Zygophyseter varolai, Bianucci & Landini, 2006

Bianucci, Giovanni & Landini, Walter, 2006, Killer sperm whale: a new basal physeteroid (Mammalia, Cetacea) from the Late Miocene of Italy, Zoological Journal of the Linnean Society 148 (1), pp. 103-131 : 106-118

ZYGOPHYSETER VAROLAI SP. NOV.

Holotype: MAUL 229 /1: skull ( Figs 3–5 ) mandible with 22 teeth in place ( Fig. 9 ), 25 loose teeth, some as fragments ( Fig. 9 ), incomplete left periotic and left tympanic bulla ( Fig 6 , 7 ), atlas, nine thoracic vertebrae, ten lumbar vertebrae and nine caudal vertebrae, 23 complete or fragmentary ribs; almost complete left scapula and fragment of right scapula ( Fig. 10 ); a small fragment of right radius, one phalanx; all remains of the same specimen. Measurements are given in Table 2.

Etymology: Named in honour of Angelo Varola who discovered, collected and restored the holotype. The species name is also in recognition of the outstanding field, laboratory and research activities on the fossil vertebrates of the Pietra leccese carried on by Angelo Varola in the last 20 years.

Horizon and locality: Cisterna Quarry near Cavallino, Salento Peninsula (Apulia, southern Italy, Fig. 1 ) in the informally named ‘Pietra leccese’, which consists of generally massive, uniformly fine-grained biomicrites ( Mazzei, 1994). The specimen was collected about 2 m below ground level, in sediments of early Tortonian (Late Miocene) age, about 8.14–10.5 Ma. This age is based on a planktonic foraminiferal association referable to the Neogloboquadrina acostaensis zone of Iaccarino & Salvatorini (1982), sensu Foresi et al. (1998). The Pietra leccese has produced a rich vertebrate fossil assemblage ( Capellini, 1878; Pilleri, 1986a; Bianucci, Landini & Varola, 1994a, 2004; Bianucci, 2001), and from the Cisterna Quarry in particular, cetacean ( Bianucci, Landini & Varola, 1992, 1994b), sirenian ( Bianucci, Landini & Varola, 2003) and fish remains ( Carnevale et al., 2002) have been recently described.

Diagnosis: A Physeteroidea approximately 6.5– 7 m in body length with a skull 1.5 m in condylobasal length and characterized by a short and tapered rostrum, presence of a peculiar anterior projection of the supraorbital process of the right maxilla, circular supracranial basin, extreme anteroposterior elongation of the zygomatic process of the squamosal, relatively small periotic, 13 pairs of large teeth (greatest diameter of root larger than 3% of the condylobasal length of the skull) in the upper jaw and 14 in lower jaw. Similar to Naganocetus in body size and large teeth, and in having the following plesiomorphic features: supracranial basin not extended anteriorly, leaving the most part of the rostrum dorsally convex; deep alveoli not reduced in number in the upper tooth row; teeth with enamel crown; and anteroposteriorly elongated temporal fossa. It differs from Naganocetus in the shorter rostrum, the probably larger number of teeth, the more elongated jugal articulated with the squamosal, the more elongated zygomatic process and the mandible less robust.

Remarks: We propose the English common name ‘killer sperm whale’ for this species, considering its probable similar feeding adaptation to the extant delphinid killer whale ( Orcinus orca ).

Description

General features of the fossil skeleton: The fossil skeleton was discovered in a quarry surface of about 16 m 2. The bones were disarticulated but displaced little from original anatomical position ( Fig. 2 ). As is usual for the Pietra leccese fossils, the bones exhibit a marked compression, particularly evident in the vertebral corpora. Moreover, the dorsal and ventral surfaces of the cranium are not well preserved because the cortical bone is only partially fossilized. The bones, especially the larger elements, were cut into several portions by the saw during stone cutting in the quarry, but almost all the cut portions were recovered.

On the basis of the preserved skeletal elements, the total length of the animal in life is estimated to have been approximately 6.5– 7 m, of which 21–23% would have been represented by the head.

General features of the skull: The skull, extracted in nine stone blocks and then restored, is almost complete, lacking only the posterior crest of the supracranial basin and portions of the left side of the exoccipital and of the right orbital area ( Figs 3 , 4 ).

The most peculiar features of the skull are the extreme elongation of the zygomatic process of the squamosal ( Fig. 5E ), the large hemispherical concavity (supracranial basin) in the dorsal surface of the cranium, and the presence of a peculiar plate of the right maxilla delimiting anterolaterally the supracranial basin ( Fig. 5B ).

The skull exhibits an accentuated asymmetry evident in dorsal view, caused by the displacement to the left side of the external nares, the decrease in size of the right naris, the lack of the right nasal, and the marked difference in shape and size between the right and left premaxillae and maxillae.

The rostrum is triangular, tapered, relatively narrow in its anterior portion (one-third of the rostrum length) and relatively short, being about onethird of the condylobasal length. Its dorsal surface is convex for the whole anteroposterior extension, except its most posterior 10 cm where the dorsal surface slopes anteromedially to delimit anteriorly the supracranial basin. In dorsal and ventral views the lateral margins of the rostrum are concave, particularly in the posterior portion, except the most posterior 10 cm which is rectilinear and parallel to the sagittal plane. The apical portion (4 cm long) of the rostrum consists exclusively of the premaxillae and exhibits a peculiar dorsoventral compression.

In lateral view the rostrum is straight, except the dorsoventrally compressed apical portion, it increases in height slightly from the anterior to the posterior portion, and it is rather low at the base.

In dorsal view the cranium is slightly wider than long and in lateral view, considering the missing dorsal portion ( Fig. 4B ), it greatly increases in height posteriorly.

Dorsal view of the skull: The premaxillae are widely exposed on the dorsal surface of the rostrum and completely cover the maxillae in the 27-cm-long distal portion of the rostrum. The width of the premaxillae is approximately the same along the whole length of the rostrum.

The two premaxillae are medially separated by a continuous mesorostral groove, the width of which decreases about half way alsong the length of the rostrum.

The exposed dorsal surface of the maxilla is almost parallel to the horizontal plane; by contrast, the dorsal surface of the premaxilla is ventrolaterally bent. The left antorbital notch is deep and U-shaped ( Fig. 5C ), delimited medially by the lateral margin of the rostrum and laterally by the anterior edge of the antorbital process. Both margins are rectilinear and parallel to the sagittal plane. As discussed below, the left antorbital notch is more posteriorly located than the right notch. We conventionally consider as base of the rostrum the transverse line passing through the left antorbital notch.

In the right lateral margin of the rostrum, about 15 cm anteriorly to the rostrum base, the maxilla exhibits a peculiar lateral extension that seem to represent an anterior projection of the supraorbital process of the maxilla ( Fig. 5B ). Consequently, the right maxillary portion of the rostrum terminates more anteriorly than the left one and the right antorbital notch is a narrow slit (as in Kogia ) clearly anterior to the left antorbital notch.

Medially to the left antorbital notch, a large and anteroposteriorly elongated infraorbital foramen is present in the left maxilla and at the same level two large foramina are in the right maxilla ( Fig. 5B ). A large premaxillary foramen is also present in the right premaxilla near and slightly posterior to the two right maxillary foramina, although apparently there is no left premaxillary foramen.

The cranium exhibits a circular supracranial basin extending on most of its dorsal surface and only in the posterior portion of the rostrum. The walls and floor of this basin consist almost exclusively of both maxillae and premaxillae widening in the cranium. This fossa on the right side is anterolaterally delimited by the anterior projection of the supraorbital process of the maxilla.

The width of the right premaxilla remains constant posteriorly to the external nares, while the width of the left premaxilla increases considerably posteriorly, and, at the level of the external nares, the left premaxilla reaches the lateral margin of the cranial fossa.

Because of the poor preservation of the dorsal surface of the supracranial basin, the sutures are not clearly visible posterior to the external nares ( Fig. 4A ), and, consequently, the posterior extension of the premaxillae and of the maxillae, and any possible posterior dorsal exposure of the frontals, cannot be described.

The external nares lie near the centre of the supracranial basin and are placed slightly to the left with respect to the midline (the right narial passage is crossed by the sagittal plane). The left narial passage is clearly larger than the right one, even if the poor preservation does not permit us to evaluate the true original dimensions of these openings. The two nares are separated by a wide mesethmoid and the left one is partially covered dorsally by a large rectangular bony plate that might represent the left nasal ( Fig. 5A ). The posterior suture between this supposed nasal and the left premaxilla is not distinguishable. The right nasal bone is missing and was probably absent originally.

The left maxilla does not completely cover the supraorbital process of the frontal, which is clearly visible in dorsal view. By contrast, the right maxilla probably completely covered the right orbit (not preserved), producing an asymmetrical extension of the supracranial basin.

The posterior crest of the supracranial basin was in a stone block lost in the quarry during fossil collection. Nevertheless, its elevation is estimated to have been about 35 cm beyond the uppermost preserved portion, based on the observations made by us before the stone block was lost.

The occipital shield had a narrow anteroposterior dorsal extension, judging by the narrow cut surface of the posterior crest and by its short preserved posterior portion.

Lateral to the occipital shield, the large and elongated zygomatic processes of the squamosals are widely visible in dorsal view with an anteromedially directed apex and a convex lateral margin delimiting the wide temporal fossa.

Lateral view of the skull: The fused lacrimal and jugal are triangular, with a wide anterodorsal portion that contributes to form the antorbital process and a thin and elongated posterior portion articulating with the zygomatic process of the squamosal ( Fig. 5E ). The jugal–squamosal suture is anteroposteriorly elongated and obliquely orientated with respect to the major axis of the skull.

The orbit is anteroposteriorly short and somewhat arched. It is at the same height as the dorsal margin of the rostrum and is dorsally delimited by a thick supraorbital process of the frontal that becomes higher posteriorly. The postorbital process is relatively narrow and elongated and it is posteroventrally directed toward the zygomatic process.

The frontal–maxilla suture is angled posterodorsally forming an angle of 35° from the axis of rostrum, and the lateral exposure of the frontal slightly widens posteriorly ( Fig. 5E ).

The zygomatic process of the squamosal is unusually elongated with respect to other odontocetes and it is rather thin, particularly in its anterior portion where it terminates with a pointed and anteriorly directed apex. In lateral view, the zygomatic process is parallel to the major axis of the skull and its dorsal margin delimits ventrally a very wide, deep and anteroposteriorly elongated temporal fossa. This margin of the zygomatic process has an asymmetrical sinusoidal shape, being concave along almost its entire length, except a strong convexity in its most posterior portion. Overall, the zygomatic process is triangular in lateral view owing to the posterior elevation of its dorsal margin.

The external auditory meatus is clearly visible in lateral view of the skull as a groove separating the postglenoid and the paroccipital process of the exoccipital.

Ventral view of the skull: In ventral view, about 4 cm posterior to the apex of the rostrum, a W-shaped suture separates the premaxillae from the maxillae. The short ventral surface of the premaxillae lacks any alveoli, while each maxilla has 13 relatively deep, single-rooted alveoli. These alveoli are joined one to another forming a gutter with indented lateral and medial margins. A straight suture between the two maxillae is visible along the mid line on the anterior portion of the rostrum ventral surface.

The ventral surface of the anterior portion of the cranium, as with that of the posterior surface of the rostrum, is poorly preserved and consequently it is not possible to estimate the extension of the palatine and pterygoid. By contrast, the ventral surface of the left orbital area is in good condition and the very deep antorbital notch, the large lacrimal fused with the jugal, the elongated and thin lacrimal, and the anteroposteriorly elongated jugal–squamosal suture are clearly visible ( Fig. 5C ). Moreover, the orbit is posteriorly delimited by an evident curved postorbital ridge.

The preserved posterior ventral surface of the cranium exhibits a relatively narrow basioccipital delimited posteriorly by rather protuberant occipital condyles and laterally by two crests forming an angle of about 30°.

Lateral to the basioccipital crest, the squamosal protrudes well out of the braincase and terminates with a narrow zygomatic process anteroposteriorly very elongated, straight, with its major axis parallel to that of the skull, and with a pointed apex. The ventral surface of the zygomatic process is almost completely occupied by a wide glenoid fossa, posteriorly delimited by a narrow postglenoid process ( Fig. 5D ). Posteromedially to the glenoid fossa the tympanosquamosal recess is relatively small, and the falciform process (perhaps not completely preserved) appears to be short and wide. A line, slightly in relief, joining the falciform process to the subtemporal crest may represent the squamosal– alisphenoid suture, even if an accurate reconstruction of this important diagnostic area of skull base is not possible because of the poor preservation.

Posterior view of the skull: In posterior view, the exoccipital exhibits an excavated surface dorsally delimited by a protuberant, thin crest. The lateral portion of the exoccipital bends anteriorly, joining the squamosal. The jugular notch is wide and semicircular in posterior view, and the paroccipital process is unusually slender and elongated.

Periotic: Only the left periotic, lacking the posterior process, is preserved ( Figs 6A–G , 7 ). It is relatively small considering the size of the skull.

The anterior process is relatively short and ventromedially bent. The ventrolateral angle (apex) is clearly visible but without a sharp tip. We separated the large accessory ossicle of the tympanic, originally fused with the anterior process of the periotic, and a large and trapezoidal fovea epitubaria is now visible. Medially to this fossa, a narrow but deep groove for the tensor tympani is visible, while anteriorly there is a small and shallow fossa for the outer lip of the tympanic bullar surface. The fossa for the malleus is posteromedially orientated and it is laterally delimited by a large and globose lateral tuberosity. In the posterolateral part of this tuberosity the small facet for the articulation of the sigmoid process of the tympanic bulla is evident.

In its medial surface, the anterior process exhibits a rather large tuberosity visible dorsally and medially but ventrally covered by the accessory ossicle (later detached during preparation; see Fig. 4D ). A small but deep groove that may represent a vestige of the anteroexternal sulcus is visible in the lateral surface of the anterior process.

The pars cochlearis (promontorium) is almost spherical with slight dorsoventral compression and it is anteroventrally bent. On its dorsal surface, the internal acoustic meatus is relatively small and exhibits a circular outline with a raised posterior rim. The internal opening of the facial canal is separated from the cochlear foramina by a low transverse septum and has a pointed anterior groove, probably for the greater petrosal nerve ( Fordyce, 1994). The aperture for the endolymphatic duct is a large, deep fossa inside which there is a narrow fissure divided by a transverse septum. The aperture for the cochlear aqueduct is small, has an almost circular and raised rim, and is slightly medially located with respect to the endolymphatic foramen. The fenestra rotunda is small, semicircular and far from the aperture for the cochlear aqueduct.

An evident pyramidal process is located 15 mm posterolaterally to the aperture of the endolymphatic duct. The dorsal surface of the periotic, laterally to the pars cochlearis (suprameatal region), is slightly concave and delimited laterally by an arched keel representing the dorsal edge of the tegmen tympani.

Tympanic bulla: Only the left tympanic bulla, lacking its posterior portion, is preserved ( Fig. 6G–L ). In ventral and dorsal view, this bone is wide, without mediolateral compression and shows a posteromedial enlargement of the posterior portion of the involucrum. The ventral surface of the preserved portion of the bulla shows a wrinkled surface without a medial furrow. The anterior margin is rectilinear without an anterior spine. The anterior opening is wide and ‘U’ shaped in anterior view. The involucrum exhibits an anterior portion dorsally orientated and a pachyostotic posterior portion laterally curved. These two portions are separated by a concavity evident in ventral and particularly in medial view. The preserved portion of the dorsolateral margin of the tympanic bulla is anteroposteriorly in contact with the periotic by a small abruptly elevated outer lip, a large accessory ossicle and a partially preserved sigmoid process.

Teeth: There are 13 teeth in each upper tooth row, all in the maxilla, and 14 teeth in each dentary; in fact, in the lower tooth row an apical tooth is present that is absent from the upper row ( Figs 8 , 9 ).

The crowns are relatively small (about 18% of total tooth length, considering also the estimated apical portion of the crown missing as a result of wear), conical, with a circular cross-section and crenulated enamel ( Varola et al., 1988: Fig. 2 ). The margin of enamel at the crown base is irregular and interfingered. The roots are fusiform and covered by a 15-mmthick cement layer ( Varola et al., 1988: pl. 2). They taper at the lower extremity and, if completely preserved, exhibit some small secondary roots. A dark irregular band, obliquely orientated with respect to the major axis of the teeth, marks the greatest diameter of the roots. This band represents the area of connection between the gum and the teeth and it is here named the ‘gingival collar’. The gingival collar separates the tooth into an upper external portion and a lower internal portion. The internal portion is the part of the tooth originally covered by soft tissue and bone. The original inclination of the teeth with respect to the rostrum or mandible can be estimated also in the isolated teeth by orientating the plane that crosses the tooth through the gingival collar, parallel to the horizontal plane ( Fig. 8A ). In support of this inter- pretation, the mandibular teeth in places have their gingival collars approximately parallel to the dorsal surface of the mandibular body ( Fig. 8B ).

The internal portions of the teeth have shallow longitudinal grooves, the function of which was probably to increase the joint between teeth and alveoli.

There is an evident difference in shape and size from anterior to posterior teeth. In fact, examining the mandibular teeth in place ( Fig. 9C, D ), we can observe that the nine anterior teeth have a root that is circular in cross-section, with the external portion anterolaterally bent. In particular, the apical tooth has an external portion that is strongly anteriorly bent, while the external portions of the other eight anterior teeth have a sinusoidal shape in lateral or medial view. Instead, the internal portion of these nine anterior teeth is strongly posteriorly bent so that each tooth is partially covered by the nearest following tooth. The five posterior teeth are smaller than the nine anterior teeth, and their external portion is posterolaterally bent. The roots of the three most posterior teeth exhibit a strong mediolateral compression.

All external portions of the teeth exhibit various degrees of wear, affecting both the crown and the root. In fact, all crowns lack their apical portion due to wear, and their height is reduced an estimated average of 45% of the original value. In one tooth, the wear caused the total loss of the crown ( Fig. 8D ). The wear of the roots, due to the opposite teeth (occlusal wear), caused a more or less deep groove in the posterolateral surface of the external portion. The action of the opposite teeth during the life of the animal also caused a lowering of the gum as can be deduced from the folding of the gingival collar ( Fig. 8B–D ). Wear is more accentuated in the anteriormost teeth.

Mandible: In dorsal and ventral view, the two jointed dentaries appear Y-shaped, with a narrow, cylindrical and relatively elongated symphyseal portion (45% of the mandibular length) and with the two bodies being posteriorly rectilinear and forming an angle of 45° ( Fig. 9 ).

In lateral view, the mandible is arched, with its dorsal profile concave, its ventral profile convex and its anterior portion slightly dorsally bent ( Fig. 9D ).

The two dentaries are not strongly sutured and in dorsal view they progressively diverge posteriorly. At the posterior margin of the symphysis, the distance between the dentaries is about 3 cm.

Each dentary has 14 teeth, all preserved in place except the six posterior of the left dentary. These teeth are very close to one another and located in an alveolar gutter similar to that in the maxilla. Eight teeth are symphyseal.

The lateral and ventral surfaces of the symphyseal portion of the mandible exhibit shallow longitudinal grooves and exhibit three mental foramina located at the same height and at level of the seventh, eighth and ninth teeth, respectively.

In the postalveloar portion, the coronoid process is not very elevated and the condyle, round in posterior view, is protuberant and located at the posteroventral angle. On the medial surface, there is a large mandibular foramen extending anteriorly 48 cm from the condyle ( Fig. 9E ).

Vertebrae: Most of the vertebrae are incomplete and were deformed during fossilization ( Fig. 10A–E ). The atlas is the only cervical vertebra preserved. It has a circular contour in anterior view and exhibits large, semicircular and moderately concave facets for articulation with the occipital condyles. The lower and upper transverse processes join to form a single, short and rather thin process. The neural arch is very low and lacks the neural spine.

Eight thoracic vertebrae are preserved but, considering the number of preserved ribs, we can assume that originally there were at least 12 thoracic vertebrae. The anteriormost preserved thoracic vertebrae have a centrum with reduced anteroposteriorly length and a wide neural arc. Among these, an almost complete and non-deformed vertebra that may represent the fifth thoracic ( Fig. 10B ) has a centrum that anteriorly measures 92 mm in height and 110 mm in width. Its anteroposterior length is only 60 mm. The neural arch is rather thin and forms a large and pear-shaped neural canal. The neural spine is missing but judging from the small broken area, it was short and thin. The transverse processes, beginning from the lateral margins of the neural arch, are distally wide, as visible in anterior or posterior views, and anterodorsally bent. The width between the transverse processes is 235 mm. A well-preserved posterior thoracic vertebra ( Fig. 10D ) has a centrum 90 mm high and 100 mm wide and 100 mm anteroposteriorly elongated. Its neural arch is rather high and its neural canal is triangular and 70 mm high. Its neural spine is bent posteriorly and its transverse processes, departing from the dorsolateral margin of the centrum, are relatively wide in dorsal or ventral view, and they are approximately parallel to the horizontal plane. The facets for tubercula of ribs, located at the distal end of the transverse processes, are elliptical. The distance between the two facets is 300 mm.

The ten preserved lumbar vertebrae have a centrum anteroposteriorly elongated (110–150 mm) and dorsoventrally compressed due, at least in part, to diagenetic processes, and they lack the longitudinal keel on the ventral surface. Their neural arcs are high and narrow and their neural spines are very elongated and bent posteriorly. The transverse processes, when well preserved, are very elongated, dorsoventrally compressed and anteroposteriorly expanded at their distal end. The distance between these processes ranges between 400 and 420 mm. Only an incomplete caudal vertebra, with a centrum 130 mm in anteroposterior length, has been removed from the matrix; at least eight caudal vertebrae remain to be removed.

Ribs: Twelve right and 11 left ribs are preserved, some complete and others more or less incomplete ( Fig. 10F–Q ). In particular, only the proximal portions of the seventh right rib and of the fifth and ninth left ribs are preserved. Moreover, only small fragments of the second and sixth left ribs are preserved and the twelfth left rib is missing. Judging by the gradual variation in shape from the first to the twelfth right ribs and by the shape of the small posteriormost rib, we consider it probable that there were originally 12 ribs. The length increases from the first to the fifth rib and decreases from the fifth to the twelfth rib, while the width progressively decreases from the first to the twelfth rib. As is typical of Cetacea , the first rib is clearly distinguished from the others by its greatest width in anterior or posterior view and by its peculiar ‘L’ shape due to the strong curvature in its proximal portion. It is anterposteriorly flattened and it widens dorsoventrally in its distal portion. Its tuberculum is relatively small and located in a narrow neck while the capitulum is wide and only slightly protuberant. The second rib is about one-third narrower than the first, is more elongated, shows a more regular curvature and lacks the distal dilation. Moreover, its neck from the tuberculum is more elongated and sturdy and its capitululum is more protuberant. A similar architecture of the proximal portion is also observed in the next six ribs, due to the double articulation with the vertebrae, while the last four ribs lack the capitulum because they were articulated only with the transverse processes of the thoracic vertebrae. Moreover, the tenth and eleventh ribs exhibit a wide and dorsoventrally compressed tuberculum. The curvature is almost the same from the third to the eighth ribs while it decreases considerably in the next four ribs.

Forelimb: An almost complete left scapula, some fragments of the right scapula, a small portion of right radius and one phalanx are preserved.

The left scapula ( Fig. 10R ) has an elongated and distally expanded acromion and a slender coracoid process. Its straight anterior and posterior margins form an angle of about 90°. Its dorsal margin is not preserved.

The preserved proximal portion of the radius exhibits a large facet for the articulation with the humerus and a smaller posterior facet for the articulation with the ulna. The preserved posterior margin of the radius is slightly concave and the anterior margin is rectilinear. The only preserved phalanx is 72 mm long. It is rather slender and has expanded proximal and distal portions. The wider proximal portion has a convex surface for articulation with the metacarpal.

Figures

Figure 1. Map of south-eastern Italy showing the location of the Cisterna Quarry, the type locality of Zygophyseter varolai gen. et sp. nov.

Figure 2. Sketch showing in plain view the relative positions of the skeletal elements of MAUL 229/1, holotype of Zygophyseter varolai gen. et sp. nov. as they were preserved. CV, caudal vertebrae; LV, lumbar vertebrae; MD, mandible; RB, ribs; SC(l), left scapula; SC(r), right scapula; SK, skull; TV, thoracic vertebrae.

Figure 3. Zygophyseter varolai gen. et sp. nov. Skull of holotype (MAUL 229/1) in (A) dorsal, (B) ventral and (C) lateral views.

Figure 4. Zygophyseter varolai gen. et sp. nov. Skull of holotype (MAUL 229/1) in (A) dorsal and (B) lateral views. The non-preserved portion is reconstructed in dorsal view and is shaded in lateral view.

Figure 5. Zygophyseter varolai gen. et sp. nov. Details (A–D) and lateral view (E) of the cranium of the holotype (MAUL 229/1). A, narial area in dorsal view; B, foramina and antorbital lamina of maxilla in dorsal view; C, left orbital area in ventral view; D, posteroventral surface. The vertical scale bar refers to A–D.

Figure 6. Zygophyseter varolai gen. et sp. nov. Left incomplete ear bones of the holotype (MAUL 229/1). A–F, periotic in (A) dorsal, (B) dorsomedial, (C, D) ventral, (E) medial and (F) anterior views. G, articulated periotic and tympanic bulla in lateral view; H–L, tympanic bulla in (H) dorsal, (J) ventral, (K) medial and (L) anterior views. D shows detail of the anterior process with the accessory ossicle not removed.

Figure 7. Zygophyseter varolai gen. et sp. nov. Incomplete left periotic of the holotype (MAUL 229/1) in (A) dorsal, (B) ventral and (C) medial views.

Figure 8. Zygophyseter varolai gen. et sp. nov. Teeth of the holotype (MAUL 229/1). A, reconstruction of the original orientation of two isolated maxillary teeth; B, three mandibular teeth in place showing the gingival collar and the occlusal wear of the roots; C, D, two isolated maxillary teeth (arrows show the wear due to the opposite teeth).

Figure 9. Zygophyseter varolai gen. et sp. nov. Mandible (A–E) and isolated upper teeth (F, G) of the holotype (MAUL 229/1). A, anterior view; B, ventral view of the anterior portion of the symphysis; C, dorsal view; D, lateral view; E, medial view of left posterior portion of right dentary; F, lateral or medial views; G, posterior views.

Figure 10. Zygophyseter varolai gen. et sp. nov. Postcranial skeleton of the holotype (MAUL 229/1). A, atlas; B, D, thoracic vertebrae; E, lumbar vertebra; in (1) anterior, (2) dorsal and (3) lateral views; F–Q, right ribs in lateral view; R, left scapula in medial view.