Reassessment of the enigmatic Late Cretaceous theropod dinosaur, Bagaraatan ostromi Author Słowiak, Justyna Institute of Paleobiology, Polish Academy of Sciences, Warsaw, Poland justyna.slowiak@twarda.pan.pl Author Brusatte, Stephen L. School of GeoSciences, University of Edinburgh, Edinburgh, UK Author Szczygielski, Tomasz Institute of Paleobiology, Polish Academy of Sciences, Warsaw, Poland text Zoological Journal of the Linnean Society 2024 2024-02-16 202 3 1 39 https://doi.org/10.1093/zoolinnean/zlad169 journal article 10.1093/zoolinnean/zlad169 0024-4082 14284228 Is B. ostromi a valid taxon? Osmólska (1996) listed eight diagnostic features for B. ostromi : (i) two surangular foramina [also considered by Holtz (2004) as an autapomorphy of B. ostromi ]; (ii) articular with an oblique posterior surface and a short retroarticular surface; (iii) caudal vertebrae with thin-walled centra; (iv) hyposphenehypantrum articulations in at least the first 16 caudal vertebrae [also listed by Holtz (2004) as an autapomorphy of B. ostromi ]; (v) prezygapophyses in proximal caudal vertebrae with ridges on the lateral surfaces; (vi) ilium with two deep depressions; (vii) femur with the anterior trochanter (anterior crest sensu Osmólska); and (viii) tibia and fibula fused distally. The status of those features is briefly discussed below. Given that we have now re-identified the hindlimb bones as belonging to other non-tyrannosaurid taxa, those features regarding the hindlimb were discussed above, hence they will be omitted in this section. The ilium with a distinct ridge on the lateral surface of the postacetabular process, demarcated anteriorly, medially, and posteriorly by depressions, is striking ( Fig. 16 ). It occurs symmetrically on both ilia, and better preserved on the left, which is less compressed. Such ridges are not found in other theropods, to our knowledge, and are not present in juvenile Ta. bataar (MPC-D 107/7) nor the other tyrannosaurid juvenile, R. kriegsteini (Sereno et al. 2009) . Therefore, they might be a diagnostic feature of B. ostromi . The ridges on the lateral surfaces of the prezygapophyses are found also in the proximal caudal vertebrae of Ty. rex & running from the anterior margin of the transverse process to the prezygapopysis ( Brochu 2003 ). Similar ridges on the prezygapophyses of anterior caudal vertebrae are also present in Ta. bataar (ZPAL MgD-I/176). Osmólska (1996) did not quantify how thin-walled the caudal vertebrae centra of B. ostromi are in comparison to other theropods. We do not recognize any clear difference between the centrum thickness of B. ostromi and other theropods. The stout hyposphene–hypantrum articulations in at least the first 16 caudal vertebrae were considered an autapomorphy of B. ostromi by Osmólska (1996) and Holtz (2004). The presence of the hyposphene–hypantrum articulation is seen in many archosaurs, is strongly correlated with body size, and is often already present at a young age, before the articulation is necessary to support the large mass of the fully grown animal (Stefanic and Nesbitt 2019 ). The hyposphenehypantrum articulations are common in theropods, and for instance, are present in the caudal vertebrae of medium-sized Ta. bataar (ZPAL MgD-I/176). The oblique posterior surface and short retroarticular surface of the articular, also listed by Osmólska (1996), are tyrannosauroid synapomorphies ( Brusatte et al. 2014 ). Figure 25. Reconstruction of Bagaraatan ostromi ZPAL MgD-I/108 including only tyrannosaurid bones found in the assemblage and life reconstruction of the dinosaur by Jakub Zalewski. The presence of two surangular foramina and the ridge on the lateral surface of the postacetabular process of ilium seem to be the only two features listed by Osmólska (1996) that distinguish B. ostromi from other tyrannosaurids. The two surangular foramina were also later listed by Holtz (2004) as unique for B. ostromi in comparison to other theropods ( Currie et al. 2003 ). There is some confusion in the literature about the size of the surangular foramen in tyrannosauroids and its phylogenetic significance and ontogenetic and individual variation. In their phylogenetic dataset of tyrannosauroids, Brusatte and Carr (2016) used a character that simply divided the size of the foramen into two states: those with a dorsoventral depth <30% or> 30% of the depth of the posterior end of the surangular. This was based on earlier characters used by Sereno et al. (2009), Carr and Williamson (2010), and Brusatte et al. (2010) . The enlarged condition was found to be synapomorphic of a clade consisting of Dryptosaurus + Tyrannosauridae, whereas the primitive smaller foramen is seen in more basal tyrannosauroids, such as Suskityrannus , Eotyrannus , Dilong , and proceratosaurids. Other authors, however, have considered the foramen differently. The size of the surangular foramen in tyrannosaurids was divided into moderate ( Go. libratus , Albertosaurus sarcophagus Osborn, 1905 , Ty. rex , and Ta. bataar ) or enlarged ( Bi. sealeyi , Daspletosaurus spp. , Te. curriei , Ŋanatotheristes degrootorum Voris et al. , 2020, Q. sinensis , and Alioramus altai ) states by Voris et al. (2021). However, the surangular foramen in Ty.rex and Albertosaurus sarcophagus also used to be classified as smaller than in other tyrannosaurids ( Carr and Williamson 2004 ). Other authors reported that the surangular foramen in Ta. bataar is smaller than in other tyrannosaurids and invariant during ontogeny (Tsuihiji et al. 2011, Voris et al. 2021). Also, the surangular foramen in ‘ Shanshanosaurus huoyanshanensis ’ was described as large ( Currie and Dong 2001 ), but later as small (Tsuihiji et al. 2011). For Ty. rex , the size of the surangular foramen was first reported as increasing ( Carr 1999 ) but later as decreasing in size through ontogeny ( Carr 2020 ). Because of this confusion, we built a dataset to examine the size of foramina in a quantitative context. In tyrannosaurids, growth of the mandible, skull, and femur is isometric and related to the body size of the individual ( Currie 2003b ). Thus, we assessed the relationship between the size of the surangular foramen and skull length (as a proxy for body size). Our results ( Fig. 22 ) show that in all taxa the size of the surangular foramen decreases during ontogeny (negative allometry) and is strongly correlated with the length of the skull. Thus, e.g. Alioramus altai (IGM 100/1844) and the similar-sized Go. libratus (TMP 1991.36.500) have surangular foramina of proportionally the same size. Although the surangular foramen–skull length correlation is statistically significant, variability in surangular foramen size is also apparent, especially in Go. libratus and Ta. bataar , for which the data are less fitted to the trend than for the other species ( R 2 =.76–.78, vs. R 2 >.88–. 94 in Daspletosaurus spp. and Ty. rex ; Fig. 22 ). Indeed, although the surangular foramen is rather enlarged in Tarbosaurus individuals (as in other tyrannosaurids; Fig. 22) bigger than MPC-D 100/66 (skull length: 45 cm ), few exceptions were found within the hypodigm. The surangular foramen of the medium-sized specimen MPC-D 107/14 is exceptionally small in comparison to other Ta. bataar individuals of similar size (e.g. ZPAL MgD-I/3 and MPC-D 107/5; Fig. 5 ). Moreover, a specimen larger than those listed above, MPC-D 100/60, shows asymmetrical surangular foramina: the left one is smaller (anteroposterior length: 23 mm ) and the right one larger (anteroposterior length: 40 mm ). The smaller surangular foramen of the left mandible can be noticed as an outlier in the Figure 22 . It would appear that there was some variability in the timing of the surangular foramen enlargement, at least in Ta. bataar . The size of the surangular foramen in B. ostromi , regardless of whether it is measured for the single (posterior only) or double (posterior + anterior) foramina, falls into the overall variability of surangular size in the tyrannosaurids generally and Ta. bataar specifically. The position of the surangular foramen in ‘ Shanshanosaurus ’ and Ta. bataar MPC-D 107/7 is similar to the position of the posterior opening in the surangular of Bagaraatan , and those individuals cluster together on the plot. In turn, if the length of the area of both surangular foramina is measured for B. ostromi , it clusters with R. kriegsteini , the surangular foramen of which was previously reported to be ‘enlarged’ ( Fowler et al. 2011 ). What might explain the strange double set of foramina in Bagaraatan ? The bone between the anterior and posterior surangular foramina in B. ostromi is very thin, and the relative position of this area and both foramina matches the surangular foramen of R. kriegsteini and Ta. bataar (ZPAL MgD-I/31). The surangular in tyrannosaurids during the early years of life was invaded by a pneumatic diverticulum ( Gold et al. 2013 ), which pneumatized the bone and formed the enlarged surangular foramen, bordered by a pneumatic pocket posterodorsal to it. Given that more basal tyrannosauroids have a small foramen without a pneumatic pocket, it is not clear whether there was any pneumatic diverticulum in this region in these species. Owing to the fact that pneumatic diverticula induce bone resorption when they contact bone ( Bremer 1940 , Witmer 1997, Wedel 2007 ), we propose that the mandible of B. ostromi exhibits local bone resorption, induced by the pneumatic diverticula, that would explain the extremely thin bone between the anterior and posterior foramen. We hypothesize that if the pneumatization process continued slightly longer, the two foramina might have merged into a single large foramen, which is the common condition in Dryptosaurus + Tyrannosauridae ( Brusatte and Carr 2016 ). This proposal is supported by the fact that the posterior surangular foramen in B. ostromi is similar in length and positioned in a similar place as in the smaller ‘ Shanshanosaurus ’ and Ta. bataar MPC-D 107/7 (skull length ~ 29 cm ) specimens. Furthermore, the area of the surangular containing the posterior and anterior surangular foramina and the thinned bone between them matches the length and position of the surangular foramen in Raptorex (skull length ~ 30 cm ). Therefore, B. ostromi (skull also ~ 30 cm long) possibly captures the precise moment of ongoing bone resorption and perforation attributable to the pneumatic diverticulum. This process probably occurred early in ontogeny, in specimens 2–3 m long, which were probably 2–3 years old at the time of death (as indicated for Ta. bataar MPC-D 107/7 by Tsuihiji et al. 2011). Apparently, around this growth stage the pneumatic diverticulum invaded the bone, and thus variability in the size, shape, and even the number of foramina is to be expected.