Tokarahia lophocephalus, 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 : 631-647

TOKARAHIA LOPHOCEPHALUS COMB. NOV.

Diagnosis

A species of Tokarahia differing from T. kauaeroa gen. et sp. nov. in possessing zygomatic processes that do not extend anterior to the occipital shield, more extremely ‘telescoped’ nasal and premaxillae that penetrate the posterior half of the frontal, a tympanic bulla without a median furrow incised as a notch in the posterior margin of the tympanic bulla in dorsal view, more widely posteromedially divergent caudal tympanic process, a more deeply excavated pit on the lateral side of the anterior process, a fenestra rotunda that is more widely separated from the aperture for the cochlear aqueduct, lacking a finely sculptured tubercle immediately dorsal to the fenestra rotunda, and exhibiting a small vertebrarterial canal in the seventh cervical vertebra.

Holotype

OM GL 412 (= old catalogue number OM c.62.1), partial skeleton, including partial cranium (now lost; Fig. 17 ), partial right periotic ( Figs 8 , 18 ), left and right tympanic bullae ( Figs 19 , 20 , fragmentary left mandible ( Fig. 21 ), seven cervical vertebrae, four thoracic vertebrae ( Fig. 22 ), and two scapulae (both lost). Marples (1956) reported that two scapulae were preserved, and Fordyce (1980: 1980) mentioned that one scapula was lost alongside the skull, but that another scapula bearing the old number OM c.62.4 belonged to the holotype individual. Two scapulae bearing the old number OM c.62.4 exist: one is the right scapula of ‘ Mauicetus ’ brevicollis , illustrated by Marples (1956: figure 6, mistakenly identified as a left scapula), embedded in a plaster block with the medial surface exposed, and the other is a less well-preserved left scapula prepared in three dimensions and missing much of the dorsal margin. Matrix adhering to the left scapula is glauconitepoor calcareous limestone matrix, similar to the right scapula of ‘ Mauicetus ’ brevicollis , and is otherwise a mirror image of the right scapula, indicating both belong to ‘ Mauicetus ’ brevicollis . Both scapulae bearing the old OM c.62.4 number, and here interpreted as the holotype scapulae of ‘ Mauicetus ’ brevicollis , bear a welldeveloped coracoid process, differing from the scapula of T. kauaeroa gen. et sp. nov.

Referred specimen

OM GL 443 (= old catalogue number OM c.78.2), isolated partial right tympanic bulla ( Fig. 23 ). This specimen formerly bore the old catalogue number OM c.62.3, the same number as the holotype specimen of ‘ Mauicetus ’ brevicollis ; however, Fordyce (1980) indicated that Marples (1956) never mentioned this specimen, and relabelled the specimen as OM c.78.2; the current catalogue number is OM GL 443. Furthermore, the holotype specimen of ‘ Mauicetus ’ brevicollis is not an eomysticetid based on the triangular transverse processes of the atlas, greatly enlarged transverse processes of the axis with vertebrarterial canals, and anteroposteriorly flattened C3–C7, differing markedly from Tohoraata waitakiensis , Tokarahia lophocephalus , and Tokarahia kauaeroa gen. et sp. nov., as well as Eomysticetus and Yamatocetus . Given the lack of any connection to ‘ Mauicetus ’ brevicollis , this specimen is separated and referred to T. lophocephalus , from which it is indistinguishable (see below).

Tentatively referred specimen

OU 22081, partial skeleton including fragmentary rostrum and braincase ( Figs 24 , 25 ), left and right periotics and tympanic bullae ( Figs 26–28 ), isolated tooth ( Fig. 29 ), incomplete mandibles ( Figs 24 , 25 ), partial atlas and axis, partial third and fourth cervical vertebrae, and sternum ( Fig. 30 ), identified as Tokarahia sp. , cf. T. lophocephalus . OU 22081 was collected from about 6–7 m above the basal Duntroonian brachiopod– Lentipecten shell bed and about 6–8 m below the lowermost occurrence of Waitakian foraminifera at Hakataramea quarry, and approximately 2–3 m below a horizon yielding Duntroonian foraminifera (= Aglyptorhynchus hakataramea type horizon), indicating a Duntroonian age for OU 22081, probably upper Duntroonian (approximately 26.0–25.2 Mya).

Type locality and stratigraphic context

The holotype skeleton of T. lophocephalus (OM GL 412) was collected by B.J. Marples in 1942 from massive glauconitic sandstone of the Kokoamu Greensand, Kokoamu Cliffs, 3 km east south-east of Duntroon, North Otago, South Island, New Zealand ( Fig. 1 ). Grid reference NZMS 260 I40:29890, near 44°52′S, 170°42′E. Fossil record number I40/f0027 ( New Zealand fossil record file, Geological Society of New Zealand). The holotype was probably collected from the diffuse brachiopod– Lentipecten shell bed in the upper part of the unit, which at the nearby type Duntroonian section at Landon Creek marks the base of the Duntroonian stage, suggesting a lower Duntroonian age (approximately 27.3– 26.0 Mya).

Description

Cranium

To reduce redundancy with the description of T. kauaeroa gen. et sp. nov., this description empha- sizes features that differ between the two or features of OU 22081 that differ from or are not preserved for OM GL 412. The holotype cranium of T. lophocephalus (OM GL 412) is lost, but morphological details present in the low-resolution half-tone plate published by Marples (1956: plate 1; reproduced in Fig. 17 ) permit a limited redescription. The skull includes a fragmentary proximal rostrum, complete frontals, squamosals, and braincase. The nasal is elongate with parallel lateral margins and a squared-off posterior end. The premaxilla appears to terminate along the lateral side of the nasal as a posteriorly tapering wedge. The maxilla is almost completely missing. The supraorbital process of the frontal is transversely wider than anteroposteriorly long. The nasals extend posteriorly to the anteroposterior midpoint of the frontal, nearly to the orbitotemporal crest, and further posterior than in T. kauaeroa gen. et sp. nov. The orbitotemporal crest is positioned near the posterior margin of the frontal. Medially, the crest is anteriorly retracted from the posterior margin. An elongate, posteriorly directed postorbital process is present. The temporal fossa is longer than wide and oval with a concave medial margin. The intertemporal region is longer than wide and bears a high sagittal crest; the position of the frontoparietal suture is uncertain. The apex of the supraoccipital shield is positioned slightly anterior to the posterior margin of the temporal fossa. The zygomatic process of the squamosal is elongate, cylindrical, anteroposteriorly directed, and medially bowed so that the lateral and medial margins are concave and convex, respectively. The supramastoid crest appears to have been absent on the zygomatic process. The zygomatic process is longitudinally twisted so that the lateral surface faces dorsolaterally. The occipital shield is triangular with a strongly developed external occipital crest. The occipital condyles are proportionally small and set out on a short neck; the exoccipital is anteroposteriorly inflated and club-like, as in Tokarahia sp. , cf. T. lophocephalus (OU 22081). Unlike Tohoraata , the exoccipital faces posteromedially. The parietal–occipital suture is unfused.

The skull of OU 22081 is poorly preserved, but includes the anterior half of the rostrum, including left and right maxilla, and left premaxilla, left and right mandibles exposed in a block in near-life position, and isolated fragments of maxilla, premaxilla, and nasals, partial squamosal, basioccipital, exoccipital, and vomer ( Figs 24 , 25 ). Bone surfaces are friable but generally pristine and in some cases bioeroded.

Premaxilla

The anterior 50 cm of the left premaxilla is exposed and associated with the palatal portion of the rostrum, and is somewhat disarticulated and shifted laterally ( Fig. 24 ). The ventral surface of the premaxilla is dorsoventrally shallow laterally and deepens medially; a medial keel is developed anteriorly, which gives the premaxilla a ventrally concave cross section. This concave trough is evident on the anterior 12 cm of the premaxilla: two elongate parallel, linear grooves are present anteriorly; these grooves are parallel with the lateral margin of the premaxilla, and are inferred to articulate with the maxilla.

Maxilla

The palatal surface of the maxilla of OU 22081 is well preserved, but the lateral margins on both sides are incomplete ( Fig. 24 ). The palatal surface is flat and lacks palatal foramina, with the exception of a single bilateral pair of large anteriorly opening greater palatine foramina (4–5 mm in diameter) positioned relatively far anteriorly and medially. They are confluent with deeply entrenched sulci (approximately 240 mm long) that transversely widen slightly anteriorly, and become diffuse 220 mm from the anterior preserved edge of the maxilla. Posterior to the greater palatine foramen on the left maxilla, two smaller foramina (1.5 mm diameter) are positioned anteroposteriorly along the medial margin and are contiguous with anteroposteriorly shorter sulci (15–20 mm in length). The pattern of palatal sulci is generally reminiscent of Eubalaena australis and Aetiocetus weltoni . The maxilla consists of delicate sheet-like dorsal and ventral laminae; no connections between these laminae are evident anteriorly, but the laminae appear to converge near the posterior portion of the preserved palate. These laminae define an apparently hollow wedge-shaped cavity that is dorsoventrally deepest anteriorly (maximum dorsoventral depth = 23 mm), and presumably forming the canal for the infraorbital maxillary soft tissues, including the maxillary artery and venous sinuses ( Walmsley, 1938: 142–143). This geometry perhaps explains the greater dorsoventral depth of the anterior portion of the rostrum in Yamatocetus canaliculatus .

Nasal

A fragment of the right nasal of OU 22081 lacks the anterior and posterior ends. It is rectangular, elongate, transversely narrow, and dorsally flat ( Fig. 25B ; Table 2), like the holotype. The dorsolateral surface bears numerous articular grooves and ridges for the articulation of the medial edge of the premaxilla, which appears to have dorsally obscured and ankylosed with the lateral portion of the nasal.

Vomer

The partial vomer of OU 22081 includes most of the ventral portion but lacks the dorsolateral edges; in dorsal aspect it is elongate and lanceolate ( Fig. 25E ; Table 2). It exhibits a shallow mesorostral groove, and is transversely convex with a low ventral crest. Posteriorly the vomer narrows and terminates to a transversely acute, conical point; approximately 70 mm from the posteri- or tip there is a slight transverse swelling. The mesorostral groove ends just anterior to this swelling. Elongate, dorsolaterally facing facets extend from this swelling to the posterior tip, perhaps representing the broken bases of the vomerine wings.

Parietal

OU 22081 includes a fragment of the parietal from the intertemporal region ( Fig. 25A ). The parietal is laterally concave and transversely narrow, suggesting a sharp sagittal crest, as in T. kauaeroa gen. et sp. nov. Posteriorly, a pair of matrix-filled foramina is present with irregular cross sections (approximately 20 mm maximum diameter), corresponding to the olfactory nerve tract ( Godfrey, Geisler & Fitzgerald, 2013). Anteriorly, a poorly preserved ethmoid recess is present.

Squamosal

The right squamosal of OU 22081 is well preserved and bears an elongate zygomatic process and a short anteroventrally inclined, anteroposteriorly flattened postglenoid process ( Fig. 25C, D ; Table 2). The dorsally arched zygomatic process is delicate, anterolaterally directed, tapers anteriorly, and curves ventrally at its anterior apex. Dorsomedially, a deep longitudinal groove is developed along the length of the zygomatic; it is irregularly excavated in places and appears to be taphonomically enlarged. In cross section the zygomatic process is subcylindrical, with an evenly convex dorsal margin; the supramastoid crest does not extend onto the zygomatic process or past the posterior margin of the temporal fossa. The zygomatic process is longitudinally twisted so that the lateral face is dorsolaterally directed; the ventral surface is flattened. The zygomatic process is medially arched, so that the lateral and medial margins are concave and convex, respectively, in dorsal view. Medially within the shallow squamosal fossa, a minute, subtriangular trough-like secondary squamosal fossa (sensu Sanders & Barnes, 2002a, b) is present. The oval glenoid fossa is concave and bears distinct margins. It is positioned medially on the squamosal, and laterally the postglenoid process and lateral part of the squamosal descend ventrally below the level of the glenoid fossa. Medially, the falciform process descends ventromedially towards the periotic fossa. A large pit is present on the posterolateral margin of the periotic fossa for the articulation of the lateral surface of the periotic. The subtemporal crest is dorsoventrally thick and anteriorly concave.

Occipital

The occipital includes the ventral portions of the exoccipitals and basioccipital ( Fig. 25C, D ; Table 2). The basioccipital is roughly tabular in ventral view and damaged anteriorly; it widens posteriorly because of the enlarged and ventrolaterally flaring basioccipital crests. The ventral surface of the basioccipital is anteriorly flat and posteriorly concave between the basioccipital crests. A deep dorsomedially oriented groove is present posterolateral to the basioccipital crest, which ventrally forms the jugular notch and separates the basioccipital from the paroccipital process. The hypoglossal foramen opens laterally within the jugular notch. The paroccipital process of the exoccipital is anteroposteriorly thick and inflated, with a convex posterior margin that faces posterodorsally. The posteri- or margin of the exoccipital is posterolaterally oriented. The occipital condyles are large and convex, and distinguished ventrally by a concave pedicle; the condyles are separated ventromedially by a deep intercondylar notch.

Periotic

The holotype periotic includes only the pars cochlearis medial to the fenestra ovalis ( Fig. 18 ; Table 3). The fenestra rotunda is large and oval-shaped, and not confluent with a dorsally ascending sulcus as in T. kauaeroa gen. et sp. nov. The posterodorsal margin of the fenestra ovalis is smooth and flat, unlike the convex and finely sculptured tubercle present in T. kauaeroa gen. et sp. nov., Tokarahia sp. (OU 21975), and Tokarahia sp. , cf. T. lophocephalus (OU 22081). The internal acoustic meatus is funnel-shaped and anterolaterally directed; the distinction between the spiral cribriform tract and the foramen singulare is not preserved. Posteriorly, the lateral rim of the internal acoustic meatus extends slightly more dorsal than the medial rim, so that the lateral wall of the meatus is visible in medial view. The caudal tympanic process is damaged but present as a short, low ridge that is posteromedially divergent from the long axis of the pars cochlearis; it defines the medial margin of the anteroposteriorly shortened and shallow stapedial muscle fossa. The stapedial muscle fossa is finely pitted. The stylomastoid fossa is flat and smooth. The posterior edge of the caudal tympanic process is positioned closely to the fenestra rotunda, forming a narrow shelf (3–4 mm wide).

Both periotics of OU 22081 are well preserved, relatively large, and robust, and bear elongate posterior and anterior processes and well-developed superior processes ( Figs 9F , 26 , 27 ; Table 3); they are very similar to T. kauaeroa gen. et sp. nov., with some exceptions. The caudal tympanic process is damaged but appears to have been short and posteromedially divergent, as in the T. lophocephalus holotype. Dorsally, the internal acoustic meatus is large and pyriform, and the crista transversa is deeply recessed; the spiral cribriform tract and foramen singulare are separated by a low crest deeply recessed within the meatus, as opposed to the high crest in T. kauaeroa gen. et sp. nov. and Tokarahia sp. , cf. T. kauaeroa gen. et sp. nov. (OU 21975). The lateral edge of the meatus is not developed into a robust triangular prominence as in T. kauaeroa gen. et sp. nov., although a small spur is present. The aperture for the cochlear aqueduct is small and subcircular, whereas the aperture for the vestibular aqueduct is transversely wider and slit-like. The posterior bullar facet is more strongly diamondshaped in ventral view, resembling the condition in Tokarahia sp. , cf. T. kauaeroa gen. et sp. nov. (OU 21975). The anterior process is triangular and dorsoventrally deeper than T. kauaeroa gen. et sp. nov., but shares a concave anterodorsal margin with it and Tohoraata raekohao . Anterior to the pars cochlearis the anterointernal sulcus is anastomosed: a dorsal branch of the sulcus splits and diverges dorsally, which splits again, and a third anteriorly placed sulcus descends ventrally and re-joins the ventral branch of the anterointernal sulcus.

Tympanic bulla

The tympanic bulla of the holotype, OM GL 443, and OU 22081 are relatively similar to the tympanic bulla of T. kauaeroa gen. et sp. nov.; the left bulla is complete and the sigmoid region of the right bulla is missing ( Figs 19 , 20 ; Table 4). The involucrum lacks an abrupt bulge on its dorsal margin, as in T. kauaeroa gen. et sp. nov. The median furrow does not form an incised notch, as in T. kauaeroa gen. et sp. nov., but forms a shallow triangular furrow between the lobes in dorsal and ventral outline. The conical process is not connected to the lateral part of the sigmoid process by a horizontal crest as in T. kauaeroa gen. et sp. nov. and Tohoraata waitakiensis . The referred bulla OM GL 443 similarly lacks the posteriorly incised median furrow of T. kauaeroa gen. et sp. nov. ( Fig. 21 ), but is slightly anteroposteriorly longer than the holotype bulla. Owing to breakage of OM GL 443, a few additional details of the tympanic cavity are worth noting. The floor of the tympanic cavity is pitted, and posteriorly the tympanic cavity forms a blind end that wraps dorsomedially around the lateral margin of the involucrum, which is developed as a rugose, laterally projecting knob. This knob is ventrally undercut by the tympanic cavity. Posteriorly, the cavity curves dorsally and narrows transversely, passing into a furrow that leads to the elliptical foramen. Both tympanic bullae of OU 22081 are well preserved ( Figs 27 , 28 ; Table 4) and nearly identical to that of the T. lophocephalus holotype (OM GL 412). The tympanic bulla of OU 22081 principally differs from that of T. lophocephalus in retaining a connection between the basal sigmoid process and the anterior conical process, although this horizontal lamina is delicate, and raises the possibility that it is damaged in T. lophocephalus . The posterior process of the bulla bears a transversely concave and anteroposteriorly short facet for the posterior process of the periotic. In medial and lateral view the posterior process is sharply triangular, bearing a triangular dorsal apex and a posteroventrally directed spur. The distal surface of the posterior process is triangular and shallowly concave. As in T. kauaeroa gen. et sp. nov. ( Fig. 12A, B ), the bulla of OU 22081 is slightly rotated when placed in articulation with the periotic ( Fig. 27D ), indicating that the outer lip of the bulla would have faced ventrolaterally when in articulation with the skull.

Dentition

A single isolated partial tooth was recovered during preparation of the palate ( Fig. 29 ). The tooth was recovered within 5 cm of the posterior part of the left maxilla; the crown is missing and may have been damaged during earlier preparation. Despite being found near the left maxilla, many bone fragments likely to represent fragments of the lateral margins of the maxillae have been separated from the skull and transposed up to 20 cm away; it is therefore not possible to identify the region in which the tooth was originally located. Furthermore, it is not possible to identify which surface is lingual or labial. The more convex margin of the tooth is likely to represent the mesial margin, as most cetacean teeth are distally recurved. The root is subtriangular in labial/lingual view. The root bears a diamond-shaped, linguolabially flattened cross section. On the flatter surface (labial or lingual), a shallow furrow parallels the distal margin. The root is zoned in cross section, and a dense outer layer (0.7– 1 mm thick) of dentine is visible surrounding a central core of dentine with parallel fibres and pores; this inner zone may reflect the remnant of a pulp cavity, which appears to have been completely filled in. This arguably does not represent a misidentified shark tooth root, because the outer surface of the root lacks pores and appears to be covered with a smooth layer of cementum. This specimen differs from all contemporaneous odontocetes in being labiolingually flattened; all contemporaneous odontocetes ( Otekaikea , Waipatia , and Squalodontidae, Fordyce, 1994 ; Tanaka & Fordyce, 2014) have tooth roots that are circular or oval in cross section, and are generally near cylindrical or conical in shape. Southern hemisphere toothed mysticetes such as mammalodontids also share tooth roots with circular cross section and differ from the tooth of OU 22081. Lastly, isolated shark teeth associated with OU 22081 have well-preserved crowns but bioeroded (or completely missing) roots, and no other odontocete elements were found, strongly suggesting that this tooth belongs to OU 22081.

Mandible

A photograph of the holotype specimen of T. lophocephalus in the field during excavation shows that parts of both mandibles were preserved ( Fig. 17B ), but only the fragmentary left mandible survives ( Fig. 22 ). The left mandible is poorly preserved and includes only the posterior portion. It is badly crushed and dorsoventrally flattened, and few morphological details are evident. Parts of both mandibles of OU 22081 are preserved in segments, including nearly the entire anterior left and right mandibles in life position, within a block of matrix, much of the posterior right mandible, and a fragment of the left mandibular condyle and posterodorsal margin ( Figs 24 , 25 ; Table 5). The mandible corresponds closely to that of T. kauaeroa gen. et sp. nov., and better preserves the mandibular terminus, symphyseal groove, and mandibular foramen. Anteriorly, the dorsal and ventral margins of the mandible are parallel; the mandibular terminus is positioned at the dorsoventral midpoint, and the anteriormost portion of the mandible (anterior 7.5 cm) is lanceolate rather than subrectangular. A deeply entrenched and well-preserved symphyseal groove (16 cm long) is present anteriorly on the medial surface near the ventral margin. The coronoid process is subtriangular but missing the dorsal tip; it appears to have been tongue-shaped, as in Eomysticetus and Yamatocetus , with a broadly concave mandibular notch. The dorsal margin of the mandible rises somewhat gradually towards the anterior margin of the coronoid process. The mandibular foramen is greatly enlarged into a large cavity and appears to have an arcuate anterior margin. The preserved fragment of the mandibular condyle indicates it is transversely narrow and anteriorly excavated by the enlarged mandibular canal.

Atlas

The holotype atlas is large and well preserved, with an incomplete neural spine and plate-like transverse processes ( Fig. 22A–C ; Table 5). The atlas is relatively anteroposteriorly elongate, similar to Eomysticetus and Yamatocetus . The condyloid facets are large, concave, and D-shaped (convex and flat lateral and medial margins, respectively). The articular surfaces are ventromedially separated at the midline by a shallow furrow. The neural canal is subrectangular with relatively flat dorsal, ventral, and lateral margins. The neural canal narrows slightly ventrally. No tubercles for the transverse ligaments are present between the odontoid portion and neural foramen, unlike Tohoraata waitakiensis . The transverse process is large, robust, rectangular in anterior view, and anteroposteriorly thick; it is positioned dorsally and measures approximately half the dorsoventral height of the anterior articular surface. The ventral margin of the atlas is evenly convex (unlike the flattened ventral margin of Tohoraata waitakiensis ), and bears a small ventral tubercle posteriorly. The neural arch is robust and dorsoventrally thick, and bears a low but incomplete neural spine; large (15 mm diameter) lateral vertebral canals are present anteriorly within the lamina. The posterior articular surface is superficially damaged but flattened and robust, without sharp margins; ventral to the neural canal it is somewhat concave for the reception of the odontoid process of the axis. In lateral aspect, the anterior margin is oblique and faces somewhat anteroventrally, whereas the posterior margin is vertical. The atlas of OU 22081 is large and robust, and is missing the left ventral portion ( Fig. 30 ; Table 6); it does not differ appreciably from the holotype.

Axis

The holotype axis is mostly well preserved, aside from the damaged pedicles and transverse process; the body of the axis is similarly anteroposteriorly thickened like the atlas ( Fig. 22D–F ; Table 6). The anterior articular surface is broad and shaped in a figure of eight, but is less medially constricted than in Tohoraata waitakiensis ; the lateral part is flat on either side of the low odontoid process. The anterodorsal surface of the odontoid process is flat and contiguous with the dorsal surface of the body within the neural canal; a slight median ridge is developed on the dorsal surface of the body. The ventral margin is evenly convex with a well-developed median tubercle, unlike Tohoraata waitakiensis . The neural foramen narrows posteriorly as the posterior portion of the body – and thus the ventral margin of the neural foramen – is posteriorly elevated. The convex dorsal margin of the posterior articular surface bulges into the neural canal, giving it a ventrally concave crescent shape. The posterior articular surface is transversely narrower than its anterior counterpart, and is shallowly concave and oval, with a centrally positioned slit-like notochordal pit. Lateral to the posterior articular surface is a flat to slightly concave surface on the posterior surface of the anteroposteriorly flattened, subrectangular transverse process. The neural arch is robust with a triangular outline, and the anterior spine and neural spine are missing. The postzygapophyses are developed as a subtriangular sheet with a small, thickened knob at its posterior apex. The axis of OU 22081 is roughly triangular and is missing nearly the entire posterior half of the body and posteroventral part of the neural arch ( Fig. 30 ; Table 7). It differs from the holotype in having a concave ventral margin without a hypapophysis. Otherwise, the neural spine is more completely preserved than the holotype and anteriorly exhibits an anteroventral spine with a small tubercle developed at its extremity.

C3–C7

The third through seventh cervical vertebrae are all present in the holotype, but the position of all but C7 are speculative ( Fig. 22I–N ; Table 8). These vertebrae are nearly identical with those of T. kauaeroa gen. et sp. nov. The transverse process bears a small, incompletely preserved vertebrarterial canal, similar to Eomysticetus and Yamatocetus , but unlike T. kauaeroa gen. et sp. nov. Two partial mid-cervical vertebrae of OU 22081 are identified as C3 ( Fig. 30E, F ) and C4 ( Fig. 30G, H ).

Thoracic vertebrae

Parts of four thoracic vertebrae are preserved in the holotype, two of which remain in articulation ( Fig. 22O ). These vertebrae have anteroposteriorly thicker and circular bodies that are nearly as long as transversely wide, a dorsally positioned pedicle, and large, dorsolaterally positioned transverse processes developed as subrectangular blocky tubercles with an anterolaterally facing articular facet for the ribs. The epiphyses are fully closed but unfused with a visible suture; a notochordal pit is present on each vertebra.

Sternum

The sternum is preserved in OU 22081 and represent- ed by a single subtriangular, dorsoventrally flattened element ( Fig. 30I, J ; Table 9). No distinct articular facets for any ribs are evident, as the anterolateral portion is dorsoventrally flattened and tapers laterally. The sternum also thins posteriorly, and becomes transversely narrower posteriorly; the posterior end appears to be broken. The posterolateral margin of the sternum is concave. The sternum is dorsally flat longitudinally and slightly transversely concave, and the ventral surface is transversely convex. On the dorsal surface, a small median groove is present anteriorly, whereas a small foramen is present anteriorly on the ventral surface.

Rib histology

A single rib fragment of OU 22081 was sectioned. In comparison with OU 22235, no obvious marrow cavity exists ( Fig. 16A, B ). Although not obvious in plain light, in crossed-polarized light nearly the entire cross section is composed of dense haversian tissue consisting of overlapping secondary osteons (90–216 μm in diameter) and interstitial fragments of secondary osteons. Large vascular channels are present, 20–140 μm in diameter. In places, pristine background non-remodelled lamellar bone is preserved. Where abundant lamellar bone exists, secondary osteons are rarer and less frequently overlapping. Few obvious primary osteons are visible. No periosteum is preserved. Along one part of the bone, a fragment of an outer diagenetically altered zone appearing opaque in thin section adheres to the bone; this zone is missing from the remainder of the cross section. This altered zone also consists of a network of opaque tubular structures under 12 μm in diameter. Although most structure in this zone is not apparent, even as vestiges, secondary osteons are clearly visible in the innermost part of this opaque zone, demonstrating that as in OU 22235 it is diagenetically altered.

Taphonomy

Several episkeletozoans are present on OU 22081. Several small coiled serpulid worm tubes (up to 3 mm in diameter) are present on the parietal fragment, left mandible fragment, periotic, the posterior process of the left bulla, and on the right bulla. A small (2 mm diameter) patch of bryozoans is present on the right side of the neural spine of the axis.

Referral of ‘ Mauicetus ’ lophocephalus and OU 22081 to Tokarahia

This species is referable to Tokarahia as it shares a relatively elongate bulla with similar proportions to T. kauaeroa gen. et sp. nov., and has medial and lateral lobes that are equally wide, unlike Tohoraata spp. The periotic also shares a posteromedially divergent caudal tympanic process with T. kauaeroa gen. et sp. nov., absent in Tohoraata and other eomysticetids ( Fig. 8 ). Additional shared features unique to Tokarahia are also preserved in Tokarahia sp. , cf. T. lophocephalus (see below). OU 22081 shares a similar tympanic bulla with T. lophocephalus and T. kauaeroa gen. et sp. nov., and aside from the aforementioned minor differences, the bulla of OU 22081 is not separable from T. lophocephalus . Furthermore, the periotic of OU 22081 exhibits a posteromedially widely divergent caudal tympanic process, as in T. lophocephalus , deviating nearly 45° from the long axis of the periotic. OU 22081 critically preserves additional skull and periotic characters not preserved or no longer accessible in the holotype specimen of T. lophocephalus . These additional features link the two species together, indicating the recognition of a single genus. These additional features uniting the two species of Tokarahia preserved in OU 22081 include a diamond-shaped posterior bullar facet, a sharp transverse crest on the posterodorsal surface of the periotic between the posterodorsal angle and the posterior pars cochlearis, and a sharp crest between the facial sulcus and stapedial muscle fossa.

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 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 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 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.

Figure 17. Holotype skull and skeleton of Tokarahia lophocephalus (OM GL 412), reproduced from Marples (1956): A, prepared skull in dorsal view; B, specimen as excavated in field.

Figure 18. Holotype pars cochlearis of Tokarahia lophocephalus (OM GL 412), whitened with ammonium chloride: A, ventral; B, medial; C, dorsal; D, lateral.

Figure 19. Holotype left tympanic bulla of Tokarahia lophocephalus (OM GL 412), whitened with ammonium chloride: A, medial; B, lateral; C, dorsal; D, ventral; E, anterior; F, posterior.

Figure 20. Holotype right tympanic bulla of Tokarahia lophocephalus (OM GL 412), whitened with ammonium chloride: A, medial; B, dorsal; C, lateral; D, ventral.

Figure 21. Holotype left mandible of Tokarahia lophocephalus (OM GL 412): A, dorsal; B, lateral; C, medial.

Figure 22. Holotype vertebrae of Tokarahia lophocephalus (OM GL 412). Atlas in (A) anterior, (B) posterior, and (C) dorsal view; axis in (D) anterior, (E) posterior, and (F) dorsal view;?C3 in (G) anterior and (H) posterior view;?C4 in (I) anterior and (J) posterior view;?C5 in (K) anterior and (L) posterior view; C7 in (M) anterior and (N) posterior view; articulated thoracic vertebrae in (O) lateral view.

Figure 23. Referred tympanic bulla of Tokarahia lophocephalus (OM GL 443), whitened with ammonium chloride: A, medial; B, lateral; C, dorsal; D, ventral.

Figure 24. Partial rostrum and mandibles of Tokarahia sp., cf. Tokarahia lophocephalus (OU 22081) in situ within block of matrix: A, photograph; B, interpretive line drawing.

Figure 25. Referred cranial elements and mandible of Tokarahia sp., cf. Tokarahia lophocephalus (OU 22081): A, fragmentary intertemporal region in dorsal view; B, right nasal in dorsal view; C, fragmentary basicranium in dorsal view; D, fragmentary basicranium in ventral view; E, vomer in ventral view; F, right mandible in medial view.

Figure 26. Referred periotic of Tokarahia sp., cf. Tokarahia lophocephalus (OU 22081), whitened with ammonium chloride: A, ventral; B, medial; C, dorsal; D, lateral.

Figure 27. Articulated left tympanoperiotic of Tokarahia sp., cf. Tokarahia lophocephalus (OU 22081), whitened with ammonium chloride. Articulated periotic and posterior process of bulla in medial (A) and ventral (B) view. Articulated tympanoperiotic in: dorsal view (C, relative to bulla); posterior view (D); medial view (E, relative to bulla); and lateral view (F, relative to bulla).

Figure 28. Referred tympanic bullae of Tokarahia sp., cf. Tokarahia lophocephalus (OU 22081): A, right tympanic bulla in medial view with posterior process; B, dorsal; C, lateral; D, ventral; E, left tympanic bulla in medial view; F, dorsal; G, lateral; H, ventral.

Figure 29. Isolated tooth of Tokarahia sp., cf. Tokarahia lophocephalus (OU 22081): A, tooth in mesial view; B, lingual or labial view; C, distal view; D, lingual or labial view; E, photograph of broken cross section.

Figure 30. Vertebrae of Tokarahia sp., cf. Tokarahia lophocephalus (OU 22081): A, atlas in posterior view; B, atlas in dorsal view; C, axis in anterior view; D, axis in dorsal view; E, cervical vertebra in anterior view; F, posterior view; G, partial cervical vertebra in anterior view; H, posterior view; I, sternum in dorsal view; J, ventral view.