Frontal.

In NRRU-F01020035, both frontals are present as wedge-shaped plates as in other allosauroids, although the anterior and lateral margins are damaged (Figs 2, 3). In NRRU-F01020036, although the left frontal lacks its anteromedial and posterolateral parts, the anterolateral margin is better preserved than those of NRRU-F01020035 (Fig 5). The size and morphological features of the frontal are quite similar between these two specimens.

In comparison with other allosauroids, the frontal is lateromedially broad even in the anterior part (Fig 2A, B) as in Sinraptor hepingensis [12] and carcharodontosaurids [17,19,21,25,54] but unlike Allosaurus and Sinraptor dongi, in which the anterior part is much narrower than the posterior part in dorsal view [10,13,15]. The bone gradually tapers anteriorly in dorsal view to form a blunt but pointed end at its anteromedial margin. The dorsal surface of the frontal except for the supratemporal fossa forms an anterior part of the skull table as a flat plane like those of other allosauroids [10,12,13,15,17,19,21,25] but unlike the narrow one of Murusraptor [31] and the sagittal crest of Shaochilong [28]. The interfrontal suture is straight in dorsal and ventral views in its anterior two-thirds but becomes intricately sculptured in the posterior one-third. Both frontals are connected even at their anterior ends, unlike most allosauroids, in which both frontals diverge before reaching their anterior ends [10,13,30]. At the posterior end of the interfrontal suture, the frontoparietal suture emerges as a deeply interdigitating line that is directed posterolaterally on the skull table forming a slight dorsal swelling. Such direction of the frontoparietal suture accompanied with the wedge-shaped anterior margin of the parietals is unknown in other allosauroids, in which the suture is directed laterally even on the skull table [10,12,13,15,30,54]. There is a pair of small foramina possibly for the exits of some vascular components at the middle of the frontoparietal suture on the skull table (Fig 2B). The frontoparietal suture enters into the supratemporal fossa with a slight lateral swelling (frontoparietal process sensu [54]) that is less distinctive than those of Sinraptor [10] and Eocarcharia [54], and becomes a straight line running laterally on the bottom of the fossa as in other allosauroids. The frontoparietal suture is located more anteriorly on the skull table than in the supratemporal fossa as in Sinraptor (Fig 4 in [12]; Fig 7 in [10]) but unlike Allosaurus, in which the one is located more posteriorly on the skull table than in the supratemporal fossa (Fig 11 in [13]; Fig 2 in [15]). At the anteromedial corner of the supratemporal fossa, the dorsal rim of the fossa slightly protrudes posterolaterally to partially roof the fossa (Figs 2A, B, 5B, C) as in some carcharodontosaurids [43,54]. At the lateral end of this rim, the lateral margin of the bone is damaged but preserves two deep pits excavating the frontal medially (Figs 2A–D, 3B, E, 5B, C, E, F). These pits probably related to the postorbital contact and a similar excavation is also seen in Eocarcharia [54] whereas the surface is rather flat or convex in other allosauroids [14,17,25,31,55]. These pits are separated by a thick lamina into the anterodorsal and posteroventral ones unlike an almost single slot of Eocarcharia [54]. In NRRU-F01020035, the ventral rim of the posteroventral pit protrudes somewhat ventrally from the remainder of the ventral surface of the bone, whereas this protrusion is broken in NRRU-F01020036.

On the ventral surface, there is an anterolaterally oriented crest aligned with the broken anterior margin of the orbitosphenoid (Figs 2E–F, 5K, L). At the lateral end of the crest, the lateral margin of the frontal bears a deep pit penetrating the frontal posteromedially to receive the posterior apex of the prefrontal as in Allosaurus [13] and Carcharodontosaurus [21]. Anteromedial to the crest, the thickness of the frontal becomes about half of the posterolateral part of the bone and exhibits a shallowly-concave surface representing the area of the contact for the olfactory bulbs (Fig 2E, F). Posterior to the crest, the medial and posterior margins of the ventral surface of the frontal are covered by the orbitosphenoid and the laterosphenoid, respectively.

Parietal.

In NRRU-F01020035, both parietals are well preserved but lacking a large part of the transverse nuchal crest (Figs 2, 3). In NRRU-F01020036, only the anteriormost part of the left parietal adhered to the frontal is preserved (Fig 5). In contrast to the frontals, the interparietal suture is obliterated (Fig 2A, B) so that both parietals are completely fused as in carcharodontosaurids [17,19,21,25,28] but unlike basal allosauroids [10,13,15] and Murusraptor [30]. In dorsal view, the parietals extend anteriorly to form a wedge invading the interfrontal suture and laterally to meet the suture against laterosphenoids. In lateral view, the parietal-laterosphenoid suture is oriented almost horizontally (Figs 2C, D, 3B, E).

In dorsal view, the parietals are largely occupied by the supratemporal fossae. The dorsal surface between supratemporal fossae forms a supratemporal crest with a flat plane as a posterior part of the skull table (Figs 2A, B, 5B, C). The lateral width of this crest is similar to that of Allosaurus (Plate 2 in [13]), which is narrower than those of Acrocanthosaurus, Carcharodontosaurus and Giganotosaurus [17] but wider than those of Shaochilong [28] and Murusraptor [30]. Posteriorly, the parietals are projected dorsally with the skull table gradually becomes facing anterodorsally and spread laterally to form a prominent dorsal margin of the nuchal crest (Figs 2A–D, 3B, C, E, F). In anterior view, the center of the dorsal margin is concave as in Allosaurus (Fig 13 in [13]) but unlike the prominent top of the crest seen in Sinraptor [10] and Giganotosaurus [25]. The dorsal margin of the nuchal crest does not extend much above the supraoccipital (Figs 2C, D, 3B, E) as in Sinraptor [10] and is tall as in some abelisauroids and carcharodontosaurs [43]. In dorsal view, there is a small tongue-like process on the midline partially overlapping the dorsal surface of the supraoccipital as in Sinraptor [10] and Allosaurus [15] but is much smaller than that of Giganotosaurus [25].

Remainder of the braincase.

The remainder of the braincase is represented only by NRRU-F01020035 lacking at least opisthotics and the parasphenoid. The foramen magnum is circular, which is 3 cm wide and 2 cm high in posterior view (Fig 3A, D). The occipital condyle is 5 cm wide and 4 cm high in posterior view. In lateral view, the angle between the skull roof and the occiput (occipitofrontal angle) is obtuse (Fig 3G) as in other allosauroids except for Allosaurus and Acrocanthosaurus [25,31,43].

The orbitosphenoids are the anteriormost elements of the braincase except for the frontals (Figs 2C–F, 3B, E). The orbitosphenoids are completely fused with each other to form a low median ridge, which probably correspond to the interorbital septum seen in Acrocanthosaurus [17], Giganotosaurus and Carcharodontosaurus [25]. The anterior part of the orbitosphenoid contacts the frontal, as in carcharodontosaurids [17,21,28,54], but unlike Sinraptor and Allosaurus, in which the one is bordered only by the laterosphenoid [10,14,56]. Although the anterior part of the orbitosphenoid may pertain to the sphenethmoid, as suggested for Carcharodontosaurus iguidensis [21] and possibly for Acrocanthosaurus [57], there is no obvious evidence of its ossification in NRRU-F1020035. In ventral view, the anterior margin of the left orbitosphenoid is aligned with the anterolaterally oriented crest (Figs 3E, F), as in Sinraptor (Fig 7E in [10]) and Allosaurus (FPDM-V-9672). However, this margin is somewhat broken in both orbitosphenoids along the same straight line. Therefore, the orbitosphenoid might extend slightly more anteriorly as in Acrocanthosaurus (Fig 16 in [17]). A sizable opening for the exit of the cranial nerve (CN) I (olfactory nerve) excavates the medial parts of the orbitosphenoids anteroposteriorly. The mesethmoid is absent in this specimen, resulting in a single opening for CN I, as in Sinraptor [10] and multiple specimens of Allosaurus [13,16,17].

Posterior to the orbitosphenoids, there is a large circular opening corresponds to the exit of CN II (optic nerve) with the path of the pituitary (Fig 3F). The flange of bone overlying and separating them in other theropods (e.g., [17,25,58]) is absent. It is uncertain whether the flange was not ossified or eroded by the weathering. Such a large opening is similar to the exit of CN II of Allosaurus [14]. However, the opening of NRRU-F01020035 is only anteriorly bordered by the orbitosphenoids, whereas is completely surrounded by the orbitosphenoids in Allosaurus [14]. The exit of CN III (oculomotor nerve) lies adjacent to the lateral margin of the exit of CN II and, being bordered anteromedially by the orbitosphenoid and posterolaterally by the laterosphenoid, marks the approximate posterior end of the suture between these elements.

The laterosphenoid is covered dorsally by the frontal and the parietal (Figs 2D, 3E) as in other allosauroids. While the laterosphenoid-prootic suture is not obvious, the probable remnant of the laterosphenoid-orbitosphenoid suture is partially visible. The laterosphenoid has a barlike lateral extension but its distal end, namely the distal cotylus that contacts the postorbital, is broken (Fig 2F). The anteromedial part of the left laterosphenoid has a pair of small foramina. The more anteromedial one corresponds to the exit of CN IV (trochlear nerve) because it is present on the remnant of the orbitosphenoid-laterosphenoid suture along with the exit of CN III as in most theropods [14,27,59]. The other, more posterolateral one probably corresponds to the orbitocerebral vein. These foramina seem to be merged in the right laterosphenoid as in Murusraptor [31] because there is only a single, slightly larger foramen that intercepts the remnant of the orbitosphenoid-laterosphenoid suture.

The pituitary (hypophyseal) fossa is situated posteroventral to the exit of CN II and excavates the braincase posteriorly (Fig 2D, F). This fossa is possibly composed of the complex of basisphenoid and parasphenoid, but there is no obvious suture line to distinguish them. Left and right exits of CN VI (abducens nerves) are visible as a pair of small foramina within the anterior part of the fossa as in Shaochilong [28], Carcharodontosaurus [23] and Giganotosaurus [25,26] unlike those on the outside of the pituitary fossa and separated by a median ridge seen in Acrocanthosaurus [18] and Shaochilong [31]. The deepest part of the pituitary fossa has a pair of exits that correspond to the internal carotid artery foramina connected to the rostral (anterior) tympanic recess as in Shaochilong [28].

The prootic is completely fused with the laterosphenoid as the suture between them is not visible (Figs 2, 3). The posteroventral region of this laterosphenoid-prootic complex is perforated by exits of CNs V and VII (trigeminal and facial nerves). There are four trigeminal foramina (exits of CN V) penetrating medially into the braincase on the right side while only three on the left side (Fig 2D, F). Among the trigeminal foramina, the anterior one is separated from the posterior foramina as in Allosaurus [14,16] and Murusraptor [30], despite the posterior foramina are merged into one in the latter two taxa. Therefore, the area anterior to the posterior foramina is probably occupied by the laterosphenoid as in these two taxa [14,16,31]. The anterior foramen is the exit of CN V₁ (ophthalmic branch of CN V), which is situated posterolateral to the exits of CNs II and III and penetrates posteromedially into the braincase. Somewhat posterior to the exit of CN V₁, there are three openings for CN V_2,3 (maxillary and mandibular branches of CN V) on the right side, whereas only two are on the left side. Based on their position, the two smaller openings on the right correspond to the smaller opening on the left because both are dorsal to the largest opening (Figs 2D, 3E). Each of these two sections possibly corresponds to each branch but cannot be determined here. Posterior to these trigeminal foramina, there is a small opening for the exit of CN VII penetrating the prootic region. When the frontal is horizontal, the exit of CN V₁ is located ventral to the nuchal crest, whereas those of CNs V_2,3 and VII are located more posteriorly. Posterodorsal to the exit of CN VII, there is a large foramen that corresponds to the columellar recess [56]. Ventral to the exit of CN VII, the rostral tympanic recess excavates the braincase anteromedially, which seems to be covered laterally by the basisphenoid if preserved. The anterodorsal end of the rostral tympanic recess is connected to an oval chamber excavating the region ventromedial to the exit of CN V₁. This chamber is almost completely exposed on the left side due to the absence of its lateral wall, which is partially preserved in the right side. Ventral to the entrance to this anterodorsal chamber is the internal carotid artery foramen as a junction to the pituitary fossa. Posterior to this foramen, there is an entrance for an internal chamber invading the braincase toward the neck of the occipital condyle (see “Endocranial cavities” for details).

Below the laterosphenoid- prootic region, the basisphenoid and parasphenoid are not preserved or are extremely poorly preserved (Figs 2D, 3E). Although the pituitary fossa could be interpreted as a remnant of the basisphenoid, this interpretation is uncertain, and the posterior part of the fossa may instead be formed by the laterosphenoid, as in Murusraptor (Fig S2 of [31]). The dorsal part of the basisphenoid recess is formed by the basioccipital as a large opening directed posteroventrally anterior to the broken basal tubera (Fig 2F). The basisphenoid recess is divided anteroposteriorly and the posterior part is further divided transversely by thin septa. The anterior part is a fossa with a faint connection to an internal chamber invading the braincase anterodorsally, toward the region just posterior to the pituitary fossa (Figs 6, 7; see “Endocranial cavities” for details). However, a large crack just across this connection indicates that the wall separating the chamber from the fossa has been lost by fracture (Fig 3G). The posterior parts are connected to the two larger chambers mentioned above, which communicate with the rostral tympanic recesses and extend into the neck of the occipital condyle.

Download:

• PNG
  larger image
• TIFF
  original image

Fig 6. Cranial endocast of NRRU-F01020035 in right lateral (A), left lateral (B), dorsal (C), ventral (D), anterior (E), and posterior (F) views.

Scale bar equals 100 mm. Abbreviations: ac, subsidiary diverticulum occupying the anterior chamber; asc, anterior semicircular canal; cc, common crus; cer, cerebral hemisphere; dp, dural peak; floc, floccular recess; fm, foramen magnum; lag, lagena; lsc, lateral semicircular canal; ob, olfactory bulb; ot, olfactory tract; ov, orbitocerebral vein; pit, pituitary; pc, complex of the caudal diverticulum of the rostral tympanic recess and the medial basioccipital diverticulum occupying the posterior chamber; psc, posterior semicircular canal; vc, venous canal; II–XII, cranial nerves.

https://doi.org/10.1371/journal.pone.0345155.g006

Download:

• PNG
  larger image
• TIFF
  original image

Fig 7. Cranial endocast with transparent braincase of NRRU-F01020035 in ventral (A) and left lateral (B) views.

Scale bar equals 100 mm. Abbreviations: bo, basioccipital; eo, exoccipital; fr, frontal; ls-po, laterosphenoid-prootic complex; ob, olfactory bulb; os, orbitosphenoid; ov, orbitocerebral vein; pa, parietal; pc, complex of the caudal diverticulum of the rostral tympanic recess and the medial basioccipital diverticulum occupying the posterior chamber; pit, pituitary; so, supraoccipital; II-XII, cranial nerves.

https://doi.org/10.1371/journal.pone.0345155.g007

The posterodorsal region of the braincase is composed of the supraoccipital, which contacts the parietal anteriorly and the exoccipital-opisthotic complexes lateroventrally (Figs 2B, D, 3D, E). The lateral parts of the supraoccipital are broken. In dorsal view, the suture line between the supraoccipital and the parietal is invaded by the tongue-like process of the parietal (Fig 2B) as in other allosauroids [10,15,25]. There is a dorsal projection of the supraoccipital (supraoccipital knob) along with the median ridge on the posterior surface (supraoccipital crest) as in other allosauroids [10,21,24,25,28] except for Acrocanthosaurus and some specimens of Allosaurus, in which the knob is divided into two by a midline fold [17]. The width of the supraoccipital knob (39.60 mm) is larger than that of the foramen magnum (29.93 mm) as in other allosauroids [43]. Although the suture line against the exoccipitals is indistinct near the dorsal margin of the foramen magnum, the remaining part of the suture indicates that the contribution of the supraoccipital to the foramen magnum is very narrow or absent (Fig 3D).

In posterior view, the exoccipitals form lateral edges of the foramen magnum (Fig 3D). Whether or not the exoccipitals are separated by the supraoccipital on the dorsal edge of the foramen magnum is ambiguous because the suture against supraoccipital is invisible in that part. In contrast, the exoccipitals are clearly separated by the basioccipital on the ventral edge of the foramen magnum. At the midheight of the foramen magnum, a horizontal suture-like line is present as in Murusraptor (Fig 6E’ in [30]). In dorsal view, sutures against the basioccipital are present just anterodorsal to the condylar surface (Fig 2B), thus exoccipitals have only a little contribution to the occipital condyle unlike Allosaurus, in which the exoccipitals occupy a conspicuous part of the condyle [13]. Dorsolateral to this suture, a large pneumatic cavity probably correspond to the caudal tympanic recess invades the base of the paroccipital process (Fig 3D, E) as in Sinraptor and many other theropods [56].

The basioccipital composes most of the occipital condyle and makes a small contribution to the floor of the foramen magnum (Fig 2B). Just below the occipital condyle, the basioccipital forms a broad, plate-like surface (“basioccipital apron” sensu [60,61]) that faces posterodorsally and is slightly wider than the occipital condyle in posterior view (Fig 3D). The basioccipital apron continues posteroventrally with forming a rough and concave posterodorsal surface toward the basal tubera, whereas the tuber itself is not preserved in this specimen. The concave posterodorsal surface becomes a deep fossa at the dorsal margin of the basioccipital apron excavating the base of the neck of occipital condyle anterodorsally (Fig 4A). This fossa probably corresponds to the subcondylar recess but is not connected with any internal pneumatic cavities unlike those of Shaochilong [28] and Murusraptor [30]. The lateral width of this fossa (21 mm) is much narrower than that of the occipital condyle (51 mm) as in most theropods [43].

Anterolateral to the occipital condyle and posteroventral to the caudal tympanic recess, there is a large fossa correspond to the paracondylar pocket [28,62] or paracondylar recess ([56,58,63]; Fig 3D, E). The dorsal part of this fossa bears two foramina separated dorsoventrally by a septum. The dorsal foramen represents the exit of the posterior branch of CN XII (hypoglossal nerve). The ventral foramen is further separated mediolaterally by a vertical septum (Fig 4A) as in Giganotosaurus [27,64]. The medial, smaller one corresponds to the anterior branch of CN XII, whereas the lateral, larger foramen is interpreted as the vagal foramen, representing the caudal opening of a subdivided metotic fissure and transmitting CNs X and XI (vagus and accessory nerves; Fig 2D). In total, there are three foramina as in Giganotosaurus [18], Shaochilong [28] and Murusraptor [31] on the right side, unlike Sinraptor and Allosaurus, in which there are only two foramina because CN XII exits only through one foramen [16,56]. The remaining ventral part of the paracondylar pocket probably corresponds to the paracondylar pneumatopore, although which is more deeply excavated in carcharodontosaurids to lead into an extensive recess below the endocranial cavity [21,25,28,43]. The anterior margins of these foramina and a fossa is formed by a thin bony web as a remnant of the crista tuberalis (metotic strut). Anterior to this bony web, there is a large triangular cavity representing the columellar recess excavating the braincase anteromedially.

The dorsal and anteroventral rims of the columellar recess are formed by a thin ventral wall of the caudal tympanic recess and a thicker lamina, respectively (Figs 2D, 3D, E). The columellar recess accomodates two foramina separated by a vertical septum, namely the crista interfenestralis (Fig 4). The anterolateral division is the fenestra ovalis (or vestibularis) excavating the braincase anterodorsally to connect with the cochlear duct. The posteromedial division is the fenestra pseudorotunda (or fenestra cochleae). The anterior end of the dorsomedial corner is the deepest part that is excavated by an opening for CN VIII (vestibulocochlear nerve). Posteriorly, the dorsomedial corner has two additional openings of the vagal canal transmitting CNs X and XI in the right side, while there is only one opening in the left side. The area ventral to them is excavated by an opening for CN IX (glossopharyngeal nerve).

Cranial endocast.

The digital cranial endocast reconstructed from the well-preserved braincase (NRRU-F01020035) is nearly complete and consists of the cavities for the brain and inner ears, canals for cranial nerves and blood vessels, and pneumatic cavities (Figs 6, 7). The endocast preserves the olfactory bulbs and cerebral hemispheres, as well as the midbrain, hindbrain, and pituitary. Some dural sinuses are visible on the endocast, but cranial nerves and blood vessels themselves cannot be observed in the CT slices and were therefore not reconstructed. The endocast measures 175 mm long from the anterior end of the olfactory bulbs to the foramen magnum and has a maximum width of 42 mm at the lateral processes of the cerebral hemispheres. The volume of the endocast is approximately 124 ml, excluding canals for cranial nerves and blood vessels.

Overall, the cranial endocast is dorsoventrally low and mediolaterally narrow, with bulges indicative of forebrain, midbrain, and hindbrain structures. The cephalic flexure (angle between the forebrain and the midbrain) and the pontine flexure (angle between the midbrain and hindbrain) are at 130 and 150 degrees, respectively, which makes the forebrain slightly upward to the hindbrain, but the two are approximately parallel. The angles of the cephalic and pontine flexures are similar to those of Sinraptor [56], Tameryraptor [19], Carcharodontosaurus [23] and Giganotosaurus [25,26]. This makes the endocast elongate anteroposteriorly, unlike those of Allosaurus, Ceratosaurus, Majungasaurus and Viavenator, which are dorsoventrally high [16,65–67]. There are no traces of sutures between the bones of the braincase on the surface of the endocast, confirming that the braincase belongs to a subadult or adult specimen.

Forebrain.

The anteriormost portion of the cranial endocast represents the olfactory bulbs and tract. Only the dorsal margin of the olfactory bulbs is visible as impressions on the ventral surface of the frontal as in other theropods [16]. Due to the absence of bony septum, the olfactory bulbs are reconstructed as a single structure expanded anterolaterally, but in dorsal view, it shows a heart shape, with its midline slightly depressed along the antroposterior axis. The olfactory tract is triangular in its cross-section with a flat dorsal surface and relatively short in comparison to these of Carcharodontosaurus [23] and Giganotosaurus [25,26]. The ventral apex of this tract becomes rounded posteriorly to merge with the cerebral hemispheres, while the flat dorsal margin continues posteriorly to the approximate midpoint of the cerebral hemispheres. The lateral process on the anterior part of the cerebral hemisphere marks the part of the endocast with the greatest mediolateral width. The venous canal, reconstructed on the right side only, extends anterolaterally from the anterior dorsolateral margin of the cerebral hemisphere as in Majungasaurus [58] and Allosaurus [16,66]. Ventral to the cerebral hemisphere is the large circular exit of CN II (optic nerve) with the path of the pituitary (Fig 7A). The dorsalmost part of the endocast, the dural peak, is at the same anteroposterior level as the caudal margins of the CN II exit and the cerebral hemispheres.

Midbrain.

There is no obvious representation of the optic lobes (optic tecta) and CNs III (oculomotor nerve) and IV (trochlear nerve). The midbrain is inclined posteroventrally along with the occiput (occipital plate) as in Carcharodontosaurus [23] and Giganotosaurus [25,26]. Posterior to the cerebrum, the dorsal surface of the endocast above the hindbrain is marked by a pronounced ridge that continues caudally from the dural peak.

Hindbrain.

Boundaries between the cerebellum, medulla, and pons of the hindbrain are not clearly defined in the cranial endocast. However, there is a distinct floccular process on each side (Figs 6, 7C), which in life housed the floccular lobe of the cerebellum, as in other theropods [10,27,66,68,69]. The floccular process extends posterolaterally to enter the region surrounded by the anterior semicircular canal. The floccular process is elongated dorsoventrally and slopes anteroventrally along the long axis of the anterior semicircular canal. The subtle swelling dorsal to the floccular process probably corresponds to a vascular component (Fig 6A, B) such as the vena capitis dorsalis of Carcharodontosaurus [23] and the caudal middle cerebral vein of Murusraptor [31] and Giganotosaurus [27]. Anteroventral to the floccular process, CN V (trigeminal nerve) is represented by a single root on the lateroventral margin of the hindbrain as in other allosauroids [16,18,23,27,31,56]. CN V extends anteroposteriorly to form an anterior exit for CN V₁ (ophthalmic division) and two or three posterior exits for CN V_2,3 (maxillary and mandibular divisions). An enlarged region at the base of these divisions indicates the location of the trigeminal ganglion [16,31]. A similar pattern, in which the ophthalmic, maxillary and mandibular branches diverge from a shared exit region, is also common in other tetanurans (e.g., Allosaurus, Struthiomimus, Dromaeosaurus, Troodon), although tyrannosaurids have ophthalmic and maxillomandibular canals that branch separately from the endocranial cavity [67]. CN VI (abducens nerve) projects anteroventrally from the ventral part of the hindbrain and enters into the pituitary fossa. CN VII (facial nerve) emerges posterior to the exits of CN V_2,3 and extends lateroventrally. CN VIII (vestibulocochlear nerve) emerges ventral to the endosseous labyrinth and anterodorsal to the passage of CN IX (glossopharyngeal nerve). CN IX has its own passage completely separated from those of CNs X and XI (vagus and accessory nerves) as in Carcharodontosaurus [23], unlike that of Allosaurus conjoined only with CN X [16] nor with CNs X and XI as in Acrocanthosaurus [18], Giganotosaurus [27] and Murusraptor [31]. CNs X and XI shares the passage as the vagal canal directed posterolaterally, although its middle part is bifurcated in the left side. The endocast has a distinct lateral process dorsomedial to the metotic fissure. There are two passages for CN XII (hypoglossal nerve), which arise from different roots and extend posterolaterally from the posteroventral aspect of the hindbrain.

Diverticulum.

In the cranial endocast, three pneumatic chambers for the diverticula located in the basicranium ventral to the hindbrain are reconstructed (Figs 6, 7). These are the anterior one and the posterior two chambers.

The anterior chamber extends anterodorsally and expands anterolaterally. There are sagittal grooves on the dorsal and anteroventral surfaces, indicating a subtle division of the anterior chamber into left and right. Although the anterior chamber has a small opening in the anterior part of the basisphenoid recess, a large crack just across this opening indicates that the wall separating two chambers has been lost by fracture (Fig 3G). Instead of the external opening, the anterior chamber has a broad communication with the posterior chambers. Therefore, the anterior chamber probably housed the subsidiary diverticulum derived from the one occupying the posterior chamber.

The posterior chambers are posteriorly elongated and posterolaterally dilated. The relative position of the posterior chamber is similar to that of the caudal diverticulum of the rostral tympanic recess (cRTr) and the medial basioccipital diverticulum (MBd) in other avetheropods [70]. In addition, the posterior chamber communicates with the posteroventral part of the rostral tympanic recess as in cRTr, and with the dorsal apex of the basisphenoid recess as in MBd (Figs 2F, 7A). Each posterior chamber is elongated posteriorly as in cRTr [70]. However, the posterior chamber of Siamraptor is oval and expanded posterolaterally, unlike the narrow form of cRTr [70]. The inflation of this chamber renders the sagittal septum separating the posterior chambers incomplete, allowing a broad confluence between them (Fig 6D). Given this unusual inflation and the topographic similarity to both cRTr and MBd, the posterior chamber of Siamraptor appears to be formed by the fusion of chambers occupied by cRTr and MBd in other avetheropods.

Inner ear (endosseous labyrinth).

The cranial endocast also preserves the semicircular canals, cochleae, and vestibule of the inner ear. The inner ear measures approximately 112.8 mm dorsoventrally and 83.5 mm anteroposteriorly. The region outlined by semicircular canals is strongly triangular in lateral view (Fig 6A, B) as in Carcharodontosaurus [23] and Giganotosaurus [27]. The anterior and posterior semicircular canals are tall, slender and approximately triangular in posterolateral and anterolateral views, respectively. The anterior semicircular canal extends dorsally above the level of the posterior semicircular canal. The lateral semicircular canal is oval in dorsal view, with its main axis oriented anteroposteriorly.