Tsaagan mangas

Brownstein, Chase Doran, 2021, Dromaeosaurid crania demonstrate the progressive loss of facial pneumaticity in coelurosaurian dinosaurs, Zoological Journal of the Linnean Society 191 (1), pp. 87-112 : 93

Tsaagan mangas

Both maxillae are preserved in the holotype of Tsaagan , although the left is more complete and still shows morphological details of its antorbital fossa ( Fig. 3A ). As in “ Bambiraptor ”, Deinonychus , Velociraptor and other dromaeosaurids, Tsaagan possesses both promaxillary and maxillary fenestrae. However, in Tsaagan , both of these fenestrae are medially bordered by the same crescentic wall of bone ( Fig. 3 A-D; Norell et al., 2006). The promaxillary fenestra is small and placed anteroventrally relative to the maxillary fenestra, which is large, ovoid to circular and clearly penetrates into the interior of the skull. A ridge of bone subtended by the promaxillary fenestra separates the promaxillary and the maxillary fenestrae ( Fig. 3A ; the promaxillary pila of Norell et al., 2006). The maxillary fenestra is large and subovoid. In addition to the promaxillary and maxillary fenestrae, a large, excavated portion of bone identifiable as an additional pneumatic recess is placed at the anteroventral edge of the antorbital fenestra. This feature recalls the additional pneumatic recess in Velociraptor that is placed ventral to the maxillary fenestra in that taxon ( Figs 2A–B , 3A ). The closely adjacent nature of the promaxillary and maxillary fenestrae in Tsaagan distinguishes the maxillary pneumatic morphology of this taxon from Acheroraptor temertyorum (Evans et al., 2013) , “ Bambiraptor feinbergi ”, Deinonychus antirrhopus , Saurornitholestes langstoni and Velociraptor mongoliensis ( Norell et al., 2006; Turner et al., 2012; see also Norell et al., 2006). The maxillary fenestra and promaxillary fenestra are also closely adjacent in Achillobator and Atrociraptor (e.g. Norell et al., 2006; Turner et al., 2012); however, in these taxa both appear as dorsoventrally elongate, subovoid structures that are only shallowly inset (e.g. Turner et al., 2012). Among dromaeosaurids, Linheraptor , which may be synonymous with Tsaagan (e.g. Turner et al., 2012), shows the same morphology (e.g. Xu et al., 2015). The closely adjacent nature of the promaxillary and maxillary fenestrae in Atrociraptor , Linheraptor and Tsaagan indicates the anterior migration of the anterior end of the maxillary antrum to become closely adjacent with the promaxillary sinus ( Fig. 3B; e.g . Witmer, 1997; Norell et al., 2006; Witmer & Ridgely, 2008; Gold et al., 2013). However, the maxilla of Atrociraptor is deep and anteroposteriorly shortened (e.g. Turner et al., 2012) and therefore probably possessed a differently shaped maxillary antrum. The antorbital fossa of Tsaagan is deeply invaginated along its entire border.

Figures

Figure 2. Craniofacial pneumaticity of Velociraptor. Skull AMNH FARB 6515 in (A, C) lateral and (B, D) medial views, with the rostrum in (E) lateral and (F) medial views. Abbreviations: aofen, antorbital fenestra; aof, antorbital fossa; mf, maxillary fenestra; mx, maxilla; pmf, promaxillary fenestra; mi, maxilla interior;?pipf, premaxillary pneumatic foramina; pmx, premaxilla; pf, pneumatic foramina of the frontal; front, frontal; nf, neurovascular foramina; rugose, rugose bone surface; smooth, smooth bone surface. In (C) and (D), light blue are neurovascular foramina, light green is the maxillary fenestra, light red is the promaxillary fenestra, tan is an additional maxillary fenestra.

Figure 3. Craniofacial pneumaticity of Tsaagan. Skull (cast of IGM 100/1015) in (A, C) lateral and (B, D) medial views. Abbreviations: aofen, antorbital fenestra; aof, antorbital fossa; pneu fen, additional maxillary pneumatic fenestra; mf, maxillary fenestra; mx, maxilla; pmf, promaxillary fenestra; pmx, premaxilla. In (B), green is the additional maxillary pneumatic fenestra, blue is the maxillary fenestra, and yellow is the promaxillary fenestra.