Tarbosaurus bataar

Hurum, Jørn H. & Karolsabath, 2003, Giant theropod dinosaurs from Asia and North America: Skulls of Tarbosaurus bataar and Tyrannosaurus rex compared, Acta Palaeontologica Polonica 48 (2), pp. 161-190 : 164-187

Tarbosaurus bataar ( Maleev, 1955a, b)

Holotype: PIN 551−1 .

Paratype: PIN 551−2 .

Type locality: Nemegt, Mongolia.

Type horizon: Nemegt Formation (?late Campanian/early Maastrichtian).

Description of the skull of Tarbosaurus bataar ZPAL MgD−I/4 with additional remarks on Tyrannosaurus rex BHI− 3033

The skull as a whole ( Figs. 1 , 2 , for measurements see Tables 1–5).—The skull of Tarbosaurus bataar in lateral view resembles that of Tyrannosaurus rex , both are deep and have powerful jaws. In dorsal view the skull of T. rex is extremely broad posteriorly but narrows towards the snout; in Ta. bataar the skull is narrower (especially in its ventral part: the premaxilla, maxilla, jugal, and the quadrate complex), and the expansion of the posterior half of the skull is less abrupt. In T. rex , the jugal flares out posteriorly so strongly that the ventral part of the lacrimal shaft is visible in dorsal view. The slender snout of Ta. bataar is reminiscent of more primitive NATs (see, e.g., Currie 2003). The modifications of the T. rex skull shape, compared to all other tyrannosaurids, concentrate in the jaw apparatus, while the braincase is less affected.

The most obvious difference between T. rex and Ta. bataar is the doming of the nasal in Ta. bataar which is high between the lacrimals and is less attached to the other bones of the skull than in most tyrannosaurids. This is because of a shift in the handling of the crushing bite in Ta. bataar which will be discussed in more detail below.

Premaxilla ( Fig. 3 ).—The premaxilla is a short, stout bone, resembling a subvertically oriented prism with rounded outer edges, and with elongated mediodorsal (supranarial) and dorsolateral (subnarial) processes that border much of the external naris. The premaxillae fit tightly together, with their medial faces sculpted in small grooves and protuberances. In T. rex BHI− 3033 there are six ridges situated on the supranarial process, while the rest of the surface possess only small grooves. The anterior margin of the premaxilla is perpendicular to the palatal plane and at a level with the external naris, where it bends strongly posteriorly. This curvature is similar to that in Albertosaurus , while in T. rex , “ Nanotyrannus ”, Albertosaurus sarcophagus ( TMP 81.9.1, Bakker et al. 1988), and in e.g., Allosaurus , the snout tip gently curves posteriorly along the premaxilla. The thin, medial, dorsoposteriorly bent nasal processes of the premaxillae separate the external nares and border them anteriorly. The processes become narrower dorsoposteriorly to fit between the processes of the nasals, that join them approximately one third of their length posteriorly. There is a pronounced depression (narial fossa) surrounding the anterior margin of each external naris. The suture between the premaxilla and the maxilla runs obliquely anteroventrally. The surface in contact with the maxilla has three main ridges pointing posterodorsally. In T. rex ( BHI− 3033) the surface is divided by a main ridge and several grooves. The premaxillary tooth row is bordered by a shallow depression on the palatal side. The bone contains four D−shaped, serrated teeth that are smaller than lateral teeth, and the tooth row arcade is more mediolaterally than posterioanteriorly oriented ( Holtz 2001). The palatal surface of the premaxilla is almost flat, forming a narrow palatal shelf. On the labial side, along the tooth row, there are three rounded vascular pits, situated between the teeth bases, and about one tooth diameter upwards from the lower margin of the premaxilla. Several smaller pits are visible still higher on the labial side of the bone. The position of the foramina corresponds exactly to the pattern seen in T. rex ( BHI− 3033).

Maleev (1974: 141) stated that on the internal side of the premaxilla, just below the naris, there is a deep, longitudinal groove for the thin anterior process of the maxilla. Obviously he meant the lower (maxillary) process of the nasal and not the maxilla itself. He also described an oval opening between the maxilla and premaxilla (ant3) as the third antorbital fenestra ( Maleev 1974: 140). Molnar (1991: 141) described a similar feature in T. rex as a dorsoventrally elongate foramen that leads through a short channel with smooth walls and opens into the oral cavity just below the palate. This internal opening is, however, not shown in Molnar’s restoration of the skull of T. rex ( Molnar 1991: fig 9A), Molnar’s (1991) and Osborn’s (1912) restorations are mislabelled—on the figure they are labelled A and B, respectively, and in the caption: B and A. In Ta. bataar the opening is barely visible on the oral side, and the shape of the external fenestra varies, from an oval to a long cleft. In T. rex it occupies one third of the maxillary contact surface of the premaxilla ( Molnar 1991), while in Ta. bataar it tends to be smaller. In a composite reconstruction of Gorgosaurus libratus ( Carr 1999: fig 5A) the foramen is named foramen subnarialis and is small in lateral view, see also Currie (2003) on subnarial foramen. Madsen (1976) described a homologous structure in Allosaurus fragilis and it is widespread in Saurichia (Sereno and Novas 1993; Sereno 1999).

The premaxillae are missing from the otherwise well preserved skull of ZPALMgD−I/3, but are present in ZPAL MgD−I/4, ZPALMgD−I/175, ZPALMgD−I/44, GIN 100/60, GIN 100/65, GIN 107/2 and GIN 107/3, as well as in PIN 551−1, PIN 551−2, PIN 552−1.

Maxilla ( Fig. 4 ).—The maxilla is the largest bone of the Ta. bataar skull. It is roughly triangular in shape, with the alveolar edge convex and the posterior edge deeply emarginated for the antorbital fenestra. The outer side of the bone is smooth within the antorbital fossa but rugose elsewhere. There are numerous pits along the labial margin, within about 10 cm above the tooth row. The blunt anterior apex contacts the premaxilla, the dorsal edge forms a suture with the nasal and, at the dorso−posterior end, contacts the lacrimal. The ventral edge contacts the jugal posteriorly. The suture to the nasal bone is very different from what can be observed in Gorgosaurus libratus ( Witmer 1997: fig. 30) where the suture is a smooth surface. In Ta. bataar the suture consists of deep transverse cavities and ridges fitting exactly into the nasal. This is also the case in T. rex ( BHI− 3033: Fig. 11A ). The anterior part, which contacts the premaxilla, has a small invagination for the promaxillary fenestra. The massive, posterodorsal process of the maxilla has the anterior end of the lacrimal as a sheath around its posterior end. This posterodorsal process is thin and plate−like in T. rex .

There is a large foramen in the middle of the maxilla. This has been called the second antorbital fenestra (ant2) by Maleev (1974) and termed maxillary fenestra by Molnar (1991) and Witmer (1997). This fenestra has an ovate−triangular shape with sharper end facing forward and opens into the lateral surface of maxilla in the anterior part of the antorbital fossa. The maximum height and length of the maxillary fenestra are the same.

On the medial surface just below the maxillary fenestra, the posterior end of a horizontal palatal shelf protrudes medially from the mid−anterior part of maxilla and meets the palatal shelf of the contralateral maxilla along the midline of the skull. The palatal shelf of the maxilla contacts the palatine along its rear end, and the front is overlain by the vomer. Molnar (1991: 142) indirectly suggested that the maxillae are entirely separated by the vomer. This is true only in ventral view of the palate. In Ta. bataar the vomer only underlies the anterior part of the palatal shelves along the midline of the skull. This is especially evident in ZPALMgD−I/3, which has been broken to the right of the midline. A similar situation occurs in T. rex . The palatal shelf is smooth, but there are several pits or depressions for dentary teeth in its proximal part, where it merges into the medial surface of the main body of the maxilla. This medial (lingual) surface bears a row of shallow grooves, against which the dentary teeth probably fitted. Further ventrally, covering the replacement teeth and bases of functional teeth, are the interdental plates. They have rough surfaces and are not fused together.

The medial surface of the maxilla shows several suboval chambers (accessory cavities or sinuses), situated along the nasal margin and anterodorsally from the maxillary fenestra. A thin layer of bone covered these chambers, but now it is broken. They follow the same pattern as described for Gorgosaurus libratus by Witmer (1997: fig. 30), with a promaxillary recess anteriorly and a maxillary antrum posteriorly (epiantral recess + maxillary antrum). The excavatio pneumatica within the ascending ramus of the maxilla is not observed. The epiantral recess is larger than in G. libratus , expanded dorsally and partly covered mediodorsally by the postantral strut. The promaxillary recess is deeper, but of the same general shape as in G. libratus , with two main cavities. Molnar (1991: fig 2, cavities a and b) described the same general shape for T. rex . The promaxillary fenestra is visible in medial view and is situated in the dorsal part of the promaxillary strut. On the ventral part of the promaxillary strut in the contact with the palatine part of the maxilla, the fenestra communicans opens between the posterior part of the promaxillary recess and the anterior part of the maxillary antrum.

In Ta. bataar the shape and size of the maxilla and its foramina are very much the same as in other tyrannosaurids ( Carpenter 1990). The maxilla of “ Maleevosaurus ” would also fit within this variability range, except for the extremely elongated dorsal ramus as restored by Maleev (1974).

Nasal ( Fig. 5 ).—As in other tyrannosaurids, the nasals of Ta. bataar are fused along the midline into a strong, elongated element, slightly compressed in the middle. The dorsal surface is convex in transverse section and rugose, with several round pits that open along both sides of the midline, especially just behind the nares and near the posterior end. The rugose texture is more pronounced in larger specimens and in the middle part of the nasals and is typical of mature specimens ( Carr 1999). The ventral surface is smooth and markedly concave in transverse section. There are several pits on the ventral surface of the bone. The first pair of pits is placed behind the nares and slightly medially from their rear end. The first pair of pits is also seen in T. rex (BHI−3033), but due to preservation only one more pit can be observed. Along their ventrolateral margins, the nasals contact the maxillae. Grooves and ridges perpendicular to the sagittal plane strengthen the contact. They extend along the whole maxillary contact, while Molnar (1991) stated that in T. rex they are restricted to the anterior one third. In the T. rex specimen available to us, BHI−3033, the ridges extend as far as in Ta. bataar , but they are less prominent ( Fig. 5B ).

In the anterior part, the nasals are widest just behind the external nares. The premaxillary processes of the nasal form the posterodorsal margin of each naris. The subnarial process is about half the length of the upper one, and contacts the dorsolateral process of the premaxilla. The nasals are separated along the midline by a thin fissure, extending backwards from the contact with the premaxilla to the widest point (i.e., within anterior 1/4 to 1/3 of their length). The suture between the nasals is visible also in its posterior part, especially on the ventral surface and in the smaller individuals. This suture extends anteriorly to the level of the lacrimal−maxilla contact. The nasal contacts the frontal posteriorly along a W−shaped suture, and in its lateroposterior—with the lacrimal. The suture against the anterior ramus of the lacrimal differs considerably in Ta. bataar and T. rex . In Ta. bataar it is an almost horizontal, smooth groove. In T. rex the complex articulation between the anterior ramus of the maxilla and lacrimal seen in Ta. bataar is shifted to the nasal. Between the main body of the nasal and a posteriorly oriented lacrimal process ( Fig. 5B ), a robust groove is present for the anteriorly bifurcated, blunt end of the lacrimal. This lacrimal process of the nasal causes the nasal bone to be wider in T. rex than in Ta. bataar and thus in dorsal aspect the nasals of Ta. bataar are more slender than in T. rex , Albertosaurus ( Carr 1999: fig. 5) and Daspletosaurus ( Russell 1970) . The lack of expansion and lack of lacrimal process on the nasal might be synapomorphies of Ta. bataar , but may be present in Alioramus , too. In more primitive theropods like Sinraptor ( Currie and Zhao 1993: figs. 3, 5) and Allosaurus ( Madsen 1976: fig. 5b, c), the expansion is less prominent than in North American tyrannosaurids, but a small lacrimal process of the nasal is present (see discussion).

The most posterior part of the nasal bone is not preserved in the studied specimen (ZPALMgD−I/4) of Ta. bataar , but it is well preserved in T. rex BHI−3033. From comparisons of the medial side of the lacrimal (see Fig. 5B ) it is likely that the same structure is present in both. The most posterior part of the nasal in T. rex is a thin posterodorsal strut that bends slightly laterally. This process fits into a small pocket in the lacrimal.

Lacrimal ( Fig. 6 ).—The anterodorsal ramus (anterior process of Currie 2003) of the lacrimal borders the antorbital opening from above, while the vertical ramus (descending or jugal ramus, or postorbital bar of Currie 2003) separates this opening from the orbit. The apex protrudes slightly backwards and meets the frontal. In larger specimens the apex and the dorsal margin is very rugose, but the pronounced lacrimal horn, found in most North American tyrannosaurids and in Allosaurus , is lacking. In T. rex ( Fig. 7A ) the apex is more inflated than in Ta. bataar . The posterior surface forming the suture to the frontal and prefrontal is divided vertically by a rugose ridge in Ta. bataar , but in T. rex (BHI−3033) this surface is nearly smooth. There is no contact between the postorbital and lacrimal in the described specimen of Ta. bataar , but see Currie (2003). The suture resembles that in T. rex , but it differs from that of Albertosaurus because of the more anteriorly positioned prefrontal in this genus ( Carr 1999: fig. 5).

On the lateral side, just above and behind the posterodorsal corner of the antorbital fenestra, there is an oval or reniform lacrimal opening, which leads to interior sinuses, divided by an internal ridge. In T. rex the dorsal ramus contains a large sinus that is at least three−chambered in its apical moiety ( Molnar 1991: fig. 3). In Ta. bataar the extent of the lacrimal sinus is similar, and fills most of the anterodorsal ramus and part of the vertical ramus.

The orbital margin of the vertical (= descending) ramus of the lacrimal is convex anteriorly, while the antorbital margin is straighter, only lightly bent in midheight. The lower third of this ramus widens ventrally, where it contacts the jugal; the anterior margin of the bone forms a ridge that continues ventrally (slightly posteriorly) on the lateral side of the lacrimal. A triangular flap of bone protrudes anteroventrally from behind the ridge (anterior process of the jugal suture). A crest separates two shallow concave areas on the medial side of the vertical ramus. The anterior area widens upwards, while the posterior area widens downwards. At least the upper part of the ramus is hollow, containing the lacrimal duct and probably a sinus extending downwards (in T. rex the descending ramus appears to be solid; Molnar 1991). The contact between the jugal and lacrimal is similar in Ta. bataar and T. rex with the anterior process of the lacrimal medial to the jugal and the posterior process with a groove for the jugal.

The horizontal ramus is longer and less inflated in Ta. bataar (ZPALMgD−I/4) than in T. rex (BHI−3033). The horizontal ramus narrows gradually towards its anterior end, where it is divided by a short mediolateral cleft. The cleft process fits into the robust posterior end of the maxilla, and separates the antorbital fenestra from the nasal. In T. rex this anterior process of the lacrimal is more clearly bifurcated ( Fig. 7A ), the cleft fits into the nasal and the contact with the maxilla is limited to a shallow groove in the anteroventral part. In Gorgosaurus libratus (TMP 91.36.500; Carr 1999: fig. 5) the suture is similar to that in T. rex , while in in Ta. bataar the suture is like that of Alioramus ( Kurzanov 1976: fig. 1).

The medial side of the horizontal ramus contacts the constricted, posterior part of the nasal. The contact surface is excavated longitudinally. In the anterior half there are two deep, parallel grooves, divided by a thin ridge that runs along the mediodorsal part of the ramus. A large foramen for blood vessels is situated in the middle of the ramus in Ta. bataar , and more anteriorly in T. rex (BHI−3033). More posteriorly, one horizontal ridge continues, and separates the lower, smooth and concave part of the medial surface from more sculpted upper part, contacting the nasal and anterior parts of the frontal. Medially, the apex has a large depression, which is absent in T. rex . Dorsal to this depression a small groove fits the position of the groove for the posterior hook of the nasal in T. rex . The horizontal ramus is hollow almost to the front end.

Prefrontal ( Fig. 8 ).— Maleev (1974) described the prefrontal as a small triangular bone on the skull roof, surrounded by frontal, lacrimal, nasal, and postorbital. A similar bone is shown in the dorsal reconstruction of T. rex skull ( Osborn 1912), and in Daspletosaurus torosus ( Russell 1970) . Molnar (1991) did not mention the bone in his detailed cranial osteology of T. rex , but did figure it in his thesis ( Molnar 1973), and Rozhdhestvensky (in Maleev 1974) noted that the triangular outlines visible on the skull roof could actually be the fractured posterolateral processes of the nasals.

The bone is clearly visible, lateral to the anterior process of the frontal, in dorsal and lateral view in the two disarticulated skulls studied ( Ta. bataar ZPALMgD−I/4 and T. rex BHI−3033). The lateral side of the prefrontal has several ridges for the attachment of the lacrimal, and supports most of the posteromedial side of the latter bone. A ventrally directed process of the prefrontal fits along the posterodorsal part of the medial side of the lacrimal. It makes the dorsal 1/4 of the medial ridge on the vertical ramus of the lacrimal in Ta. bataar . This ventral process of the prefrontal is smaller in T. rex . The bone can also bee seen in Ta. bataar ZPAL MgD−I/38, where the nasals are absent, and both prefrontals are separated from the frontals by a wide fissure.

Postorbital ( Figs. 8 , 9 ).—The postorbital is a triradiate bone, with the largest ramus extending ventrally to meet the jugal bone. The posterior (intertemporal of Currie 2003) ramus meets the squamosal, and the medioventral ramus joins the frontal and possibly the parietal and laterosphenoid (see Currrie 2003). The orbital margin of the postorbital is strongly arched and a protruding rugosity extends parallel to its upper part, forming a crescent−shaped postorbital horn. This anteriorly concave crescent is most rugose and pronounced in the largest specimens, but can be seen in all specimens studied. Molnar (1991) described a groove rimmed with tubercles along the dorsal margin of the orbit as typical of the T. rex postorbitals ( Fig. 11B ). Such a feature has not been observed in Ta. bataar . Instead, there is a rim that runs along the posterior edge of the orbit, and flanks a smooth surface perpendicular to the lateral plane. This surface is widest in the posteriormost part of the orbit and narrows anteriorly, where the rim fades away into a vertical sheet of bone, protruding into the orbit. The position and curvature of this smooth, concave surface seems to fit the posterior part of the eyeball and its muscles.

The lower part of ventral ramus forms an anteroventrally oblique contact surface with the jugal. A thin, vertical sheet of bone extends further anteriorly into the orbit, which is a typical feature of most large specimens of tyrannosaurids (see discussion in Currie 2003).

Squamosal ( Figs. 9 , 10 ).—The squamosal is a large bone and consists of a hollow thin−walled body with anteroventral (quadratojugal), anterodorsal (postorbital or intertemporal ramus), and mediodorsal processes. In lateral view the bone occupies the posterodorsal corner of the side of skull, and borders the upper part of the lower temporal fenestra posteriorly. The squamosal forms a long, deep groove for the postorbital along the anterodorsal process. The bone meets the quadratojugal on the ventral surface of its anteroventral process and the quadrate in a deep groove on its posteriormost end. The anteroventral process of the squamosal is almost horizontal and is flexed slightly downwards in its distal part in ZPALMgD−I/4. It al − most divides the lower temporal fenestra into separate openings. The process extends along the dorsal margin of the quadratojugal, which partly overlaps it laterally in the anterior and posterior part of the contact area. This overlapping of the quadratojugal is greater than in Albertosaurus , Gorgosaurus , and Daspletosaurus , and is more like that of T. rex BHI−3033.

In lateral view, the anterodorsal process is inclined at an angle of 45 degrees to the anteroventral process and is curved (convex dorsolaterally). This process is deeper than the anteroventral one, overlaps medially the posterior ramus of the postorbital, and surrounds its end from above and below. Within the angle between the anterodorsal and the anteroventral processes of the squamosal, there is a thin “flap” of bone, offset slightly medially from both processes. This forms a gently arching posterior margin of the upper part of the lower temporal fenestra.

The body and the anterodorsal process of the squamosal are deeply concave anteromedially. The nuchal process extends along the posterior edge of the upper temporal fenestra towards the nuchal crest and joins the parietal. This vertically deep part of squamosal lies almost in the plane of nuchal crest. Posteriorly, the body of the squamosal is covered by the exoccipital/opisthotic. Generally, the squamosal of Ta. bataar ZPALMgD−I/4 is very similar to that of T. rex , as described and figured by Osborn (1912) and Molnar (1991). It differs from that of Albertosaurus and Daspletosaurus ( Russell 1970; Carr 1999), where the anteroventral process is more curved and about the same tickness as the anterodorsal process. The body and mediodorsal processes of the squamosals in Albertosaurus and Daspletosaurus are located more posteriorly in respect to nuchal crest, but this may be an allometric difference.

Jugal ( Fig. 12 ).—The jugal is compressed and consists of three rami in the parasagittal plane. The bone is only slightly concave on the medial side. In ventral view, T. baatar lacks the strong lateral flexure observed in T. rex . Molnar (1991) concluded that in some specimens of T. rex (illustrated in Osborn 1912) this flexure is an artefact, but in the specimen available to us (BHI−3033) the flexure is real ( Fig. 7B ). The jugal extends from the antorbital fenestra to the anteroventral corner of the lower temporal fenestra. The lateral side of the jugal has a round opening (jugal foramen), located below the posterior margin of the orbit. This opening led to internal chambers (sinuses). Generally, the surface of the jugal is mostly smooth; only the slightly protruding part of it, the cornual process, situated ventrally to the jugal foramen, is rugose in larger specimens. There are two small foramina situated at the border between the ascending and posterior rami on the lateral side. In T. rex there is one large foramen or two smaller foramina. On the dorsal side, at the base of the ascending ramus, there is one foramen.

The anterior ramus widens anteriorly and is divided distally by a deep incision into medial and lateral flaps, the latter being subdivided into smaller upper and larger lower processes. The narrowing, posterior tip of the maxilla fits between them and continues along the ventral edge. The anterior ramus abuts the lower ramus of the lacrimal dorsally, and the ectopterygoid ventromedially. The ascending process of the jugal abuts the postorbital along a shelf in the upper 3/4 of its oblique anterior margin, and its posterior, subvertical margin forms most of the anterior border of the lower temporal fenestra. The jugal−postorbital contact in Ta. bataar is a straight, oblique line (in Daspletosaurus and Albertosaurus the line is distinctly bent; Russell 1970). A large indistinct depression covers most of the lateral side of the ascending process. This depression is notably smaller in T. rex (BHI−3033; Fig. 7B ). The posterior ramus (subtemporal process of Currie 2003) is divided by a triangular notch into two processes, of which the dorsal is markedly smaller than the ventral. They embrace the anterior end of the quadratojugal, which laterally overlaps almost the entire ventral process of the posterior ramus. The ventral process is thickest on the ventral side but thins dorsally. In T. rex (BHI−3033, Fig. 7B ) the process is more convex and of even thickness.

Quadratojugal ( Fig. 9 ).—The quadratojugal is oriented vertically. In lateral aspect, it is wide ventrally, narrow in midheight, and wide dorsally. The elongated dorsal process overlaps the anteroventral process of the squamosal laterally and the quadrate posteromedially. The ventral process extends anteriorly to fit between the forked posterior ramus of the jugal and overlaps most of the ventral one. The posterior part of ventral process of the quadratojugal overlaps the quadrate just anterodorsally to the quadrate condylus and in the posterodorsal corner of the bone. The bone covers the quadrate laterally, but it joins it only in the upper and lower part of the thicker, posterior margin of the quadrate (“shaft” in Molnar 1991). The middle part of the quadratojugal (consisting only of the shaft) is thus separated from the quadrate by an opening (paraquadrate foramen), which is lens−shaped in posterior view. The shaft of the quadratojugal in Ta. bataar is more slender than it is in T. rex (BHI−3033; Fig. 11C ). The main difference pertains to the articulation between the jugal and the anteroventral process of the quadratojugal. In both taxa the quadratojugal fits between the forked posterior part of the jugal, with a dorsal thickening of the process, but the shape is different. In Ta. bataar the process is almost straight but slightly bent laterally in the anteriormost part. In T. rex (BHI−3033) it is extremely concave medially and covers a larger part of the posterior ramus of the jugal laterally.

Palatine ( Fig. 13).—The palatine is roughly triangular in both dorsal and lateral view. The bone is hollow, thin−walled with a smooth surface, except for rugose patches on the inflated dorsal surface.

There are four processes protruding from the triangular main body, the anterolateral maxillary process, the dorsomedial vomeropterygoid process (vomerine process of Currie 2003), the ventroposterior pterygoid process, and the dorsoposterior jugal−lacrimal process. The dorsal side of the palatine contains two large openings (palatine recesses) placed laterally along the deep groove for the maxilla. A smaller foramen is placed more medially. In T. rex (BHI−3033) the large openings are similar in shape to those in Ta. bataar , while the medially placed foramen is larger.

The maxillary margin is slightly convex anterolaterally, with a deep groove for the posterior part of the palatine pro−

HURUM AND SABATH—SKULLS OF TARBOSAURUS AND TYRANNOSAURUS COMPARED 177

lower pterygoid process cess of the maxilla. There is a deep pocket where the dorsal process surface meets the maxillary groove anteriorly. This is absent in T. rex (BHI−3033). In both Ta. bataar and T. rex the palatine extends to the level of the fifth alveolus from the back. In Ta. bataar the medial side of the pterygoid process has several small ridges, while in T. rex this surface is almost smooth with a central ridge on the most anterior part of the surface. The vomeropterygoid process is very similar in both, while the jugal process is deeper in Ta. bataar .

palatine recesses deep groove for maxilla maxillary process jugal-lacrimal process vomeropterygoid process 5 cm

The curvature of the choanal margin of the palatine of Ta. bataar is more pronounced than in T. rex and the posterior margin of the choana is almost perpendicular to the long axis of the skull. This resembles the situation in Daspletosaurus torosus (and to some extent in Allosaurus fragilis, Madsen 1976 : pl. 2B), but the choanae are broader and shorter in Ta. bataar than in all above species. The shape of the choanae is, however, variable within a single individual (GIN 107/1). Also, the palatine−pterygoid contact in relation to the vomer is more like in Albertosaurus and Daspletosaurus ( Russell 1970) than in T. rex ( Molnar 1991) . Another feature shared by Ta. bataar and Daspletosaurus while barely present in T. rex is the pterygopalatine fenestrae. They are markedly smaller than the choanae in Ta. bataar , as in Daspletosaurus . The palatopterygoid fenestra in Ta. bataar seems to be broader posteriorly, while it is anteriorly broader in Daspletosaurus . They are contained exclusively between the palatines and pterygoids, and situated anteromedially from the suborbital fenestra. The suborbital fenestrae are bigger and, as in Allosaurus , also the ectopterygoids, jugals and maxillae participate in forming their posterolateral margins.

The greatest surprise was the dorsoposterior jugal−lacrimal process visible on the skull in lateral view. The process lies medially to the most anteroventral part of the lacrimal and fits into the concave area of the bone. This is a similar situation to that described for Gorgosaurus (Currie 2003) .

Ectopterygoid ( Fig. 14 ).—This stout, triangular bone has a protruding, hook−like lateral process, posteriorly bent and reaching the anteromedial part of jugal with the convex side of its tip. The medial part of the bone articulates with the pterygoid. The body of the ectopterygoid in Ta. bataar is hollow as in other tyrannosaurids, but the oval opening on its ventral surface is smaller than in T. rex (BHI−3033).

The anterior edge of the ectopterygoid “hook” forms the posterior margin of a triangular suborbital fenestra. The fenestra is bordered medially by the pterygoid, and anterolaterally by the outer part of the posterior edge of the palatine as well as the ventral side of the anterior end of the jugal arch (the posteriormost part of the maxilla and anteriormost part of jugal). The shape, size, and position of this foramen is very similar to that in T. rex BHI−3033, but it differs from that of Daspletosaurus , where it is less triangular and more curved. In Allosaurus the foramen is triangular, but proportionally larger than in the tyrannosaurids.

In dorsal view the ectopterygoids of Ta. bataar ZPAL MgD−I/4 and T. rex BHI−3033 are somewhat different. A ridge placed in the root of the hook is directed medially in T. rex , while in Ta. bataar it starts in two grooves on the distal part of the hook and turns posteromedially. In the posterior end of the main body the articulation to the pterygoid differs. A large groove in T. rex is equivalent to two small grooves divided by a ridge in Ta. bataar . In ventral view the bone is very similar in the two.

Vomer ( Fig. 15 ).—The vomer of Ta. bataar consists of a large rhomboid plate at the anterior end (recognized as an autapomorphy of Tyrannosauridae by Molnar 1991) and long, laterally flattened stem which is bifurcated posteriorly. The shape of the vomer is similar to that of T. rex ( Molnar 1991: fig. 5). The bone is not preserved in Ta. bataar ZPALMgD−I/4, partial in ZPALMgD−I/3, but is well preserved in GIN 107/1. In the lat − ter, the anterior rhomboid plate, ca. 20 cm long and almost 10 cm wide, is distorted and has been displaced towards the right side of the skull. This displacement shows that the palatal shelves of the maxillae meet anteriorly on the midline, and the vomer would normally overlap them ventrally. The anterior tip of the vomer definitely extends ventrally well onto the palatal surface of premaxillae, a situation not obvious in T. rex ( Molnar 1991) . The stem of the vomer in Ta. bataar lacks two pits (?dental fossae) observed in one specimen of T. rex ( Molnar 1991) . There is a long medial groove, representing the suture line of vomers, on its ventral side, extends over the rhomboid plate, but vanishes anteriorly. The posterior ends of the vomer are overlapped in the sagittal plane by the vomerine processes of the pterygoids. Immediately in front of this contact and just behind the internal nares, the vomers extend to the medial portion of the palatine.

Pterygoid ( Fig. 15 ).—The pterygoid is not preserved in ZPALMgD−I/4, but is seen in other specimens (GIN 107/1, GIN 107/2, GIN 100/70, PIN 551−2, ZPALMgD−I/3). The general form of the pterygoid in Ta. bataar is similar to that of T. rex described by Molnar (1991). It consists of a flat, horizontal palatal plate and a vertical, plate−like quadrate process. From the posterior part of the palatal plate a short, blunt, posterior process protrudes to contact the basisphenoid and a triangular lateral process to contact the ectopterygoid, while the medial margin of the palatal plate extends anteriorly to form a vomerine process. The vomerine process is thin, gently curved and its proximal part is inclined anteromedially, but the distal part is oriented anteriorly. Thus the gap between the vomerine processes of the contralateral pterygoids narrows anteriorly (though they do not meet). The process starts as a ridge on the proximal part, but flattens laterally and lies in the same plane as the palatal plate.

The posterior process of the pterygoid is short and inclined slightly mediodorsally, to meet the basipterygoid processes of basisphenoids. The ventral side of the palatal plate is flatter than illustrated by Maleev (1974) and, thus it is very similar to that of T. rex BHI−3033 ( Fig. 15B 2 , B 3 ). There are, however, differences in their shape with the medial margin in Ta. bataar being more sigmoidal and gently curved laterally in the anterior part. As in T. rex , the lateral margin is concave. A marked difference between the pterygoids of Ta. bataar and T. rex is the shape of the anterior edge of the palatal plate. In T. rex it is an oblique line, inclined slightly anteromedially−posterolaterally, joining the vomerine process at an angle of about 130 degrees ( Molnar 1991: fig. 6). In Ta. bataar the edge is curved, anteriorly convex. Thus it recedes medioposteriorly and meets the vomerine process at an angle of about 50–70 degrees to the long axis of the skull, so the medial part of anterior edge of the palatal plate of the pterygoid does not contact the palatine. A triangular pterygopalatine fenestra opens between the pterygoid and palatine, bordered posteriorly by the medial part of the palatal plate, medially by the lateral side of the vomerine process and anterolaterally by the palatine.

The vertical, quadrate process of the pterygoid is compressed at the base and then very wide dorsally. It is inclined posterolaterally, so that it is parallel to the quadrate. It broadly underlaps the anteromedial part of the quadrate. The posterior margin of that process is less curved than in T. rex as reconstructed by Molnar (1991).

Epipterygoid ( Fig. 16 ).— Maleev (1974) did not mention the epipterygoid, though it is often preserved in Ta. bataar where it articulates with the quadrate process of the pterygoid along a planar joint. The bone is preserved in ZPALMgD−I/4 and also seen in ZPALMgD−I/3 and GIN 100/70. The epipterygoid is thin and forms a vertically elongated triangle with a rod−like upper part. In Ta. bataar the basal part of the epipterygoid is flat, with slightly concave lower margin, and it overlaps the dorsal part of the vertical process of pterygoid, just anterior to the area overlapped by the quadrate. The epipterygoid narrows dorsally. The anterior edge of the rod−like process is slightly bent posterodorsally where it forms a ridge on the distal, swollen part. This tapers gently to form a tip oriented dorsally and slightly posterolaterally.

Molnar (1991) suspected that in T. rex the distal (upper) end of the epipterygoid contacted the laterosphenoid. In Ta. bataar there is no evidence for such a contact. If the quadrate and pterygoid are properly oriented in GIN 100/70, and in ZPALMgD−I/3, then the upper end of the articulated epipterygoid points slightly away from the braincase. Only a rotation of quadrate and pterygoids to make them incline more mediodorsally would allow a contact between the epipterygoid and the laterosphenoid.

Quadrate ( Fig. 9 ).—The quadrate is broken in ZPAL MgD−I/4, but the dorsal part of it contacting the squamosal and the articulation surface for the articular is well preserved. The middle part of the quadrate is separated from the quadratojugal by the large paraquadrate foramen. The upper contact surface between the quadrate and quadratojugal is almost vertical and lies in the parasagittal plane, while the lower is oblique, as the quadrate bulges medially in its ventral part, forming a large condyle. A slightly widened, concave posteroventral end of the quadratojugal overlies the lateral surface of the condylar part of the quadrate.

The condyle itself is massive, and its two convex elliptical articular surfaces are separated by an oblique groove, as in other large theropods ( A. fragilis , T. rex ). The dorsal end of the quadrate has a saddle−like articular contact surface for the squamosal as in T. rex ( Molnar 1991) . The quadrate extends anteriorly into a deep, thin, flat pterygoid process, which distally bends slightly medially, and contacts the quadrate process of the pterygoid. The proximal part of the pterygoid process of the quadrate bears two concavities medially. These are separated by a rounded ridge oriented anterodorsally and merging into the flat medial surface of the anterior process of quadrate. Above the condyle there is a large, dorsoventrally elongated foramen in the anteromedial wall of the bone, leading to the internal sinus.

In lateral view, the bone is hardly visible, being hidden behind the quadratojugal. Only the posterior end of the condyle can be seen.

Braincase

Frontal ( Fig. 17 , Table 1).—The frontals are surrounded anteriorly by the nasals and prefrontals, laterally by the lacrimals and postorbitals, and posteriorly by the parietals. Their ventral surface forms the roof of the brain cavity (the telencephalic part), and joins with the parasphenoid ventrally, surrounding the brain from below. The smooth dorsal surface of the frontals is flat between the lacrimals, but further back it slopes down, along a semicircular anterior border of the upper temporal fenestra. The fenestrae are separated by the sagittal crest. Its anterior part is formed by the frontals, and the posterior part by the parietals. The interfrontal suture and sutures with parietals and postorbitals are indistinct and hardly traceable on the skull surface. Only the anterior part of the interfrontal suture is clearly visible, especially in younger individuals. The frontal is separated from the orbit by the lacrimal−postorbital contact. The sutural contact of the postorbital/frontal consists of a complex pattern of ridges and grooves common to both Ta. bataar and T. rex . In Ta. bataar two ridges divide the groove for the posterior part of the lacrimal, while this groove is deep and smooth in T. rex . This difference is also reflected in the shape of the posterior apex of the lacrimal (see lacrimal description). The contribution of the frontals to the skull roof and the shape of the frontal in dorsal view are both size related traits, and newly collected specimens of Ta. bataar show the changing trend (Currie 2003).

HURUM AND SABATH—SKULLS OF TARBOSAURUS AND TYRANNOSAURUS COMPARED 181

5 cm

Parietal ( Fig. 17 , Table 2).—Both parietals are fused together along the sagittal crest, which merges posteriorly with the transverse nuchal crest. The profile of the sagittal crest is concave, with the lowest point at the level of maximum lateral constriction of parietals when seen in dorsal view. The posterior part of the sagittal crest rises more steeply than the anterior part. The crest is thin, sharp, and its sides are almost parallel. They slope steeply downwards, especially in the isthmus between the upper temporal fenestrae. The nuchal crest is rather massive, higher than the sagittal crest and can be divided into two symmetrical alae, separated by a medial groove. Each is laterally expanded and has rounded edges; the dorsal edge is thick and convex in posterior aspect, while the lateral is thinner and concave. Along the dorsal edge the surface is rugose and the pattern extends onto the external and posterior side of the edge. The posterior wall of the ala is slightly concave. The size and shape of the nuchal crest depends on the age of the individual. Thus in a juvenile specimen of Ta. bataar , the alae in dorsal view are slightly recessed posteriorly (GIN 100/70), while in adults (e.g., GIN 107/2) they lie exactly in a transverse plane, as they do in T. rex and Nanotyrannus (in D. torosus and G. libratus they are inclined forward). In GIN 107/2, which probably represents a fully adult individual, they are not only thicker and more rugose, but also relatively smaller with regard to the whole occipital part of the skull. This is also true in large specimens of T. rex (e.g., BHI−3033). GIN 100/65 has two pronounced ridges on the posterodorsal side of the parietals. The ridges form a V−shape with its apex facing forward, and extending between the posterior 1/4 part of the sagittal crest and the upper edge of each nuchal crest. In other specimens (right side of GIN 100/2) such structures are hardly visible or absent.

The parietals occur evenly anterolaterally to meet the frontals along the transverse flexure marking the anterior part of the upper temporal fossae and where the parietals form their concave anterior walls. Maleev (1974) described the parietals as fitting into incisions in the squamosal, but this is not the case and they are barely in contact with the squamosal.

Ethmoid complex ( Fig. 17 ).—The ethmoid extends along its midline of the skull roof, below the frontals. It reaches the parasphenoid posteroventrally and is partly overlapped by the nasals anterodorsally. The ethmoid bone of Ta. bataar ZPALMgD−I/4 fits the description and figure of T. rex given by Osborn (1912: fig. 8). It is not preserved in BHI−3033. The ventral side of the bone is transversely concave and bears a sagittal septum in its anterior part. The ethmoid is wider and shallower anteriorly. The bone is also visible in front of the braincase in ZPALMgD−I/3. No contacts with lacrimals have been observed.

Exoccipital−opisthotic ( Figs. 17 , 18 ).—As in all theropods, these bones are competely fused in tyrannosaurids. The exoccipital−opisthotics lie dorsolaterally from the basioccipital. The major part of each exoccipital−opisthotic is a plate extending laterally from the foramen magnum and bordered dorsally by the supraoccipital, parietals and squamosals. The plate is inclined posterolaterally. In T. rex , it contains an internal sinus occupying a dorsolateral portion of the exoccipital−opisthotic ( Russell 1970; Molnar 1991). The exoccipital formed the posterior part of the braincase and is perforated by the foramina for cranial nerves IX–XII ( Maleev 1974: fig. 12).

Maleev (1974) described separate “exoccipitalia” and “paroccipitalia, or opisthotica”, but did not mention that they are fused. The lateral parts of the exoccipital−opisthotic form large, winglike paroccipital processses, extending posteroventrally.

Basioccipital ( Fig. 17 ).—The dorsal part of the basioccipital forms most of the occipital condyle (except most of its dorsal surface). In juveniles, the condyle is subspherical in shape, but in adults becomes reniform with a flattened dorsal part. The foramen magnum above the condyle is pear−shaped, elongated dorsally in ZPALMgD−I/3, and rounded. It is proportionally smaller in larger specimens.

The ventral part of the basioccipital forms a rectangular plate descending on the posterior surface of the basisphenoid. The descending part of the basioccipital is slightly concave transversely. Ventrally, the descending part of the basioccipital broadens and forks into two basitubera. Dorsolaterally, it is not possible to see the suture to the exoccipital−opisthotics.

Supraoccipital ( Fig. 17 ).—The supraoccipital is situated dorsal to the foramen magnum. The bone forms a triangle with its apex oriented ventrally, and an incision in the middle of the dorsal, thickened edge. Ventrolaterally, it fuses with the exoccipitals−opisthotics. Dorsally, it has a deep groove for the medioventral part of the parietal. The posterior surface of the supraoccipital bears a vertical medial ridge in its dorsal part. In Ta. bataar ZPALMgD−I/4 the transverse width of the supraoccipital is less than the transverse width of the occipital condyle, while it is wider in T. rex BHI−3033.

Basisphenoid ( Fig. 17 ).—The basisphenoid extends as a subtriangular plate in the transverse plane. Its posterodorsal edge is sutured to the ventral edge of the paroccipital processes and ventrolateral edges form the basipterygoid processes. The processes have been broken in ZPALMgD−I/4 and MgD−I/3, revealing internal sinuses. The fractured basipterygoid process shows several oval sinuses separated by thin septa oriented anteromedially−posterolaterally. The presence of sinus chambers in the basisphenoid has been noted in T. rex by Osborn (1912) and Molnar (1991), as well as in G. libratus by Russell (1970), who presented a diagrammatic reconstruction of the extent of those sinuses. In Russell’s reconstruction ( Russell 1970: fig. 4), only a large, single sinus is visible in the ventral part of the basisphenoid, while several are present in Ta. bataar .

Sclerotic ring.—The sclerotic ring is only preserved in a subadult specimen with the skull length of about 0.8 metre, GIN 100/70. Here the sclerotic ring has an external diameter of about 65 mm and internal diameter of 30 mm. It consists of 15 plates. The sclerotic ring is still relatively large in this juvenile individual, and the orbit is almost oval. The “keyhole shape” of the orbit in adults defines the space for the eye at the upper end. The eye in juveniles would also occupy proportionally more of the orbit.

Mandible

The mandibles of Ta. bataar and T. rex were recently described by Hurum and Currie (2000). Here we repeat some of their observations and add a more thorough description of each bone. In Ta. bataar , as in all tyrannosaurids, the posterior part of the mandible is deep and connected with the dentary by an intramandibular joint. The posterior part deepens progressively in ontogeny. The posterior end is transverse, without a retroarticular process, though in the largest specimens a stout posteromedial process may develop on the articular. There is a small anterior surangular foramen. The mandibular fenestra is also small and becomes dorsoventrally compressed in ontogeny.

Dentary ( Fig. 19 , Table 3).—The ventral margin of the dentary is almost straight, slightly concave. The symphyseal margin ends below the third tooth and rises anteriorly and obliquely to reach the anterior end of the tooth row. It is slightly convex in lateral view and its transition into the ventral margin of the dentary varies from gentle curvature (as in ZPALMgD−I/4), to a rather sharp flexure (like in PIN 551−3, or GIN 107/1; where a rugosity exists at that point on the left dentary). A rugose zone extends on the mesial margin of symphyseal surface. In lateral view, the dentary of Ta. bataar (ZPALMgD−I/4) is more pointed anteriorly than in T. rex (BHI−3033), but this might be an allometric difference.

The dentary bears a row of circular pits parallel to the tooth row along the labial margin, some 2–3 cm below the teeth, and spaced each 1–2 cm (slightly more than one per tooth) in the posterior part and more frequent in the anterior end. Other, less numerous pits open below at the external surface of the dentary. On the lingual side at the level of tooth 2 to 12, it bears a gently down−curved Meckel’s groove at 1/3 of its height from its ventral edge. At its anterior end this groove is the same as in T. rex (BHI−3033), but its posterior part is moved dorsally. The Meckel’s groove runs below, and parallel to, a more prominent groove, marking the lower margin of the supradentary. The dentary is rather stout and massive in its anterior part, bearing larger teeth. The posterior part is rather thin, though much deeper than the anterior one. It is strengthened by two bars of bone, forking at the level of 10th to 11th teeth. Here is the apex of a triangular fossa, separating dorsal and ventral bars on the labial side of dentary (the anterior end of Meckelian fossa), from which a groove continues anteriorly. The triangular Meckelian fossa between the bars is overlain by the splenial, which is only partially preserved in ZPAL MgD−I/4 and rarely preserved in situ.

The posterodorsal part of the dentary has a long medial process and a shorter lateral process separated by a deep pocket. The dorsolateral process (the intramandibular process of the dentary, Currie and Zhao 1993), fits into a slot on the medial side of the surangular ( Hurum and Currie 2000).

The thin, ventroposterior margin of the dentary is often broken away. In better preserved specimens, like ZPAL MgD−I/4, it runs obliquely posteroventrally from behind the end of mandibular tooth row and ends on the ventral margin of the mandible, where the lower jaw is deepest. In Ta. bataar , the posteroventral part of the dentary has an abrupt, square end, concave medially in transverse section where it meets the angular. Its posterior end articulates with a perfectly matched vertical ridge on the lateral surface of the angular. In T. rex and G. libratus , there is a smooth groove on the angular with no abutting ridge for the posteroventral end of the dentary ( Hurum and Currie 2000).

Supradentary/coronoid ( Figs. 19 , 20 ).—The supradentary is a flat bone, extending on the labial side of the dentary along the tooth row. It covers the interdental plates. The lateral surface of the bone bears shallow concavities, corresponding to the interdental plates ( Maleev 1974) and weak ridges that fit between the teeth ( Hurum and Currie 2000). As in T. rex , the supradentary of Ta. bataar extends backwards from the septum between the 2nd and 3rd teeth, to the end of the tooth row. Here it thins to fit between the last teeth bases, the splenial and prearticular, and is fused to the coronoid ( Hurum and Currie 2000). The tyrannosaurid supradentaries are bent ventrally as in Allosaurus ( Madsen 1976: pl. 9D, erroneously labelled “C” on the drawing), but much deeper dorsoventrally, so that they resemble a “stretched crescent”, with the ventral edge more curved than the dorsal edge, rather than a strip of bone.

The coronoid of Ta. bataar is a small, thin, flat, triangular bone, clearly visible on the lingual side of the right mandible of ZPALMgD−I/4 and GIN 107/1, fitting between the pre − articular and surangular. Its shape, size and location correspond strictly to that of T. rex as illustrated by Osborn (1912). The anterodorsal side of the triangle contacts the surangular, the large, flat medial surface is partially covered by the anterodorsal section of the prearticular (the sharp, elongated anterior apex of coronoid is squeezed between the two bones and bends slightly upwards). The posteroventral side is free and hangs over the Meckel’s groove and the surangular. Molnar (1991: 155) describes the coronoid as “lying at the anterodorsal angle of the Meckelian fossa”. This correctly describes the appearence of the coronoid in a fully articulated mandible, but it should be remembered, that the fossa extends much farther anteriorly and is covered by the prearticular and the splenial.

Splenial ( Figs. 19 , 20 ).—The splenial is only partially preserved in ZPALMgD−I/4. A complete splenial is present in the right mandible of GIN 107/1, in ZPALMgD−I/134 the main body of the right splenial is preserved, and only the dorsal part is missing in ZPALMgD−I/5. The splenial is a trian − gular bone, flattened so that it lies in a parasagittal plane, with the long base oriented almost parallel to the ventral margin of dentary. In T. rex the ventral margin is more inclined and the anterior tip of the splenial is placed higher, because of the more dorsal position of the posterior part of the Meckel’s groove on the dentary. The lingual surface of the splenial lies at the same level as the lingual surface of the anterior part of the dentary. The ventral edge of the splenial is thicker than the rest of the bone and in close contact with the ventral bar of bone in the posterior part of the dentary. The anterodorsal edge contacts the dorsal bar of the dentary. Thus, the splenial tightly covers the triangular fossa, and the anterior part of the Meckelian fossa in the dentary, except for the foramen, adjacent to the ventral margin of splenial. The foramen is oval (ellipsoid) with its long axis oriented anteroventrally−posterodorsally. Its anterior end almost reaches the ventral bar of the dentary at the level of 11th tooth, while the posterior lies at the level of the last alveolus (and the posterior end of supradentary). The width of that foramen is about 2/3 of its length, and is roughly equal to the distance separating it from the anterodorsal margin of splenial. The posterodorsal edge of splenial is deeply convex, contacts the prearticular along its upper part and has a small groove for the prearticular. The dorsal apex is blunt. The posterior bifurcated tip of the posteroventral process meets the angular. The posteroventral process of ZPAL MgD−I/4 has a foramen directed dorsoventrally which is not seen in any other specimens. The anterodorsal side of the triangular splenial has a marked lip, and in lateral view this lip has a groove that fits the supradentary. The shape of the lip differs in different tyrannosaurids, the two extremes being the gradual, not very pronounced lip of Tyrannosaurus and the abrupt lip of Tarbosaurus ( Fig. 20A 2 , B).

Maleev (1974: fig. 19) illustrated the splenial of Ta. bataar , as the right side splenial of PIN 551− 2 in lingual view. It is however the lingual view of a left splenial. In lateral view a longitudinal ridge along the ventral margin would be visible; the ridge marks the flexure between the flat surface of bone, and the obliquely sharpening “blade” of the contact surface for the ventral bar of the dentary, as can be seen in isolated splenials of ZPALMgD−I/34. The facet is widest in the middle, and thins toward both ends, as the ridge that borders it is curved. Moreover, the anterior margin of the splenial illustrated by Maleev is uneven, because of fractures in the dorsal part and the tip of the anterior process is broken. The ventral margin of the bone below the splenial foramen is also incomplete, giving the impression of a wide gap, while the foramen, though much larger than in Allosaurus ( Madsen 1976: pl. 9), does not break the ventral margin of bone. The longitudinal opening in the posterior process shown by Maleev seems to exaggerate the concavity of the splenial.

Angular ( Fig. 19 ).—The angular in Ta. bataar consists of a posterior flat, wing−like plate, flaring posterodorsally in the parasagittal plane and of an anterior finger−like stem, oriented dorsoanteriorly. The posterior plate is slightly concave medially and convex ventrolaterally. In lateral aspect, it forms the posteroventral margin of the mandible and is bordered dorsally by the surangular and anteriorly by the dentary. In medial aspect, most of the posterior plate of the angular is hidden behind the prearticular. Only a small triangular part of the angular is visible near the flexure of the ventral margin of the mandible. The angular in medial aspect is bordered by the prearticular dorsally and by the splenial anteriorly. The stem fits between the prearticular and the splenial which covers its distal part medioanteriorly. In its proximal part the lateral surface fits against the medial side of the posteroventral corner of the dentary. The vertical ridge on the lateral surface of the angular for the posteroventral end of the dentary is only seen in Ta. bataar . In T. rex and G. libratus , there is a smooth groove on the angular. In both Ta. bataar and T. rex , the anteromedial groove for the posteroventral part of the splenial is deep ( Hurum and Currie 2000). A small groove dorsoanterior to the main splenial groove is only found in Ta. bataar and indicates the different shape of the medial side of the posteroventral part of the splenial.

The medial side of the angular in both T. rex and Ta. bataar contains two ventral ridges. One starts at the anterior end of the stem, separates the prearticular and splenial grooves and continues posteriorly as the ventral margin of the angular. The other is shorter and placed dorsal to the posterior part of the first ridge. Both ridges have corresponding grooves on the prearticular. The groove in the prearticular for the long ridge is smooth in T. rex , while in Ta. bataar it contains two notches into which grooves on the long ridge interlock.

Molnar (1991) described the angular of T. rex as being “much like that of Allosaurus fragilis ”. The main difference between the tyrannosaurid and allosaur angular is the shape of its anterior part. In Allosaurus ( Madsen 1976: pl. 7D, E) the anterior process of articular is an almost flat, blade−like triangle. In tyrannosaurids it forms a thin, fingerlike stem with a ridge along the medial side (Maleev 1976: fig. 17; Molnar 1991: pl. 15).

Surangular ( Fig. 19 ).—The surangular is a large, flat bone forming most of the posterior part of the mandible in lateral aspect. It consists mostly of a thin plate oriented parasagittally. Its height is the reason why the mandible of tyrannosaurids is much deeper than in other theropods. It narrows anteriorly and posteriorly. The bone is slightly convex dorsolaterally, and the upper margin is bent medially to form a thick medial flange which dorsally borders the large Meckelian fossa on the labial side of the mandible. Another, shorter ridge runs on the lateral side in the posterior part of the bone between the dorsal margin and the surangular fenestra, close to the jaw articulation. The surangular fenestra perforating the bone near its posterior end, is elliptical and fully visible in lateral aspect. Thus it is not hidden by the hanging flange of the lateral ridge of the surangular as in T. rex ( Osborn 1912: fig. 1), but less in BHI−3033. In tyrannosaurids the surangular is overlapped anteriorly by the dentary along an oblique (dorsoanterior−ventroposterior) line. The opposite was reconstructed by Carr (1999: figs. 5, 6).

The surangular has two anterodorsal processes, a large medial, and a smaller lateral process. The long medial process fits deeply into the pocket between the pair of posterodorsal processes of the dentary, thereby restricting the mediolateral movement between the two bones ( Hurum and Currie 2000). The surangular is overlapped by the angular ventrolaterally.

Posteriorly the posterior, thickest part of surangular interfingers with the articular and medially it meets the prearticular, forming the anterior part of the mandibular articular surface. It is possible to see the sutures, but impossible to prepare them apart in MgD−I/4. In T. rex (BHI−3033) the bones are less fused and it is possible to disarticulate them.

Prearticular ( Fig. 19 ).—The prearticular consists of the posteroventral body and anterodorsally directed ramus, with approximately perpendicular axes, both lying in a parasagittal plane. The body occupies the posteroventral margin of the mandible. The body of the bone stretches obliquely from the area of the mandibular joint, where it partially fuses with the articular, along the ventral margin of the surangular. The bone is located medioventrally to both angular and surangular. The body of the bone is bar−like, oval in cross−section and thinnest in the mid−length. A low, blunt ridge runs along the body of the prearticular, on the lateral side. It begins on the dorsal side at the posterior end of the bone and forms a posteroventral rim of the Meckelian fossa. The ridge then continues anteroventrally towards the medial side, and near the lowest point of the bone on the medioventral side, its shape continues further on the ventral flexure of the angular.

The anterodorsal ramus is a mediolaterally flattened sheet of bone, ascending along the posterior end of splenial, covering the Meckelian fossa posteriorly to the splenial. The ramus is of even width in its proximal and medial part, but thins distally, and terminates dorsally near the upper edge of the mandible in contact with the coronoid. The posterodorsal part of the splenial covers the anterodorsal margin of the prearticular ( Hurum and Currie 2000).

When the angular is removed, the body of the bone is covered by six slightly anterodorsally directed ridges and five grooves in lateral view. The most ventral ridge is the ventral guide for the groove for the long ridge observed on the angular. Anterior to the ridge is a notch that fits into the angular. Dorsal to the first ridge on the prearticular is the second ridge that forms the ventral border for the groove for the second, shorter ridge on the angular. A third short, but very prominent ridge on the prearticular marks the dorsal limit for the posterior part of the short ridge on the angular. There are three long, weaker ridges dorsal to the three prominent ridges that do not have any contact with other bones. The pattern of the three main ridges is the same in T. rex , except for the notch in front of the first ridge that is not present. The three dorsal, weaker ridges are not found in BHI−3033.

Articular ( Fig. 19 ).—The articular is a short, stout bone, forming the posteriormost part of the mandible, including the transversely expanded facet of the mandibular joint. The glenoid fossa consists of two concave surfaces, separated by an oblique ridge. The articular is strongly fused with the prearticular anteromedially. Anterolaterally, the articular contacts the surangular. The posterior end is expanded into an oval (semicircular) shallowly concave surface, facing slightly medially.

The tyrannosaurids lack the retroarticular process. This is also the case of Ta. bataar , though in the large and probably gerontic specimen GIN 107/2, there are prominent ventromedially bent processes, protruding posteriorly from both articulars. This feature is however unique among the studied specimens and may be considered an individual variation, related to age or some pathology. Normally, only a low ridge or short medial process is observable on the medial edge of the posterior surface of the articular.

Dentition

The terms tooth counts, tooth positions, and numbers of alveoli are synonymized in the following description. Each premaxilla of Ta. bataar bears four teeth. The teeth are smaller than those of the maxilla and more slender. Each premaxillary tooth is compressed so that the longer axes of their D−shaped cross sections are perpendicular to the tooth row (the extrapolated long axes of the teeth cross sections would meet near the anterior end of the vomer).

The maxilla bears 12–13 teeth in Ta. bataar , 12 in ZPAL MgD−I/4. The tooth count may differ in both maxillae of the same individual (e.g., in GIN 107/1 there are 12 teeth in the left maxilla and 13 in the right). Such differences in tooth number are independent of growth stage of the individuals, as noted by Russell (1970) for Albertosaurus and Daspletosaurus . In T. rex (BHI−3033) there are 11 teeth in the maxilla (11–12 in others).

In Ta. bataar the dentary bears 15 (rarely 14) teeth. The base of the mandibular dental row, occupying almost the whole upper edge of dentary, is concave. The anterior part is almost straight, while the posterior rises markedly. The teeth, however, are smaller in the posteriormost part of the row, so the tips of fully grown mandibular teeth lie on an almost straight line, sloping down at both ends (the first and last two teeth do not reach the medium level). The longest teeth are at the positions 3 to 9. There are 12–14 dentary teeth in T. rex .

In Tarbosaurus bataar (ZPALMgD−I/4) the teeth have up to 85 mm long crowns. The serration count is 3 serrations per mm on the premaxillary teeth, 2 serrations per mm on the maxillary, and 2 serrations per mm on the dentary teeth. In Tyrannosaurus rex (BHI−3033) the teeth have up to 100 mm long crowns. The serration count is 3 serrations per mm on the premaxillary teeth, 2 serrations per mm on the maxillary, and 2 mm on the dentary teeth.

Figures

Fig. 1. Skull of Tarbosaurus bataar ZPAL MgD−I/4 in lateral (A) and dorsal (B) views.

Fig. 2. A. Generalized skulls of Tarbosaurus bataar in lateral (A1) and dorsal (A2) views. B. Tyrannosaurus rex in lateral (B1) and dorsal (B2) views compared (not to scale). Light grey, lacrimal; dark grey, nasal.

Fig. 3. Right premaxilla of Tarbosaurus bataar ZPALMgD−I/4 mirrored to be comparable to other bones form the left side of the skull in lateral (A, B) and medial (C) views.

Fig. 4. Left maxilla of Tarbosaurus bataar ZPAL MgD−I/4 in lateral (A, B) and medial (C, D) views.

Fig. 11. Tyrannosaurus rex BHI−3033. A. Maxilla in medial view. B. Postorbital in lateral view. C. Quadratojugal in lateral (C1) and medial (C2) views.

Fig. 5. A. Nasal of Tarbosaurus bataar ZPALMgD−I/4 in lateral (A 1) and ventral (A2, A3) views. B. Ventral view of the nasal of Tyrannosaurus rex BHI−3033.

Fig. 6. Left lacrimal of Tarbosaurus bataar ZPAL MgD−I/4in lateral (A, B) and medial (C, D) views.

Fig. 7. Tyrannosaurus rex BHI−3033. A. Left lacrimal in lateral (A1) and medial(A2) views. B. Left jugal in lateral (B1) and medial (B2) views.

Fig. 8. Left prefrontal, frontal, postorbital and parietal of Tarbosaurus bataar ZPAL MgD−I/4 in lateral (A, B) and medial (C, D) views.

Fig. 9. Left squamosal, quadrate, quadratojugal, postorbital of Tarbosaurus bataar ZPAL MgD−I/4 in lateral (A, B) and medial (C, D) views.

Fig. 10. A. Right squamosal of Tarbosaurus bataar ZPALMgD−I/4 in dorsal (A 1), ventral (A2, A3), and lateral (A4) views. B. Mirrored left squamosal of Tyrannosaurus rex BHI−3033 in ventral view. A1–A3 and B, anterior is up.

Fig. 12. Left jugal of Tarbosaurus bataar ZPAL MgD−I/4 in lateral (A, B) and medial (C, D) views.

Fig. 14. Left ectopterygoid of Tarbosaurus bataar ZPAL MgD−I/4 in dorsal (A, B) and ventral (C, D) views.

Fig. 15. A. Ventral view of the skull of Tarbosaurus bataar GIN 107/1 (A1) and explanatory drawing of the same (A2). B. Tyrannosaurus rex BHI−3033; lateral view of vomer (B1), lateral view of pterygoid (B2), dorsal view of pterygoid (B3). Not to scale.

Fig. 16. Left epipterygoid of Tarbosaurus bataar ZPALMgD−I/4 in lateral (A) and medial (B) views.

Fig. 17. Braincase of Tarbosaurus bataar ZPALMgD−I/4. A, B. Lateral view, left side.C, D. Lateral view, right side.E. Partly reconstructed occipital view.

Fig. 18. Left exoccipital−opisthotic of Tarbosaurus bataar ZPAL MgD−I/4 in occipital (A) and medial (B) views.

Fig. 19. Mandible of Tarbosaurus bataar ZPALMgD−I/4. A. Lateral view, B. Medial view. C. Medial view with prearticular and splenial removed. Based on Hurum and Currie (2000), except for a new reconstruction of the splenial.

Fig. 20. A. Reconstructed splenial of Tarbosaurus bataar (based on both ZPALMgD−I/34 and MgD−I/4) in medial (lingual) (A 1) and lateral (A2) views. B. Lateral view of the left splenial of Tyrannosaurus rex BHI−3033. C. Medial view of the supradentary of Tarbosaurus bataar ZPALMgD−I/4. D. Medial view of the supradentary of Tyrannosaurus rex BHI−3033.