Tyrannosauridae, Osborn, 1905

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 : 35

Juvenile Tyrannosauridae indet.

Owing to its small body size and similarity to other juvenile tyrannosaurid specimens from the Nemegt, it is likely that B. ostromi is a juvenile tyrannosaurid. We tested this hypothesis further by determining whether B. ostromi shows juvenile features that have been well documented in Ty. rex , whose ontogenetic osteological changes have been chronicled in detail by Carr (2020). We recognized that B. ostromi shows 22 juvenile and only five adult mandible features found in Ty. rex by Carr (2020). Four of the 22 juvenile features were found only in early juveniles, and the remaining 18 in late juveniles.

The mandible characters recognized both in B. ostromi and juveniles of Ty. rex are as follows: (i) size of the first three alveoli of the dentary increasing posteriorly; (ii) shallow dentary in lateral view; (iii) shallow coronoid region of the surangular; (iv) no ridge delimiting the caudoventral fossa of the angular caudal process; (v) first two dentary alveoli much smaller than the latter alveoli; (vi) eighth tooth is the mesiodistally longest in the dentary; (vii) the alveoli decreasing posteriorly in mesiodistal length from the sixth to seventh alveolus; (viii) single large pit medial to the Meckelian fossa; (ix) low angle of the ‘chin’; (x) lightly textured ‘chin’ region; (xi) distance of the ventralmost dentary foramen from the dorsal margin of the dentary to the total height of the bone> 40%; (xii) the lateral extension of the surangular shelf horizontal; (xiii) surangular shelf not slanted; (xiv) small surangular foramen; (xv) caudal extent of the coronoid process declining before it reaches the glenoid; (xvi) presence of an embayment on the caudal margin of the surangular foramen; (xvii) cleft between the caudal glenoid process dorsoventrally short and shallow; (xvii) caudal end of the surangular shelf fading below the glenoid region; (xix) lateral scar on the surangular present; (xx) caudal glenoid process as tall as the rostral process; (xxi) lateral scar on the surangular rugose and shallow; and (xxii) dorsal orientation of the anterior glenoid foramen ( Carr 2020).

The prevalence of features shared by B. ostromi and juvenile Ty. rex supports the identification of B. ostromi as a young tyrannosaurid. The less numerous adult Ty. rex features ( Carr 2020) found in B. ostromi are listed below, with comments regarding variability within Ta. bataar . First, the second dentary tooth is> 75% of the mesiodistal length of the third dentary tooth. The proportions between the first three dentary teeth in Ta.bataar seem to be variable. In the subadult ZPAL MgD-I/175, the second dentary tooth is <75% of the mesiodistal length of the third dentary tooth, and in the adults (ZPAL MgD-I/5) it is between 70 and 78%. Second, the combined mesiodistal lengths of the first two alveoli of the dentary are greater than the mesiodistal length of the third alveolus, as in all examined individuals of Ta. bataar (subadults ZPAL MgD-I/45, ZPAL MgD-I/46, and ZPAL MgD-I/175; and adults ZPAL MgD-I/4 and ZPAL MgD-I/5; Table 1). However, the difference in all Ta. bataar specimens is greater (~ 4 cm) than in B. ostromi (1 cm). Third, there is no deviation in the ‘chin’ region, which is not recognized in any examined Ta. bataar specimen (ZPAL MgD-I/45, ZPAL MgD-I/46, ZPAL MgD-I/175, ZPAL MgD-I/4, and ZPAL MgD-I/5), nor it has been described in juvenile Ta. bataar MPC-D 107/7 (Tsuihiji et al. 2011). Fourth, the caudal surangular foramen is positioned far anteriorly to the glenoid, as in all Ta. bataar individuals (ZPAL MgD-I/4, ZPAL MgD-I/5, and ZPAL MgD-I/31), including the juvenile MPC-D 107/7 (Tsuihiji et al. 2011). Fifth, the glenoid fossa is short and deep in adult Ty. rex and B. ostromi . A shallow and long glenoid fossa can be recognized in ‘ Shanshanosaurus huoyanshanensis ’ see ( Currie and Dong 2001), but already in the slightly larger MPC-D 107/7 and B. ostromi , as in young and adult Ta. bataar (ZPAL MgD-I/4, ZPAL MgD-I/5, and ZPAL MgD-I/31), it is narrow and deep. Those features possibly indicate some species-dependent variability, similar to the proportion of the antorbital fenestra, which does not shorten as much during the ontogeny of Ta. bataar as it does in Ty. rex (Tsuihiji et al. 2011) .

As it is clear that the B. ostromi holotype belongs to a juvenile tyrannosaurid, the question becomes: can we identify which species it belonged to? We can first make comparisons with the other Nemegt tyrannosaurids: Ta. bataar and Alioramus spp. The mandible of B. ostromi is generally similar to juvenile Ta. bataar or putative juveniles of that species, like ‘ S. huoyanshanensis ’ and R. kriegsteini (see Currie and Dong 2001, Sereno et al. 2009, Fowler et al. 2011, Tsuihiji et al. 2011). The dentary is straight in the dorsal and ventral view, shallow, slender, and thickens and tapers dorsally at the anterior end. Bagaraatan ostromi , like MPC-D 107/7, but in contrast to Ta. bataar specimens and Alioramus altai , does not show any pneumatic pocket behind the surangular foramen (Tsuihiji et al. 2011, Brusatte et al. 2012). The cervical vertebrae of B. ostromi strongly resemble the middle or posterior cervical vertebrae of ‘ S. huoyanshanensis ’. They share the posterodorsal rather than dorsal inclination of the neural spines, and have less flexed centra than in adult, large tyrannosaurids ( Currie and Zhiming 2001). The fusion of some bones occurred early in the ontogeny of tyrannosaurids, e.g. the juvenile Ta. bataar already has fused nasals (Tsuihiji et al. 2011). However, the articular remains unfused with the surangular in B. ostromi , similar to ‘ S. huoyanshanensis ’, juvenile Ta. bataar (MPC-D 107/7), and Alioramus altai (see Brusatte et al. 2012). In contrast, in Raptorex and larger Ta. bataar individuals the articular is fused to the surangular. Moreover, early in ontogeny partial fusion of the pelvis was reported in Raptorex (see Fowler et al. 2011) and young Ty. rex (BMR P 2002.4.1, ‘Jane’; Parrish et al. 2013). In contrast, the pelvic bones are unfused in B. ostromi , juvenile Ta. bataar (MPC-D 107/7), and subadult Alioramus altai (see Brusatte et al. 2012) However, an early fusion of the cranial sutures might not necessarily be associated with an early fusion of postcrania, because these functional units could be subjected to developmental plasticity or separate evolutionary pressure depending on the ecology and preferred or available diet. The co-ossification of postcranial sutures and fusion between bones among tyranosaurids require further study. However, owing to their high variability, also in juveniles, we do not find them to be an adequate indicator for growth stage in tyrannosaurids.

We can more thoroughly compare B. ostromi with young juvenile Ta. bataar , because no young juveniles of Alioramus spp. are known thus far. Given that both tyrannosaurids occur in the Nemegt Formation and that B. ostromi lacks diagnostic features of either Alioramus spp. (see Brusatte et al. 2012) or Ta. bataar (see Hurum and Sabath 2003), which is mostly attributable to the fragmentary nature of the holotype skeleton, we cannot assign ZPAL MgD-I/108 to any particular species. Some subtle features suggest that B. ostromi might be a juvenile of Ta. bataar : e.g. (i) already strongly expanded anterior end of the dentary; (ii) ‘chin’ well demarcated; and (iii) lack of the pneumatic pocket next to the surangular foramen. However, because those features might potentially be a result of intraspecific variability or be more widespread among juvenile tyrannosaurids than currently suspected, we cannot clearly determine whether B. ostromi is a juvenile of Ta. bataar or Alioramus spp. Thus, we consider B. ostromi to be an indeterminate juvenile representative of the Tyrannosauridae. This assessment might be modified in the future, when more juvenile individuals of tyrannosaurid taxa are known (particularly young individuals of Alioramus spp. ) and when the growth series and variability at early life stages are better understood.