Bagaraatan ostromi Osmólska, 1996

Słowiak, Justyna, Brusatte, Stephen L. & Szczygielski, Tomasz, 2024, Reassessment of the enigmatic Late Cretaceous theropod dinosaur, Bagaraatan ostromi, Zoological Journal of the Linnean Society 202 (3), pp. 1-39 : 3-23

Bagaraatan ostromi Osmólska, 1996

Holotype

ZPAL MgD-I/108: incomplete right mandible (dentary, angular, surangular, prearticular, and articular), left and right incomplete ilia, nearly complete left pubis, partial right pubis, proximal end of left ischium, left pedal phalanx IV-1, two cervical vertebrae, 25 caudal vertebrae, and two haemal arches.

Note on diagnostic characters

We provide a full diagnosis below, because we must first describe all the bones of the Bagaraatan series before untangling which different taxa they belong to. However, we note here that this holotype individual can be referred to the Tyrannosauridae because of eight features: (i) presence of the dentary ‘chin’; (ii) transition between the anterior and ventral edges of the dentary placed below the fourth alveolus; (iii) dorsoventrally narrow Meckelian groove deeply inset into the medial side of the dentary; (iv) extremely reduced retroarticular process of the articular; (v) prominent surangular shelf; (vi) convex anterior margin of the pubis; (vii) cervical vertebrae with a hypapophysis; and (viii) thick posterior centrodiapophyseal laminae.

Locality and age

Northern Sayr, Nemegt, Ömnögov, Mongolia; Nemegt Formation.

Description

Mandible: Only two fragments of the left mandible are preserved: the anterior part of the dentary with poorly preserved supradentary, and a piece that includes articulated posterior parts of the surangular, angular, and prearticular, and the incomplete articular.

Dentary: The dentary is slender in general outline and shows an anterior expansion in comparison to the midregion (28 mm deep at the third vs. 25.5 mm deep at the ninth alveolus; Figs 1 , 2 ), which is D-shaped in cross-section. Also, the dentary is labiolingually expanded anteriorly: the anterior end is wide labiolingually (measuring 16.3 mm) in comparison to the posterior part of the preserved dentary (12.1 mm width; Fig. 1C ). The anterior tip of the dentary is missing; however, clearly it was positioned higher than the level of the tooth row (its preserved base is already dorsal relative to the rest of the bone; Fig. 1A, B ). The anteroventral margin is relatively straight and strongly inclined posteroventrally, creating an angle of 135° with the ventral margin of the dentary. This creates a distinct ‘chin’ (i.e. slightly protruding region at the place where the anteroventral and ventral margin meet) between the anterior and ventral surfaces, which is positioned underneath the third and fourth alveoli. The ‘chin’ underneath the fourth alveoli is commonly seen in juvenile tyrannosaurines ( Carr 2020: character 117) and Alioramus altai ( Brusatte et al. 2012) , but in adults the ‘chin’ is placed ahead of the fourth alveolus, as Ta. bataar ( Fig. 3 ), and Ty. rex ( Brusatte and Carr 2016: character 171). A low angle of the symphyseal region relative to the ventral margin is found in juvenile tyrannosaurids with narrow jaws, contrasting with the steeper rostroventral margin of deep-jawed adult individuals, where the ‘chin’ migrates further anteriorly ( Figs 2 , 3 ; Carr and Williamson 2004, Carr 2020).

The dorsal margin of the dentary is strongly concave in lateral view, even in the anterior part, which is a feature of derived tyrannosaurids ( Brusatte and Carr 2016: character 177) that is also seen in juveniles and subadults ( Currie and Dong 2001, Tsuihiji et al. 2011, Brusatte et al. 2012, Funston et al. 2020b). The ventral margin of the dentary is only very slightly convex ( Figs 1A–C , 2E, F ). The lateral surface is smooth; the neurovascular foramina pierce the bone along an anteroposterior sulcus (i.e. dentary groove; Figs 1A , 2E ) 7.8 mm below the tooth row. The foramina are more numerous in the anterior part of the dentary, close to the symphysis ( Figs 1F , 2B ). The dentary groove is an ontogenetically variable feature in tyrannosaurids, being sharp and deep in juveniles and shallow in mature individuals (Fig. 3; Brusatte et al. 2016, Carr 2020). On the ventral side of the dentary, a second row of foramina, parallel to the ventral margin, is present. Anteriorly, those foramina are larger and closely spaced; posteriorly, the foramina are smaller and widely spaced ( Figs 1A, F , 2B, E ).

The medial side of the dentary is smooth, with a deep and narrow groove that extends anteroposteriorly between the interdental plates and the rest of the dentary ( Figs 1B , 2F ). The interdental plates are poorly preserved, but their triangular shape is visible in medial view. The symphysis is elongated, aligned anterodorsally, and has a nearly smooth surface (bearing only minute, very subtle striations). A ‘chin’ is present, as in other tyrannosaurids ( Brusatte and Carr 2016: character 172), including small juveniles ( Funston et al. 2020b), with the exception of Q. sinensis ( Lu et al. 2014, Foster et al. 2021). The ventral margin of the symphysis ends below the fourth tooth alveolus, where a single anterior Meckelian foramen is present ( Figs 1B , 2F ). The position is similar to that in Ta. bataar (e.g. ZPAL MgDI/4 and ZPAL MgD-I/175; Fig. 3 ) and other tyrannosaurids ( Brusatte et al. 2010, Funston et al. 2020b), but in Alioramus altai the foramen is positioned further posteriorly, below the fifth tooth alveolus ( Brusatte et al. 2012). The anterior Meckelian foramen is located anterior to the anterior end of the Meckelian groove, which is shallower anteriorly and cuts more deeply into the dentary posteriorly. The deep and sharp inset of the Meckelian groove is a characteristic of tyrannosaurids and close relatives ( Brusatte and Carr 2016: character 178) and is seen in small juveniles of Ty. rex ( Carr 2020) and other juvenile tyrannosaurids ( Funston et al. 2020b). Anteriorly, the groove is positioned in the middle of the medial surface of the dentary, but posteriorly it is positioned in the upper third of the dorsoventral height of the dentary. The distance between the Meckelian groove and the tooth row also shortens posteriorly (from 13.25 mm anteriorly to 8 mm posteriorly). In dorsal view, the preserved part of the dentary is straight ( Figs 1C , 2A ), similar to Alioramus altai ( Brusatte et al. 2012) and juvenile Ta. bataar (Tsuihiji et al. 2011) .

The dentary shows 11 tooth alveoli. Nine dentary teeth are broken off, but nine complete tooth alveoli are preserved, along with most of a tiny mesial-most alveolus at the front of the jaw, and the anterior end of the 11th alveolus at the back. The preserved part of the first alveolus is exceptionally small in comparison to the other alveoli, whereas the second is larger than the first, but still smaller than the remaining teeth and with a circular outline ( Figs 1F , 2B ; Table 1). This indicates that the first two teeth in the jaw were smaller and more circular in cross-section than the remaining teeth, as is common in tyrannosauroids ( Brusatte and Carr 2016: character 175), including small juveniles ( Funston et al. 2020b). The alveoli posterior to the first two are elongated mesiodistally and have an eight-shaped outline in dorsal view ( Figs 1C , 2A ). The labiolingual width is the largest at the third alveolar position, and the anterior and posterior alveoli are narrower. The anteroposterior length of the alveoli decreases anteriorly, such that the 10th alveolus is the longest. These alveoli indicate that the associated teeth are ziphodont, with labiolingual widths <60% mesiodisal lengths, as is the case in most theropods and juvenile tyrannosaurids, but differing from the labiolingually widened incrassate teeth of large adult tyrannosaurids ( Brusatte and Carr 2016: character 201).

Supradentary: As correctly noted by Osmólska (1996), only a small, poorly preserved splinter of the supradentary is present in articulation, dorsal to the dentary and lingual to the interdental plates, at the level of the fifth to the seventh teeth ( Figs 1B , 2F ). Some uninformative, miniscule scraps of bone are also present posteriorly. As preserved, the supradentary appears to be dorsoventrally narrow, covering less than one-fifth of the mandible height.

Splenial: We could not confirm the presence of a triangular, slightly hooked anterodorsally anterior part of the splenial suggested by Osmólska (1996). The triangular element is most likely to be a cracked and inset ventral bar of the dentary.

Surangular: Only the posterior part of the left surangular is preserved ( Figs 1A–E , 4 ). The surangular is a generally thin, plate-like bone, which expands labiolingually at the dorsal margin. Lateroventrally, the surangular is covered by the flat and mediolaterally thin angular ( Figs 1A , 4A ). The angular ends very close to (only 4 mm below) the surangular foramen. In ventral view, the connection between the surangular, articular, and prearticular is visible. The ventromedial edge of the surangular contacts the prearticular. This contact is visible externally in the posterior part, but more anteriorly the surangular is partly obscured by the angular; it continues only as a narrow splinter along the posterior half of the preserved part of the angular. As preserved, the contacts between the bones in that area appear split as a result of compaction, hence their precise layout might be displaced, and thus it is possible that in vivo the surangular was either not exposed from under the angular or that the exposure was slightly larger but now is obscured and/or partly eroded. In any case, the deformation most probably was not substantial. The angular tightly covers the surangular, such that the margin between those bones is barely visible laterally but well marked ventrally.

The most conspicuous aspect of the surangular is the presence of two surangular foramina: one smaller (2.3 mm × 1.5 mm) and positioned anterodorsally, and the second larger (diameter: 5.63 mm × 3.68 mm) and placed posteroventrally to the first one ( Figs 1A , 4A ). Both are elongate, ovoid in shape rather than circular, with the long axes directed posterodorsally. The bone is extremely thin between those foramina ( Figs 1E , 4F ). This condition is different from most tyrannosaurids, where a single surangular foramen is enlarged, such that its dorsoventral depth is> 30% of the depth of the surangular ( Brusatte and Carr 2016: character 179). This is the case in Nemegt tyrannosaurids, like alioramins ( Brusatte et al. 2009, 2012, Lu et al. 2014), Ta. bataar (e.g. ZPAL MgD-I/4 and ZPAL MgD-I/31; Fig. 5 ), and the young juvenile Raptorex (Sereno et al. 2009) . However, in some other juvenile tyrannosaurids there is a single surangular foramen, but it is small ( Currie and Dong 2001, Tsuihiji et al. 2011), and it has been determined that the size of the foramen changes during the ontogeny of Ty. rex ( Carr 2020: character 126). Both surangular foramina in B. ostromi are located in a fossa below the lateral surangular shelf. There is no pneumatic pocket posterodorsal to the surangular foramen, whereas nearly all other tyrannosaurids have one ( Brusatte and Carr 2016: character 183), although it is absent in some specimens of Ta. bataar (e.g. ZPAL MgD-I/4 and ZPAL MgD-I/31; Fig. 5; and MPC-D 107/7, Tsuihiji et al. 2011). In B. ostromi, the two foramina are separated by a laterally convex, lateroposterodorsally inclined, dorsally thickening (up to ~ 5 mm), and gently posteroventrally bowed bar ( Figs 1A , 2A ). The bar buttresses the posterior part of the lateral surangular shelf.

There is a lateral surangular shelf above the foramina, close to the dorsal margin of the bone ( Figs 1A, E , 2A, B, F ). Its lateral protrusion is not as prominent as in Alioramus altai ( Brusatte et al. 2012) or Ta. bataar (e.g. ZPAL MgD-I/4, ZPAL MgD-I/5, and ZPAL MgD-I/31; Fig. 5 ), but the lateral protrusion of the surangular shelf is subtle, as in some juvenile tyrannosaurids ( Currie and Dong 2001, Tsuihiji et al. 2011, Foster et al. 2021). In lateral view, the shelf extends straight anteroposteriorly, paralleling the long axis of the mandible, as in tyrannosaurids, but differing from the anteroventral or anterodorsal orientation in most other theropods ( Brusatte and Carr 2016: character 182) The smooth surface of the adductor fossa dorsal to the shelf faces almost equally dorsally and laterally. This is similar to both species of Alioramus (Kurzanov 1976, Brusatte et al. 2012) and juvenile tyrannosaurids ( Currie and Dong 2001, Tsuihiji et al. 2011), but differs from the strongly laterally facing state in large adult tyrannosaurids ( Brusatte and Carr 2016: character 184). In older Ta. bataar , the fossa is immediately medial to the shelf, extends medioventrally, and forms a depression (more pronounced in smaller specimens), but more medially the adductor fossa curls up and faces strongly laterally (e.g. ZPAL MgD-I/31, ZPAL MgD-I/4, and ZPAL MgD-I/5; Fig. 5 ). The dorsally pointing posterior edge of the adductor fossa is more pronounced than in Alioramus altai ( Brusatte et al. 2012) . There is a triangular fossa on the lateral surface of the surangular shelf anteroventral to the glenoid, a distinguishing feature of derived tyrannosauroids ( Brusatte and Carr 2016: character 185). The glenoid on the surangular (lateral glenoid socket of Osmólska 1996) is a deep and anteroposteriorly narrow transverse concavity bound anteriorly and posteriorly by dorsally extended processes (the preglenoid process and conelike process, respectively; Figs 1 , 4 ). This is similar to Alioramus altai ( Brusatte et al. 2012) and juvenile Ta. bataar (Tsuihiji et al. 2011) ; in larger Ta. bataar the glenoid is anteroposteriorly wider (e.g. ZPAL MgD-I/4, ZPAL MgD-I/5, and ZPAL MgD-I/31; Fig. 5 ). Posteromedially to the glenoid fossa (lateral glenoid socket sensu Osmólska 1996), in dorsal view, a deep and narrow fossa is present (medial glenoid socket sensu Osmólska 1996). In Ta. bataar , the two glenoid fossae are not marked by the upraised lateral margin of the surangular. Two glenoid depressions are present in that species, but similar in depth and separated by a gradual elevation (e.g. ZPAL MgD-I/4). In B. ostromi , the medial glenoid is much deeper than the lateral glenoid. There is a fossa on the lateral surface of the surangular, ventral to the glenoid, as is seen in derived tyrannosauroids ( Brusatte and Carr 2016: character 186). This fossa is smooth, as in Alioramus altai ( Brusatte et al. 2012) , not rugose, as in Ta. bataar (ZPAL MgD-I/4, ZPAL MgD-I/5, and ZPAL MgD-I/31). Distal to the glenoid, behind the posterior dorsal (conelike) process, a second, groove-like concavity (cleft of Osmólska 1996) is present, bound posteriorly by a small but well-defined dorsal projection, which continues medially as a sharp, distinct ridge ( Figs 1C , 4A ). This feature occurs in both Alioramus altai and Ta. bataar .

The retroarticular process of the surangular is tiny, corresponding to the small corresponding process on the articular ( Fig. 1A, B ). This is a feature of tyrannosauroids ( Brusatte et al. 2014: character 76). This process is straight and slopes posteroventrally, similar to Alioramus altai ( Brusatte et al. 2012) and Q. sinensis ( Lu et al. 2014) . In Ta. bataar (ZPAL MgD-I/4 and ZPAL MgDI/5), it is oriented vertically. The medial hook process is nearly perpendicular to the prearticular axis of the surangular and constitutes almost 50% of the width of the surangular.

Angular: Only the left posterior part of the angular is preserved ( Figs 1D , 4A, C ). It is plate-like, laterally convex, securely sutured, and tightly covers the surangular. Its margins are marked in the lateral view by a shallow groove, historically marked with a pen, making exact observation difficult ( Figs 1A , 4A ). The dorsal margin of the posterior plate of the angular is convex below the anterior of the two surangular foramina and concave below the distal margin of the posterior foramen, where the dorsoventral height of the angular decreases posteriorly. The distance between the dorsal margin of the angular and the ventral margin of the posterior surangular foramen is short (4.2 mm). The posterior margin is convex, pointing slightly upwards, and the ventral margin is straight and contacts the surangular posteriorly and the prearticular anteriorly. The posterior tip of the angular is not complete. The preserved posterior end of the angular extends past the level of the posterior margin of the posterior surangular foramen.

Prearticular: The posterior process of the left prearticular is preserved and is tightly articulated with the articular posteriorly, the angular ventrally, and the surangular dorsally, laterally, and posteroventrally ( Figs 1B–E , 4C, D ). The posteromedial tip of the prearticular is broken off. The preserved part of the prearticular is medially concave in ventral view. The ventral margin between the prearticular, angular (anteriorly), and surangular (posteriorly) runs sigmoidally in ventral view, and only posteriorly does the margin between the bones curve medially (note that the bones are slightly split along the ventral surface of the mandible, but that does not seem to distort their general layout). The articular, surangular, and angular are tightly articulated with the prearticular. The prearticular is not fused to the surangular and articular, similar to juvenile Tarbosaurus ( Currie and Dong 2001, Tsuihiji et al. 2011) and Alioramus altai ( Brusatte et al. 2012) and in contrast to large Ta. bataar (ZPAL MgD-I/4 and ZPAL MgDI/5). The posteroventral margin of the prearticular is pointed downwards (similar to Alioramus altai ), whereas in Ta. bataar it is oriented posteriorly. The distal concave margin contacting the articular is shallower than in Ta. bataar .

Articular: The articular is almost complete, lacking only the ventromedial part. It is tightly articulated with the prearticular anteromedialy and contacts the surangular laterally. The posterior surface is smooth, gently concave, and elliptic, more than twice as tall as it is wide. The retroarticular process is extremely reduced ( Figs 1 , 4D, E ), as in all Tyrannosauroidea, but differing from the much larger processes in dromaeosaurids and other theropods ( Brusatte et al. 2014: character 76). The attachment site for the jaw muscles on the articular is mediolaterally narrower than the glenoid articular surface, and there is a very narrow nonarticular region between the glenoid and the muscle attachment. Both features are characteristic of most tyrannosauroids, but not other theropods (Rauhut et al. 2010, Brusatte and Carr 2016: characters 189 and 190).

Antarticular: We could not confirm the presence of a separate antarticular suggested by Osmólska (1996). As preserved, the structure in question is a cracked medial edge of the surangular.

Postcranial skeleton: Cervical vertebrae: Two incomplete amphiplatyan cervical vertebrae are preserved ( Fig. 6 ). They are similar in structure and size: the anteroposterior length of the anterior cervical centrum ( Fig. 6A–F ) measures 35.8 mm, and the posterior cervical centrum ( Fig. 6G–L ) measures 36.5 mm. The articular surfaces of the centrum are oval, slightly concave, and shallow dorsoventrally. The height to width ratio of the centra is 0.7 and 0.6 for the anterior and posterior cervical vertebra, respectively. The centra are concave laterally, and they thicken close to the parapophyses, which are oval in lateral view and directed laterally ( Fig. 6B, H ). On the lateral sides of the centra, pleurocoels (lateral pneumatic fossae) are present. Above the pleurocoels, the posterior centrodiapophyseal laminae are thick and laterally offset, and they demarcate a deep infradiapophyseal fossa anteriorly, as in all tyrannosaurids, but differing from the thinner laminae of more basal tyrannosauroids ( Brusatte and Carr 2016: character 213). Sutures between the centra and neural arches are open. Small, eroded hypapophyses on the anterior region of the ventral surface of the cervical vertebrae are present, as in tyrannosaurids and close relatives ( Brusatte and Carr 2016: character 214), including juveniles, such as the Alioramus altai holotype ( Brusatte et al. 2012).

The cervical vertebrae are similar to the mid- or posterior cervical vertebrae of the juvenile tyrannosaurid ‘ Shanshanosaurus huoyanshanensis ’, because both exhibit flat ventral surfaces of the centrum, which are also narrow-waisted, biconcave, and with a large and single pleurocoel on the lateral side ( Currie and Dong 2001).

Caudal vertebrae: Twenty-one caudal vertebrae were found in articulation ( Figs 7–14 ). Four distal caudal vertebrae and two haemal arches were also found ( Figs 14 , 15 ) but cannot be fitted to the articulated tail. The first preserved caudal is taller dorsoventrally than long ( Fig. 7A–F ; Table 2 ), whereas the second is roughly equal in height and length ( Fig. 7G–L ), and all successive centra are longer than tall ( Figs 8–14 ; Table 2 ). The transverse processes disappear starting from the 15th preserved caudal ( Fig. 12M –R). In Ta. bataar (ZPAL MgD-I/4, ZPAL MgD-I/175, and ZPAL MgD-I/177), the height of the centrum is similar to its length in the fifth caudal vertebra, and the transverse processes disappear starting from the 18th caudal vertebra. Thus, we estimate that the preserved articulated part of the tail ZPAL MgD-I/108 represents the fourth to 24th caudal vertebrae. Moreover, in the first preserved caudal, the transverse processes are oriented posteriorly ( Fig. 7A–F ), which is a typical condition of the proximal caudal vertebrae of tyrannosaurids.

The neural arches of the caudal vertebrae in ZPAL MgDI/108 are co-ossified with the centra in all bones, but the remnant of the suture is visible in the proximal centra, up to the 18th caudal ( Fig. 12M –R). This suture is also present in the proximal caudal vertebrae of other tyrannosaurids, including Ta. bataar , and also in some other theropods, such as ornithomimids (e.g. Gallimimus bullatus Osmólska et al., 1972 , ZPAL MgD-I/94).

The caudal centra are all amphicoelous; only the first preserved caudal of ZPAL MgD-I/108 is somewhat concave anteriorly and flat posteriorly ( Fig. 7A–F ). In both Ta. bataar (ZPAL MgD-I/4, ZPAL MgD-I/175, and ZPAL MgD-I/177) and Ty. rex ( Brochu 2003) , the caudal vertebrae are amphicoelous, and the first four centra are somewhat concave anteriorly. This supports the identification of the first preserved caudal of ZPAL MgD-I/108 as the fourth caudal vertebra (Fig. 7A–F). The lateral surfaces of the centra do not have any pleurocoels or other pneumatic features, and on the ventral surfaces there are no ridges ( Figs 7–14 ). The articular surfaces for the haemal arches are present at the posteroventral end of the centra; these are well visible and similar in shape to those in Ta. bataar (e.g. ZPAL MgDI/175).

The neural arches are generally incomplete. The robust and rectangular neural spines of the proximal caudal vertebrae lack their dorsal ends, but even as preserved they project beyond the level of the posterior limit of the respective centra, as in most other tyrannosaurids ( Brusatte and Carr 2016: character 229). The ontogenetic component to this character was noticed by Carr (2020). In Ty. rex , the spinous processes of the caudal vertebrae do not extend behind the level of the posterior edge of the centrum, as in juvenile Ta. bataar (Tsuihiji et al. 2011) and, apparently, the Raptorex holotype (Sereno et al. 2009). In adult Ty. rex , the spinous process of the caudal vertebrae extends posterior to the centrum ( Carr 2020), as in Ta. bataar (ZPAL MgD-I/3 and ZPAL MgD-I/175). The neural spines of ZPAL MgD-I/108 are inclined posteriorly along the tail, similar to Alioramus altai ( Brusatte et al. 2012) and Q. sinensis ( Lu et al. 2014) and in contrast to Ta. bataar, in which the neural spines project more vertically (ZPAL MgD-I/4, ZPAL MgD-I/175, and ZPAL MgD-I/177). Further distally, the neural spines become more strongly inclined posteriorly, and from the 16th caudal vertebra they become short dorsoventrally and elongated anteroposteriorly ( Figs 12–14 ). The dorsal expansion present on the posterodorsal end of the neural spine in other tyrannosaurids ( Brusatte et al. 2012) is not preserved in B. ostromi , and thus its presence cannot be confirmed.

The transverse processes are mostly incomplete in the caudal series of B. ostromi . Proximal caudal vertebrae have anteroposteriorly long and dorsoventrally thin, distally narrowing transverse processes. From the ninth caudal vertebra onwards, the transverse processes are still thin and flat, and directed laterally.Then, the 15th caudal vertebra shows reduced transverse processes, much shorter and narrow anteroposteriorly. The 16th and 17th caudal vertebrae have minute transverse processes, and the 18th and further caudal vertebrae lack the transverse processes ( Figs 12–14 ). On the anteroventral surface of each transverse process, where the process meets the prezygapophysis, there are two laminae that define a deep, triangular concavity. This is present in most other tyrannosaurids, including juvenile specimens such as the Alioramus altai holotype ( Brusatte et al. 2012), but absent in more basal tyrannosauroids and other theropods ( Brusatte and Carr 2016: character 231). A triangular depression was noticed in Alioramus altai ( Brusatte et al. 2012) at the region where the transverse process meets the neural spine, but in B. ostromi it is proportionally wider and shallower. In Ta. bataar , the depression is rather broad and shallow, regardless of the size of the animals (ZPAL MgD-I/3, ZPAL MgD-I/4, and ZPAL MgD-I/175); however, the depth and width of the depression depend on the preservation: in the caudal vertebrae of Ta. bataar ZPAL MgD-I/3, the depression is narrow and deep on the left side but shallow and wide on the right side. The depth and breadth of the fossa is best explained by taphonomic deformation, and thus its taxonomical value is limited.

The prezygapophyses of the proximal caudal vertebrae are positioned more vertically than in Ta. bataar (ZPAL MgD-I/3, ZPAL MgD-I/4, and ZPAL MgD-I/175) and Alioramus altai ( Brusatte et al. 2012). Further distally, the prezygapophyses point more anteriorly, and from the 17th caudal onwards they are longer and project even more anteriorly ( Figs 12–14 ). The surface and shape of the articular surfaces of the prezygapophyses is not visible owing to their tight articulation with the postzygapophyses or damage. The postzygapophyses are positioned behind the centrum, and their articular surfaces face lateroventrally, more laterally than in in Ta. bataar (ZPAL MgD-I/3, ZPAL MgD-I/4, and ZPAL MgD-I/175) and Alioramus altai ( Brusatte et al. 2012) .

Owing to the close articulation between the caudal vertebrae, the hypantrum between the prezygapophyses is not visible. The hyposphene between the postzygapophyses is large and rectangular in B. ostromi , similar to Ta. bataar (ZPAL MgD-I/3, ZPAL MgD-I/4, and ZPAL MgD-I/175) and in contrast to the delicate hyposphene found in Alioramus altai ( Brusatte et al. 2012) .

Ilia: The ilia are incomplete; the left and right ventral postacetabular processes, part of the left proximal preacetabular process, and apparently, two fragments of the dorsal edge of the left ilium blade are preserved ( Fig. 16 ). Osmólska (1996) mentioned (but did not illustrate) a thin bone fragment found some distance from the remainder of the pelvis, with an even natural dorsal edge and dense, perpendicular striations on one of the surfaces, which she interpreted as the dorsal edge of the ilium.The material catalogued under ZPAL MgD-I/108 includes two fragments fitting that description ( Fig. 16K–N ). Given the presence of other dinosaur species in the association and the lack of articulation with the remainder of the skeleton, their affinity to B. ostromi is uncertain, although possible.

Thebaseofthepreacetabularprocesswaspositionedabovethe pubic peduncle, as marked by the attachment site of the muscle iliofemoralis internus, the cuppedicus fossa ( Fig. 16A, B ), characteristic for tyrannosaurids and other tetanurans ( Hutchinson 2001, Carrano and Hutchinson 2002). Dorsally, the cuppedicus fossa is a wide and slightly concave area, which curls down laterally and forms the ventral margin of the preacetabular process. The dorsal margin of the preserved element of the preacetabular blade is crushed diagenetically. Above the ventral margin of the preacetabular blade, a depression is present.

Above the acetabulum, on the lateral surface of the right iliac blade ( Fig. 16G–J ), an eroded linear ridge is present ( Fig. 16I ). This structure is present in all tyrannosauroids, including the juvenile MPC-D 107/7 (Tsuihiji et al. 2011), but excluding R. kriegsteini and Q. sinensis ( Lu et al. 2014, Brusatte and Carr 2016: character 258). Possibly, the absence of this feature in the latter two might reflect an individual or growth variation.

The right postacetabular process is taphonomically compressed mediolaterally, and its pubic peduncle and the supraacetabular crest are eroded ( Fig. 16C–F ). The ischial peduncle is robust, and the acetabular surface is flat. Distally, the ischial peduncle is laterally, ventrally, and medially surrounded by a shallow depression. Further posteriorly from the ischial peduncle, ventrally, a large and deep brevis fossa is present. It is concave, wide mediolaterally, and gradually widens posteriorly, from 11 mm anteriorly to 28 mm distally. Such widening occurs also in Alioramus altai ( Brusatte et al. 2012) . There is no foramen at the base of this fossa, as in Ta. bataar (ZPAL MgD-I/4), but the foramen is present in Alioramus altai ( Brusatte et al. 2012). The medial and lateral walls of the brevis fossa are formed by the medial and lateral flanges of the postacetabular process. The lateral flange is thicker than the medial flange, as in Ta. bataar (ZPAL MgD-I/3) and Alioramus altai ( Brusatte et al. 2012) . The brevis fossa is visible in lateral view only anteriorly; further posteriorly it is concealed by the lateral flange of the postacetabular process. Above the beginning of the brevis fossa, the lateral flange of the postacetabular process continues dorsally as a dorsal, ~24-mm-long crest described by Osmólska (1996), surrounded by anterior and posterior depressions.

On the medial surface of the right ilium of ZPAL MgD-I/108, parts of three sacral ribs are present: one above the acetabulum, the second above the pubic peduncle, and the last positioned on the medial flange ( Fig. 16C–J ). Owing to the position of the sacral ribs, we agree with Osmólska (1996) that they belong to the third to fifth sacral vertebrae. If so, the laterally exposed brevis fossa terminates posteriorly at the level of the anterior part of the fifth sacral vertebra.

Pubes: The left pubis (proximal part and shaft preserved) is more complete than the right (where only the proximal part is preserved; Fig. 17). The articulation facet for the ilium is preserved in the left pubis (Fig. 17A–E). The contact with the pubic peduncle of the ilium is clear: the lateral margin is laterally extended with a rugose surface. In dorsal view, the pubic portion of the acetabulum is wider transversely, but shorter anteroposteriorly, than the ischial part. Below the acetabulum, the pubis narrows medialolaterally and forms a thin plate. The pubic tuberosity is incomplete, but it is present as a distinct convex structure, as in many tyrannosauroids, including juveniles such as the Raptorex holotype, but it does not have the highly rugose from of large subadult and adult tyrannosaurids, such as Ta. bataar (ZPAL MgD-I/3 and ZPAL MgD-I/5) ( Brusatte and Carr 2016: character 270). In B. ostromi , the tubercle is essentially level with the obturator notch, as in tyrannosaurids ( Brusatte and Carr 2016: character 271). Ventral to the pubic tuberosity and the articulation surface with the ischium, the pubis narrows anteroposteriorly and slightly widens transversely. Here, the main shaft of the pubis is anteriorly concave when seen in lateral view ( Fig. 17A ), as in tyrannosaurids generally, but differing from the straighter condition in the juvenile Raptorex holotype ( Brusatte and Carr 2016: character 269). On the posteromedial surface of the bone, the beginning of the pubic apron is preserved as a sigmoidal crest running along the medial surface of the pubic shaft ( Fig. 17B, D, E ). Its shape is similar to Ta. bataar (ZPAL MgD-I/175). The medial surface of the pubic apron is missing. The pubic shaft is circular in cross-section, starting from the region where the pubic apron appears, and remains circular until the end of the preserved part of the pubis (although the lateral surface of the pubic shaft is missing). In distal view, the proximal part of the pubis (above the shaft) is less bowed laterally than in Ta. bataar (ZPAL MgD-I/3 and ZPAL MgDI/175). This, however, can be accounted for by fact that the these two subadult individuals of Ta. bataar are twice the size (~ 7 m in length) of B. ostromi .

Ischium: Only the proximalmost left ischial plate, including the peduncles, is preserved ( Fig. 17A–D ). The articular surface of the pubic peduncle is tightly articulated with the ischial peduncle of the pubis. The pubic peduncle is separated from the ischial peduncle by an elliptic concavity. In dorsal view, the concavity is walled laterally by a wide and low margin (5 mm wide mediolaterally in the narrowest place), which expands anteriorly and posteriorly until reaching the peduncle margins, forming an hourglass-shaped margin (Fig. 17C). Medially, the concavity is walled by a straight, mediolaterally thin, and dorsally extended lamina, which is also present in other tyrannosaurids. The articularsurfaceofthepubicpeduncleis26 mmtallproximodistally and 20 mm wide mediolaterally. The lateral surface of the preserved part of the proximal ischium is concave, whereas the medial surface is only slightly concave. The articular surface of the iliac peduncle is 31 mm wide mediolaterally and 23.5 mm long anteroposteriorly. The lateral margin of the iliac peduncle is strongly extended laterally. In dorsal view, it is elliptical and has a concave articular surface with the ischial peduncle of the ilium, similar to other tyrannosaurids ( Brusatte et al. 2012).

Pedal phalanx: The left phalanx IV-1 is 33 mm long ( Fig. 18 ), its length to width ratio is 1.5. The proximal articular surface is wider (22 mm) than tall (19 mm; unlike Ta. bataar , where the proportions are the opposite: ZPAL MgD-I/29, ZPAL MgD-I/175, and ZPAL MgD-I/206), however, the dorsal and plantolateral margins of the phalanx are incomplete. The proximal articular surface is concave, in a similar manner to Ta. bataar individuals. The medial margin of the articular surface is slightly concave, and the opposite lateral margin is convex. In the dorsal and planar view, the phalanx IV-1 of ZPAL MgD-I/108 is rectangular, only slightly narrowed in the middle. In the lateral and medial view, the phalanx is triangular in overall shape, clearly narrowing (stronger on the lateral than medial side) immediately before the distal condyles. In dorsal view, a supracondylar basin is present, immediately behind the slightly elevated margin of the distal articular surface. The supracondylar basin is only slightly wider mediolaterally than long proximodistally (the length to width ratio is 0.8; in Ta. bataar specimens, the basin is much wider than long, and the ratio is ~0.4), and in comparison to Ta. bataar individuals, the basin is shallower. The lateral condyle is smaller than the medial condyle, and the lateral ligament pit is shallower in comparison to the medial one, as in all Ta. bataar individuals studied (ZPAL MgD-I/3, ZPAL MgD-I/4, ZPAL MgD-I/5, ZPAL MgD-I/29, ZPAL MgD-I/175, ZPAL MgD-I/206, and ZPAL MgD-I/331). The distal margin of the medial condyle is circular, its dorsal end does not form a pointed posteriorly tip, as in young Ta. bataar (ZPAL MgD-I/29), but in contrast to larger individuals (ZPAL MgD-I/3, ZPAL MgD-I/4, ZPAL MgD-I/5, ZPAL MgD-I/175, ZPAL MgD-I/206, and ZPAL MgD-I/331), where the tip is present. In dorsal view, the distal margin of the medial condyle is pointing anteromedially. The medial condyle is higher plantodorsally and wider medialolaterally than the lateral condyle. The distal condyles are separated by a cleft (which is acute and narrower in comparison to Ta. bataar individuals) along the entire articular surface. The rounded margin of the lateral condyle in lateral view is not complete on the plantar side. On the dorsal side, the margin of the articulation surface is smooth, only slightly lifted up. In larger individuals of Ta. Bataar , the dorsal end of the articular surface in lateral view is clearly demarcated.

The pedal phalanx IV-1 of young Ta. bataar ZPAL MgD-I/29 shows the same length to width ratio as ZPAL MgD-I/108. In subadults of Ta. Bataar , the ratio is 1.3 (e.g. ZPAL MgD-I/175), and in adults it is 1.2 (e.g. ZPAL MgD-I/206). Despite the fact that the phalanx IV-1 of B. ostromi is more slender than in subadult and adult Ta. bataar , it is short and wide, as is typical for tyrannosaurids, in contrast to the elongated and slender pedal phalanges of ornithomimids (length to width ratio is 1.7 for Ga. bullatus ZPAL MgD-I/94), caenagnathids (length to width ratio is 2.1 for Elmisaurus rarus Osmólska, 1981 ZPAL MgD-I/98), or troodontids (length to width ratio is 1.7 for Borogovia gracilicrus Osmólska, 1987 , ZPAL MgD-I/174).

Figures

Figure 1. Mandible of Bagaraatan ostromi ZPAL MgD-I/108 in lateral (A), medial (B), dorsal (C), and posterior (D) view.E, the preserved part distal part of the mandible in anterior view.F, dentary in anterior view.

Figure 2. Details of the dentary of Bagaraatan ostromi ZPAL MgD-I/108 in dorsal (A), anterior (B), posterior (C), ventral (D), lateral (E), and medial (F) view.

Figure 3. Comparison of Bagaraatan ostromi (A, B) and Tarbosaurus bataar (C–J) dentaries.A, B, ZPAL MgD-I/108 in medial (A) and lateral (B) view.C, D, ZPAL MgD-I/75 in medial (C) and lateral (D) view. E, F, ZPAL MgD-I/175 in medial (E) and lateral (F) view. G, H, ZPAL MgD-I/4 in medial (G) and lateral (H) view.I, J, ZPAL MgD-I/5 in medial (I) and lateral (J) view.Note that the anterior margin of the dentary steepens with ontogeny and that the dentary becomes taller dorsoventrally. The ‘chin’ is distinct, and the anterior end of the dentary is expanded through the ontogeny.

Figure 4. Details of the posterior mandible of Bagaraatan ostromi ZPAL MgD-I/108 in lateral (A), dorsal (B), ventral (C), medial (D), posterior (E), and anterior (F) view.

Figure 5. Comparison of Bagaraatan ostromi (A) and Tarbosaurus bataar (B–F) distal end of the mandible in lateral view.A, ZPAL MgD-I/108. B, ZPAL MgD-I/31. C, ZPAL MgD-I/3. D, MPC-D 107/14. E, ZPAL MgD-I/4. F, ZPAL MgD-I/5. Note the exceptionally small surangular foramen in the medium-sized individual MPC-D 107/14 (D) in comparison to similar-sized ZPAL MgD-I/3 (C). Asterisks indicate the posterior surangular foramen.Note that the placement of the double surangular foramina in B.ostromi is similar to the enlarged posterior surangular foramen in Ta.bataar. During ontogeny, the surangular shelf faces more laterally and the glenoid becomes longer anteroposteriorly.

Figure 6. Cervical vertebrae of Bagaraatan ostromi ZPAL MgD-I/108. A–F, anterior cervical vertebrae in anterior (A), left lateral (B), posterior (C), right lateral (D), dorsal (E), and ventral (F) view.G–L, posterior cervical vertebrae in anterior (G), left lateral (H), posterior (I), right lateral (J), dorsal (K), and ventral (L) view.

Figure 7. Anterior caudal vertebrae of Bagaraatan ostromi ZPAL MgD-I/108. A–F, fourth caudal vertebra in anterior (A), left lateral (B), posterior (C), right lateral (D), dorsal (E), and ventral (F) view.G–L, fifth caudal vertebra in anterior (G), left lateral (H), posterior (I), right lateral (J), dorsal (K), and ventral (L) view.

Figure 8. Anterior caudal vertebrae of Bagaraatan ostromi ZPAL MgD-I/108. A–F, sixth caudal vertebra in anterior (A), left lateral (B), posterior (C), right lateral (D), dorsal (E), and ventral (F) view. G–L, seventh caudal vertebra in anterior (G), left lateral (H), posterior (I), right lateral (J), dorsal (K), and ventral (L) view.

Figure 9. Anterior caudal vertebrae of Bagaraatan ostromi ZPAL MgD-I/108. A–F, eighth caudal vertebra in anterior (A), left lateral (B), posterior (C), right lateral (D), dorsal (E), and ventral (F) view.G–L, ninth and 10th caudal vertebrae in anterior (G), posterior (H), dorsal (I), left lateral (J), right lateral (K), and ventral (L) view.

Figure 10. Middle caudal vertebrae of Bagaraatan ostromi ZPAL MgD-I/108. A–F, 11th caudal vertebra in anterior (A), left lateral (B), posterior (C), right lateral (D), dorsal (E), and ventral (F) view. G–L, 12th caudal vertebra in anterior (G), left lateral (H), posterior (I), right lateral (J), dorsal (K), and ventral (L) view.

Figure 11. Middle caudal vertebrae of Bagaraatan ostromi ZPAL MgD-I/108. A–F, 13th caudal vertebra in anterior (A), left lateral (B), posterior (C), right lateral (D), dorsal (E), and ventral (F) view.G–L, 14th caudal vertebra in anterior (G), left lateral (H), posterior (I), right lateral (J), dorsal (K), and ventral (L) view. M–R, 15th caudal vertebra in anterior (M), left lateral (N), posterior (O), right lateral (P), dorsal (Q), and ventral (R) view.

Figure 12. Middle caudal vertebrae of Bagaraatan ostromi ZPAL MgD-I/108. A–F, 16th caudal vertebra in anterior (A), left lateral (B), posterior (C), right lateral (D), dorsal (E), and ventral (F) view. G–L, 17th caudal vertebra in anterior (G), left lateral (H), posterior (I), right lateral (J), dorsal (K), and ventral (L) view.M–R, 18th caudal vertebra in anterior (M), left lateral (N), posterior (O), right lateral (P), dorsal (Q), and ventral (R) view.

Figure 13. Posterior caudal vertebrae of Bagaraatan ostromi ZPAL MgD-I/108. A–F, 19th caudal vertebra in anterior (A), left lateral (B), posterior (C), right lateral (D), dorsal (E), and ventral (F) view.G–L, 20th caudal vertebra in anterior (G), left lateral (H), posterior (I), right lateral (J), dorsal (K), and ventral (L) view.M–R, 21st–23rd caudal vertebrae in anterior (M), left lateral (N), dorsal (O), ventral (P), posterior (Q), and right lateral (R) view.

Figure 14.Posterior caudal vertebrae of Bagaraatan ostromi ZPAL MgD-I/108. A–F, 24th caudal vertebra in anterior (A), left lateral (B), posterior (C), right lateral (D), dorsal (E), and ventral (F) view.G–L, indeterminate posterior caudal vertebra in anterior (G), left lateral (H), posterior (I), right lateral (J), dorsal (K), and ventral (L) view.M–R, indeterminate posterior caudal vertebra in anterior (M), left lateral (N), posterior (O), right lateral (P), dorsal (Q), and ventral (R) view.S–Y, indeterminate posterior caudal vertebra in anterior (S),left lateral (T), posterior (U), right lateral (W), dorsal (X), and ventral (Y) view.Z–Eʹ, indeterminate posterior caudal vertebra in anterior (Z), left lateral (Aʹ), posterior (Bʹ), right lateral (Cʹ), dorsal (Dʹ), and ventral(Eʹ) view.

Figure 15. Two incomplete haemal arches of Bagaraatan ostromi ZPAL MgD-I/108. A–F, haemal arch A in anterior (A), left lateral (B), posterior (C), right lateral (D), dorsal (E), and ventral (F) view.G–L, haemal arch B in anterior (G), left lateral (H), posterior (I), right lateral (J), dorsal (K), and ventral (L) view.

Figure 16. Ilium of Bagaraatan ostromi ZPAL MgD-I/108. A, B, fragment of left preacetabular process in lateral (A) and medial (B) view. C–F, left postacetabular process in lateral (C), medial (D), dorsal (E), and ventral (F) view.G–J, right postacetabular process in dorsal (G), ventral (H), lateral (I), and medial (J) view.K–N, two fragments of ilium blade in lateral (K, M) and medial (L, N) view.

Figure 17. Pubes of Bagaraatan ostromi ZPAL MgD-I/108. A–E, left pubis with proximal end of the ischium in lateral (A), medial (B), dorsal (C), posterior (D), and anterior (E) view.F, G, right pubis in lateral (F) and medial (G) view.

Figure 18. Left pes phalanx IV-1 of Bagaraatan ostromi ZPAL MgD-I/108 in dorsal (A), lateral (B), anterior (C), plantar (D), medial (E), and posterior (F) view.