolaterally concave lateral margin ( Fig. 4 ; Table 2). The occipital shield is transversely concave and deeply concave anteriorly where the subvertical nuchal crests converge. A high external occipital crest is present and continues posteriorly almost all the way to the foramen magnum. Most of the supraoccipital is subhorizontal, in contrast with the more steeply ascending supraoccipital of Tohoraata raekohao and Tohoraata waitakiensis . In lateral view, the nuchal crest is dorsally elevated above the apex of the occipital shield. The exoccipital and basioccipital are still embedded in matrix and obscured by postcrania.

Squamosal

Both squamosals remain in burial position and are disarticulated from the occipital complex; the left squamosal is close to life position, and the right squamosal is anteriorly and medially shifted ( Fig. 4 ; Table 2). Ventral surfaces are not exposed. The zygomatic process is elongate and transversely and dorsoventrally tapers towards its apex. In dorsal view, the zygomatic process is twisted longitudinally so that the lateral surface faces dorsolaterally, and is also bowed medially so that the medial margin is convex and the lateral margin is concave. In transverse cross section, the zygomatic process is broadly rounded. The medial surface of the zygomatic process bears a longitudinal groove, as in most other New Zealand Eomysticetidae ; it is unclear whether this feature is anatomically natural or a consequence of bioerosion, perhaps owing to a naturally weak or porous region of bone. The supramastoid crest is a posterodorsally directed shelf that does not extend onto the base of the zygomatic process. The posterior meatal crest is developed as an elongate low ridge that is obliquely oriented and extends dorsally onto the dorsolateral surface of the squamosal; it occupies about 75% of the dorsoventral thickness of the zygomatic process. The anterior meatal crest delineates the ventral margin of the shallow, triangular sternomastoid fossa.

Periotic

Both periotics are preserved ( Figs 5–8 ; Table 3). The periotic is relatively gracile and similar to Eomysticetus , with elongate anterior and posterior processes, and a relatively small hemispherical pars cochlearis that is not dorsally elongated. The ventral surface of the pars cochlearis is smoothly convex, and lacks a prominent anteromedial corner in ventral view. Posteriorly the fenestra rotunda opens within a small fossa; in posterior view it is teardrop-shaped with a dorsally oriented apex continuous with a minute sulcus extending dorsally to the aperture for the cochlear aqueduct. The pars cochlearis is dorsoventrally deeper posteriorly at the level of the fenestra ovalis, and becomes shallow- er anteriorly; anteriorly the dorsal surface is obliquely oriented and anterodorsally facing. Shallow, discontinuous, and subparallel ridges define the indistinct promontorial grooves.

The internal acoustic meatus is encircled by a low rim that rises posteriorly, and is highest posterolaterally, so that in medial view the lateral surface of the internal wall of the internal acoustic meatus is visible ( Fig. 5 ). This posterolateral prominence is triangular in medial view, and in anterior view, extends dorsomedially; similarly, the foramina within the internal acoustic meatus are also dorsomedially orient- ed so that they are not visible in dorsal view. The internal acoustic meatus is teardrop-shaped and transversely narrows anteriorly to a V-shaped slit, and being widest posteriorly. The spiral cribriform tract and foramen singulare are separated by a low crest that is about as high as the crista transversa; both crests are recessed approximately 5 mm into the meatus. The foramen singulare is the smallest foramen within the meatus; the spiral cribriform tract and dorsal opening of the facial canal are of similar size, and both are somewhat transversely compressed and oval. The aperture for the cochlear aqueduct is small, circular, and positioned medially on the dorsal face of the pars cochlearis, but is not aligned with the spiral cribriform tract and facial canal, as in some Cetotheriidae . The aperture for the vestibular aqueduct is encircled by a low, sharp ridge of bone; both the peripheral ridge and aperture are recessed within a common fossa.

Anterior to the fenestra ovalis, an oval incisural flange ( Boessenecker & Fordyce, 2015) is closely appressed to the anterolateral base of the pars cochlearis ( Fig. 5A ); it is separated from the pars cochlearis by a finely incised, minute sulcus, and an additional sulcus defines the lateral margin of the incisural flange at the base of the lateral tuberosity and mallear fossa. The sulcus on the lateral side of this flange extends anteriorly, where it joins the foramen leading to the anterointernal sulcus; the anterointernal sulcus is anteroposteriorly oriented and extends to the anteroventral angle. Aside from this sulcus, there is no distinct origin for the tensor tympani insertion; the rest of the medial surface of the anterior process is flat. The anterior process is transversely compressed and bears a sharp anterior keel; in medial view the process is nearly triangular, except for the triangular anterodorsal angle. The anteroventral angle is the anteriormost point of the anterior process, whereas the anterodorsal angle is positioned midway between the tip of the anterior process and the pars cochlearis. The anterior margin of the anterior process slopes anteroventrally from the anterodorsal angle to the anteroventral angle, and is anteriorly concave, as in Tohoraata raekohao . The ventral margin of the anterior process is slightly convex, and bears an elongate, triangular anterior bullar facet for the articulation of the accessory ossicle. In transverse cross section, the anterior process is triangular and tapers dorsally. Anterior to the lateral tuberosity, the lateral surface of the anterior process is somewhat convex.

The lateral tuberosity is triangular in ventral view, projects laterally, and is anteroposteriorly compressed and sharp at its apex; posteriorly, a welldefined rectangular facet for the articulation with the anterior face of the spiny process of the squamosal is present. The distinct and subtriangular mallear fossa is situated on the posteromedial part of the lateral tuberosity, being positioned medial to its apex rather than posterior to it, as in Basilosauridae . A shallow pit is present on the lateral surface of the periotic, adjacent to the lateral tuberosity; a very shallow transversely oriented anteroexternal sulcus extends dorsally from this pit. A more deeply incised sulcus and fissure is formed within the dorsal half of the shallow furrow, emanating from a small foramen; this deeply incised sulcus forms a deep notch in the superior process and terminates at the anterolateral portion of the suprameatal fossa. It is unclear whether one – or both – of these structures is homologous with the anteroexternal sulcus. The ventral opening of the facial canal is small with a transversely narrow, V-shaped opening; it opens slightly further anteriorly than the larger, oval fenestra ovalis. The facial canal opens posteriorly into a shallow facial sulcus that extends posteriorly to the level of the stapedial muscle fossa, where it curves ventrally towards the facial crest. The stapedial muscle fossa is deeply concave and has a somewhat Measurements given to nearest hundredth of a millimetre.

rugose, pitted surface. It is defined medially by the short caudal tympanic process, which lacks a posterior shelflike crest. The caudal tympanic process is oriented posteromedially.

The posterior process is relatively long (about 150% of pars cochlearis length). The posterior bullar facet is large, diamond shaped, and tapers proximally and distally. With the exception of a few faint longitudinal grooves posterolaterally, the posterior bullar facet is smooth and transversely convex. The posterior process is dorsally cylindrical, and separated from the medial and lateral edges of the posterior bullar facet by deep longitudinal grooves. The posterodorsal angle is shaped as a blunt corner, approximately forming a 90° angle between its dorsal and posterior margins. At the level of the internal acoustic meatus, the dorsal margin of the superior process is concave, and forms a saddle between the posterior apex of the superior process (= posterodorsal angle) and the anterior apex (= anterodorsal angle). A deep suprameatal fossa is developed, and is floored by bone with a porous, woven texture. The posterior part of the lateral face of the periotic is slightly convex and bears numerous minute pores. The posteroexternal foramen is slit-like, and opens into a dorsoventrally oriented groove positioned posterolateral to the posterodorsal angle.

The tentatively referred specimen OU 21975 ( Fig. 9 ) shares with T. kauaeroa gen. et sp. nov., T. lophocephalus , and Tokarahia sp. , cf. T. lophocephalus , a diamondshaped posterior bullar facet and a posterodorsal corner nearly forming a 90° angle ( Figs 8D , 16 ; Table 3). OU 21975 shares with T. kauaeroa gen. et sp. nov., to the exclusion of all other eomysticetids, a pars cochlearis that is anterodorsally excavated so that the pars cochlearis increases in height posteriorly. In general, this periotic is more massive than the T. kauaeroa gen. et sp. nov. holotype (OU 22235), and is rugose with a slightly higher and deeper superior process and suprameatal fossa (respectively). The shorter anterior process is less acutely pointed in medial view, with a less concave anterior margin. Unlike other Tokarahia spp. , a large tubercle with vertical striations is developed on the dorsal margin of the fenestra rotunda; however, a dorsally extending sulcus is present as in T. kauaeroa gen. et sp. nov. and Tokarahia sp. , cf. T. lophocephalus (OU 22081). The prominence on the posterolateral margin of the internal acoustic meatus is more extremely elevated than in other Tokarahia spp. , particularly in comparison with the T. lophocephalus holotype periotic. Although shared with Tokarahia sp. , cf. T. lophocephalus (OU 22081), a small dorsal tubercle is present between the apertures for the cochlear and vestibular aqueducts, unlike the T. kauaeroa gen. et sp. nov. holotype (OU 22235). In comparison with the T. kauaeroa gen. et sp. nov. holotype (OU 22235), the endocranial opening of the facial canal is more circular and the crista transversa is less recessed into the internal acoustic meatus, and significantly less recessed than in Tokarahia sp. , cf. T. lophocephalus (OU 22081). It further differs from OU 22081 in lacking a superficial bridge of bone that dorsally roofs the internal acoustic meatus. Similar to the T. kauaeroa gen. et sp. nov. holotype (OU 22235), but unlike Tokarahia sp. , cf. T. lophocephalus (OU 22081) and the T. lophocephalus holotype, the fenestra rotunda, and aperture for the cochlear aqueduct are very closely positioned. The caudal tympanic process is intermediate in posteromedial divergence between T. kauaeroa gen. et sp. nov. (OU 22235) and Tokarahia sp. , cf. T. lophocephalus (OU 22081). The pit on the lateral surface immediately dorsal to the lateral tuberosity is more deeply excavated than in the T. kauaeroa gen. et sp. nov. holotype (OU 22235), but is similar to Tokarahia sp. , cf. T. lophocephalus (OU 22081); similarly, a trough-like anteroexternal sulcus is present as in OU 22081 but unlike OU 22235.

Tympanic bulla

The tympanic bulla is relatively large and elongate, with well-differentiated medial and lateral lobes; in ventral and dorsal view the bulla has a cordate outline, tapering anteriorly and widest posteriorly ( Figs 10– 13 ; Table 4). The involucrum is relatively large, dorsoventrally deepest posteriorly, and is shallow anteriorly. In dorsal aspect, the involucrum abruptly narrows anteriorly, where it is formed as a transversely narrow sharp ridge. At the midpoint of the bulla, faint transverse creases are present on the involucrum. On the ventromedial surface of the involucrum, an elongate oval facet with rough surface texture is present; this would have lain close to the medial edge of the basioccipital crest in life. The dorsomedial surface of the involucrum is generally smooth, but becomes rough ventromedial to the involucral ridge ( Oishi & Hasegawa, 1995). The involucral ridge separates smooth bone inferred to mark the peribullary sinus dorsally from roughened bone embedded in soft tissues ventrally. The involucral ridge expands posteriorly into the aforementioned oval facet, and is then posteromedially contiguous with the transverse crest on the posterior margin of the medial lobe. In medial view, the posterior margin of the medial lobe bears a slight posteroventral corner formed by the transverse crest, but it is positioned on an otherwise broadly rounded margin, unlike the angular margin in Basilosauridae . In posterior view, this transverse crest is ventromedially oriented. The median furrow forms a well-defined notch in dorsal view. The lateral lobe extends somewhat further posterior than the medial lobe, and further ventrally, so that it is visible in medial view below the medial lobe.

The inner posterior pedicle is formed as a large tubercle; lateral to the pedicle is a deeply incised, V-shaped elliptical foramen. The conical process is low and slightly dorsoventrally thickened. The outer posterior pedicle is a low and anteroposteriorly short (∼ 7 mm long) ridge. The ventral side of the conical process bears a shallowly incised tympanic sulcus; radiating striae emanate from the sulcus. The sigmoid process is oval in medial view and separated from the conical process by a deeply incised sigmoid fissure; the fissure is dorsally vertical and curves anteroventrally to form a horizontal cleft. The mallear ridge is low and convex, and is separat- ed from the sigmoid process by a shallow furrow. Anterior to the malleus, a well-developed sulcus for the chorda tympani arises from the anterior process along the medial edge of the outer lip. Anteriorly this sulcus passes onto the dorsal surface, defining a medial lamina of the outer lip that is posteriorly directed and tonguelike, perhaps articulating with the accessory ossicle as in Odontoceti. The tympanic cavity exhibits a transversely narrow opening that widens anteriorly towards the oval, anteromedially oriented musculotubal canal. Internal to the lateral furrow, a sharp internal ridge is present at about the same position as the low transverse ridge on the floor of the tympanic cavity; these ridges divide the tympanic cavity into anterior and posterior compartments.

The posterior process of the tympanic bulla is subtriangular in medial view; it bears an acute anterior apex where it attached to the inner posterior pedicle ( Fig. 12 ). In medial view, the posterior process is fanshaped with concave anterodorsal and anteroventral margins. Posteriorly the process is dorsoventrally expanded and bears a dorsal spur adjacent to the facet for the periotic. The facet is transversely concave and bears longitudinal sulci. Anterior to the dorsal spur, a transversely thin crest descends from the spur to the anterior apex of the posterior process of the tympanic bulla. When articulated, the anterior part of the posterior process extends anteriorly to the level of the fossa incudis. Furthermore, when the periotic and bulla are placed in articulation, the orientation of the two elements is distinctly different from that of archaeocete and toothed mysticetes. When the articulated tympanoperiotic is oriented with respect to the tympanic bulla, the dorsal surface of the periotic faces dorsomedially ( Fig. 13 ). Because the lateral surface of the periotic articulates along a vertical butt joint in archaic mysticetes, this configuration implies that the tympanic bulla of Tokarahia is somewhat rotated with respect to archaeocetes and toothed mysticetes, so that the lateral surface of the outer lip would face ventrolaterally, intermediate between the non-rotated bulla of archaeocetes (and toothed mysticetes) and the dorsomedially rotated bulla of crown mysticetes ( Bouetel & Muizon, 2006).

Mandible

Both mandibles are preserved; the left mandible is present in life position and tightly articulated with the lateral margin of the damaged left maxilla, whereas the posterior end of the right mandible remains in approximate life position but it is flipped around by 180° ( Fig. 4 ; Table 5). In dorsal view, the mandible is slightly bowed laterally, but not as strongly curved as extant Balaenopteridae ; it lacks a posteriorly recurved section, and is evenly curved along its length. Anteriorly the mandible is transversely narrow, but at mid-length the body is transversely thick and nearly cylindrical in cross section. A sharp ventral crest is present only along the anterior 30 cm of the mandible, and the rest of the mandible has a broadly rounded ventral margin in cross section. The dorsal edge of the mandible is sharp along the entire length of the body but bears a welldeveloped alveolar groove, approximately 1–1.5 cm in width. Several parallel and anteriorly directed foramina, with associated sulci up to 10 cm in length and 3 mm wide, open within the groove.

A longitudinal furrow lies on the dorsolateral margin of the body, laterally adjacent to the alveolar groove. Within this furrow, six mental foramina open into anteriorly directed sulci up to 5 cm long and 5–8 mm wide. The anterior tip of the mandible lies about threequarters from the ventral margin. The anterior margin of the mandible is sharply triangular, but not acutely spear-shaped, as in OU 22044 and OU 12918. The anterodorsal part of the anterior tip of the mandible bears a longitudinal groove with three (possibly four) anteriorly directed foramina. The anteriormost foramen is largest (approximately 5 mm wide), and positioned ventrally just dorsal to the anterior apex of the mandible, and each foramen posterior to this is successively smaller and higher on the anterodorsal margin. Posteriorly, the lateral surface of the mandible becomes flattened, leading towards the region of the coronoid process. The coronoid process has an anteroposteriorly broad base and is broadly triangular in lateral view; the apex is damaged, so it is unclear whether the apex was triangular or broadly rounded, as in Yamatocetus ; in Tokarahia sp. , cf. T. lophocephalus (OU 22081), the coronoid process is broadly rounded. The mandibular condyle and angular process are not preserved.

Atlas

The atlas is large, robust, and anteroposteriorly thick; only the posterior side is exposed ( Fig. 4 ; Table 6). The neural canal is wide dorsally and narrows ventrally but is bilobate, as in T. lophocephalus . The neural arch is dorsoventrally flattened, evenly dorsally convex, and lies far anteriorly with a posterolaterally directed lamina. Anteriorly the lamina is perforated by a 1 cm wide, transverse foramen. The transverse processes are damaged, but appear to have been robust and transversely short, and not perforated by a vertebrarterial canal. The ventral margin of the atlas is truncated, as in Tohoraata waitakiensis . The vertebral epiphyses of the atlas are fused.

Axis

Because the axis is exposed on its side (in lateral view) and obscured by other bones, few details of its morphology are evident ( Fig. 4 ; Table 7). The neural spine is high, slightly posterodorsally inclined, and rectangular in lateral view. A small postzygapophysis extends posteriorly from the neural arch; ventral to this the arch is posteriorly excavat- ed, where the pedicle is anteroposteriorly narrower than the lamina and spine. The vertebral epiphyses appear to be fused.

C3–C7

All four mid-cervicals are present, but only C3 can be confidently identified because it remains in articulation with the axis; the others are too incomplete to be identified to position, but one is identified as?C4 ( Fig. 4 ). The body of these vertebrae is round and near circular, and slightly wider than deep. All exhibit a pointed ventral margin, fused epiphyses that lack notochordal pits, and ventrally positioned ventrolaterally projecting transverse processes. In?C4 the transverse processes are oriented more transversely than ventrolaterally. The pedicles are subrectangular in lateral view, plate-like, dorsolaterally oriented, and exhibit dorsoventrally compressed postzygopophyses; between the body and the postzygapophysis, the posterior margin of the pedicle is concave. The neural arch is triangular, giving the neural canal a wide suboval shape with a triangular dorsal margin, unlike T. lophocephalus . One cervical vertebra possesses dorsally positioned and plate-like transverse processes contiguous with the pedicles, identifying it as C7 ( Fig. 4 ; Table 8).

Thoracic vertebrae

Nine thoracic vertebrae are preserved ( Figs 4 , 14 ), and the T1 remains in articulation with C7 ( Fig. 4 ). Aside from having an anteroposteriorly thicker body than the C7, no morphological details are available. Four isolated thoracic vertebrae are preserved, and four thoracic vertebrae are preserved in articulation in a block in association with 11 ribs and the radius ( Fig. 14A ). The thoracic vertebrae exhibit an anteroposteriorly more elongate body, approximately as long as transversely wide. The pedicles are subvertically oriented, leading towards anterodorsally positioned knob-like transverse processes. The neural foramen is small and ovalshaped, with a triangular dorsal margin. Tall, transversely narrow, rectangular, and posterodorsally inclined neural spines are preserved in the articulat- Measurements given to nearest hundredth of a millimetre.

*Incomplete measurement (owing to breakage or incomplete preparation). Values in bold denote estimated measurements.

Measurements given to nearest millimetre.

*Incomplete measurement (owing to breakage or incomplete preparation). Values in bold denote estimated measurements.

ed thoracic vertebrae. The vertebral epiphyses of the thoracics are unfused and missing from the isolated thoracic vertebrae, and are present but incompletely fused in the articulated vertebrae.

Ribs

Parts of 17 ribs are preserved, both left and right ( Fig. 14A, F–G ). The anteriormost ribs have dorsoventrally expanded and anteroposteriorly flattened proximal portions, with dorsally prominent but flattened tubercles. Smaller, posteriorly inclined secondary tubercles lie slightly distal. The head is missing from these anterior ribs. The tubercle is elevated far above the broken head. Distally, the anterior ribs are transversely expanded and anteroposteriorly flattened, with flat (not concave) posterior surfaces. The posterior ribs have a cylindrical distal portion, and are more strongly curved in the proximal one-third than the anterior ribs. The proximal primary tubercles are smaller, but closer to the head, whereas the secondary tubercles are larger than in the anterior ribs. The neck is smaller and the head is indistinct, and the proximal end is less anteroposteriorly flattened and dorsoventrally expanded than the anterior ribs.

Sternum

The sternum is well preserved and relatively small, and is missing its posterior extremity ( Fig. 4 ; Table 9). It appears to be a single element, as there are no articular surfaces for other sternal bodies, similar to OU 22044 and OU 22081. The sternum is Measurements given to nearest millimetre.

*Incomplete measurement (owing to breakage or incomplete preparation). Values in bold denote estimated measurements.

Measurements given to nearest millimetre.

*Incomplete measurement (owing to breakage or incomplete preparation).

Measurements given to nearest millimetre.

*Incomplete measurement (owing to breakage or incomplete preparation).

approximately triangular and dorsoventrally flattened; it bears a pair of anterolaterally directed, dorsoventrally flattened semicircular processes for articulation with a single pair of ribs. The anterior and posterolateral margins of the sternum are concave; the posterior process of the sternum may have been longer, but is broken.

Scapula

Both left and right scapulae are large and well preserved ( Fig. 15A, B, F ; Table 10). The distal end of the scapula is relatively small in comparison with the anteroposteriorly broad proximal blade. The vertebral border of the scapula is evenly curved and dorsally convex. The anterior and posterior ends of the scapula are evenly curved, without pointed apices. Immediately dorsal to the acromion, the supraspinous region is not distinctly concave and is developed as a convex region between the spine and anterior margin, which are nearly contiguous. The acromion is well developed as a transversely flattened, tongue-shaped, anteriorly directed process that is slightly longer than dorsoventrally deep. Ventrally, the supraspinous fossa is developed as a concave trough on the basal, medial surface of the acromion, forming a longitudinal trough between the acromion and anterior border. The acromion is positioned slightly dorsal to the glenoid fossa. The glenoid fossa is directed slightly posteroventrally; a coracoid process is not developed.

Humerus

The right humerus is missing its proximal end, whereas the left humerus is nearly complete and missing the deltopectoral crest; the humeral head is disarticulated ( Fig. 15C–H ; Table 10). The humerus has basilosauridlike proportions, and is approximately twice as long as the width of the capitulum. The capitulum is Measurements given to nearest millimetre.

*Incomplete measurement (owing to breakage or incomplete preparation).

posterodorsally directed, convex, and transversely compressed. The deltopectoral crest is damaged, but appears to have been transversely compressed and present along the proximal two-thirds of the humerus. The lateral surface is somewhat flattened, whereas the medial surface bears a somewhat rugose prominence near the proximal end of the diaphysis, as in Yamatocetus . Distally the medial surface is flat and the lateral surface is more convex. The distal epiphyseal sutures are closed and nearly obliterated. The distal end bears two distinct fat facets for the radius and ulna that meet at an angle, unlike Basilosauridae . The radial facet is nearly twice as long as the ulnar facet. The posterior margin of the humerus is straight.

Radius

The left radius is preserved in a large block, in association with articulated thoracic vertebrae and ribs ( Fig. 14A ; Table 10). The distal epiphyseal suture is closed, and the proximal end is damaged. The distal half of the radius is anteroposteriorly broader and transversely flat in comparison with the more cylindrical proximal end. The radius is slightly bowed anteriorly. In cross section the radius transversely tapers anteriorly. The distal articular surface is posteroventrally oriented. The interosseus crest appears to be present and sharp, but damaged.

Ulna

The right ulna is nearly complete, lacking only the distal epiphysis, and the proximal half of the left ulna is preserved ( Fig. 15F, I–N ; Table 10). The olecranon is hatchet-shaped, posterodorsally directed, with a posteroventrally positioned apex. The articular surface for the humerus is dorsally facing, oval shaped, and anterodorsally directed; it does not extend onto the olecranon process, but the anterior surface of the olecranon would have braced the humerus and limited extension of the humero-antebrachial joint. The shaft of the ulna narrows just distally to the olecranon process, and the distal three-quarters of the shaft is rectangular and transversely narrows slightly. No obvious interosseous crest is present. The distal epiphyseal surface bears a punctate texture.

Rib histology

The sectioned rib has a lenticular cross section with a large and narrow marrow cavity ( Fig. 16B, D ); the sectioned rib fragment is relatively straight, and similar in dimensions and curvature to the distal two-thirds shaft of other preserved ribs. The marrow cavity is not open, but consists of a network of large vascular channels with trabecular struts that form a cancellous zone. The marrow cavity separates two zones of cortical bone: one on the strongly convex side of the bone, and the second on the flat side of the bone. The cortex on the convex side is nearly completely remodelled and composed of dense haversian tissue formed by overlapping secondary osteons (125–180 μm in diameter) and fragments of secondary osteons ( Fig. 16D ). Vascular channels in secondary osteons are generally smaller towards the outer cortex (>20 μm), and increase in diameter towards the inner cortex (up to 90 μm). The cortex–marrow transition zone consists entirely of former vascular channels deeply infilled by endosteal lamellae, becoming increasingly remodelled and overprinted by secondary osteons towards the inner cortex. The thickness of endosteal lamellae decreases toward the marrow cavity; in the marrow cavity, trabeculae consist of endosteal lamellae parallel with margins of the vascular channels. In contrast, vascular channels on the opposite side of the marrow cavity along the flat side of the rib lack thickened endosteal laminae. The cortex of the flat side of the rib consists of less densely remodelled bone with numerous secondary osteons that do not often overlap; few fragments of secondary osteons exist. Nonremodelled lamellar bone parallel with the flat margin of the rib is present as background tissue. Lamellar bone lacks obvious primary osteons. Periosteum is absent from the convex margin, and secondary osteons are truncated at the margin, indicative of resorption; in contrast, ghosts of periosteal lamellae are visible amongst an opaque diagenetically altered zone (300– 400 μm) along the outer margin of the flat side of the cross section. The altered zone consists of a densely criss-crossing network of tubular structures under 6 μm in diameter. Secondary osteons from the flat side generally have larger diameter vascular channels than the convex side. This pattern is overall suggestive of growth by the addition of lamellae on the flat side, with partial remodelling, formation of large vascular channels, and then subsequent filling by the addition of endosteal lamellae, intense remodelling, and eventual resorption along the convex side.

Scapula, anteroposterior length 67.0 Scapula, dorsoventral depth 46.5 Distal scapula, anteroposterior length 13.5 Scapula, dorsoventral depth of acromion 6.6 Humerus, total length 38.5 Humerus, anteroposterior width of distal end 11.0 Humerus, transverse width of distal end 5.6 Ulna, total length 43.3 Ulna, total length (to humeral articulation) 35.8 Ulna, transverse width at humeral articulation 4.1 Ulna, anteroposterior length at olecranon 15.1 Ulna, transverse proximodistal length of olecranon 13.7 Radius, total length 37.4

Measurements given to nearest millimetre.

Figures

Figure 1. Location and stratigraphy of Tokarahia-bearing localities, South Island, New Zealand. A, map of South Island with inset showing position of (B). B, map of the Waitaki Valley region, showing the positions of three Tokarahia localities. C, excavation at type locality of Tokarahia kauaeroa gen. et sp. nov., Island Cliff, North Otago (© R.E. Fordyce). D, Oligocene stratigraphy at Kokoamu Cliffs, modified from Gage (1957) and Boessenecker & Fordyce (2015). E, Oligocene stratigraphy at Island Cliff, modified from Gottfried & Fordyce (2001). F, Oligocene stratigraphy at Hakataramea Quarry, modified from Gottfried et al. (2012).

Figure 2. Excavation of the Tokarahia kauaeroa gen. et sp. nov. holotype skull and skeleton: A, exposure of the skull and mandibles in a ventral-up position;B, removal of the large jacket containing the skull.

Figure 4. Holotype (OU 22235) skull, mandibles, vertebrae, and sternum of Tokarahia kauaeroa gen. et sp. nov. A, orthogonal image derived from photogrammetry. B, interpretive line drawing.

Figure 5. Holotype right periotic (OU 22235) of Tokarahia kauaeroa gen. et sp. nov., whitened with ammonium chloride: A, ventral; B, dorsal; C, medial; D, lateral; E, anterior; F, posterior.

Figure 6. Detail of internal acoustic meatus of Tokarahia kauaeroa gen. et sp. nov. holotype (OU 22235) right periotic: A, photograph; B, interpretive line drawing.

Figure 7. Holotype left periotic (OU 22235) of Tokarahia kauaeroa gen. et sp. nov., whitened with ammonium chloride: A, ventral; B, dorsal; C, medial; D, lateral.

Figure 8. Comparison of eomysticetid periotics in ventral view, whitened with ammonium chloride: A, holotype left periotic of Eomysticetus whitmorei (reversed); B, holotype right periotic of Tohoraata raekohao; C, holotype right periotic of T. kauaeroa gen. et sp. nov.; D, referred right periotic of Tokarahia sp., cf. T. kauaeroa gen. et sp. nov.; E, holotype right pars cochlearis of Tokarahia lophocephalus; F, referred right periotic of Tokarahia sp., cf. T. lophocephalus.

Figure 9. Referred right periotic (OU 21975) of Tokarahia sp., cf. Tokarahia kauaeroa gen. et sp. nov., whitened with ammonium chloride: A, ventral; B, medial; C, dorsal; D, lateral.

Figure 10. Holotype right tympanic bulla (OU 22235) of Tokarahia kauaeroa gen. et sp. nov., whitened with ammonium chloride: A, medial; B, lateral; C, dorsal; D, ventral; E, posterior; F, anterior.

Figure 11. Holotype left tympanic bulla (OU 22235) of Tokarahia kauaeroa gen. et sp. nov., whitened with ammonium chloride: A, medial; B, lateral; C, dorsal; D, ventral.

Figure 12. Holotype (OU 22235) left and right periotics, right tympanic bulla, and posterior processes of the tympanic bulla of Tokarahia kauaeroa gen. et sp. nov., whitened with ammonium chloride: A, left periotic with articulated posterior process of bulla in ventral view; B, right periotic with posterior process of bulla in ventral view; C, left periotic with articulated posterior process of bulla in medial view; D, right periotic with articulated posterior process of bulla in medial view; E, right tympanic bulla with posterior process in medial view; F, left posterior process of bulla in ventromedial view; G, and dorsal view; H, right posterior process of bulla in dorsal view; I, and ventromedial view.

Figure 13. Articulated tympanoperiotic of Tokarahia kauaeroa gen. et sp. nov. holotype (OU 22245): A, anterior; B, posterior; C, dorsal (relative to bulla); D, medial (relative to bulla); and E, lateral view (relative to bulla).

Figure 14. Axial skeleton and radius of Tokarahia kauaeroa gen. et sp. nov. holotype (OU 22235): A, associated and articulated ribs, thoracic vertebrae, and radius; B, thoracic vertebra I in anterior view (A) and posterior view (B); D, E, thoracic vertebra B and C in anterior view; F, G, left ribs in posterior view (F) and anterior view (G).

Figure 15. Appendicular skeleton of Tokarahia kauaeroa gen. et sp. nov. holotype (OU 22235): A, right scapula in medial view; B, right scapula in lateral view; C, left humerus in lateral view; D, left humerus in medial view; E, left humerus in anterior view; F, left scapula, humerus, and ulna in approximate articulation in lateral view; G, right humerus in lateral view; H, right humerus in medial view; I, left ulna in medial view; J, left ulna in anterior view; K, left ulna in lateral view; L, right ulna in lateral, M, medial, and N, anterior view.

Figure 16. Rib histology of Tokarahia: A, complete thin section of Tokarahia sp., cf. Tokarahia lophocephalus (OU 22081); B, complete thin section of Tokarahia kauaeroa gen. et sp. nov. (OU 22235); C, photomicrograph of cortex of OU 22081; D, photomicrograph of cortex of OU 22235.