Oksoko avarsan

The four known specimens of Oksoko avarsan (MPC-D 100/33, MPC-D 102/11, MPC-D 102/12, and MPC-D 102/110) compriseatleastsixindividuals.MPC-D 100/33 andMPC-D 102/12 each represent a single partial skeleton, whereas MPC-D 102/11 includes a relatively complete skeleton (MPC-D 102/11.a) and three cranial bones of another individual (MPC-D 102/11.b). The main block of MPC-D 102/110 includes two relatively complete skeletons and a partial skeleton of a third individual. These individuals are sub-numbered MPC-D 102/110.a, MPC-D 102/110.b, and MPC-D 102/110.c, in descending order of completeness. MPC-D 102/11 and MPC-D 102/110 were probably collected from the same assemblage (Funston et al. 2020), which has a minimum number of four individuals based on right quadrates. As discussed by Funston et al. (2020), the provenance of this assemblage is uncertain because the specimens were confiscated from poachers, who excavated the skeletons illegally. Several lines of evidence suggest that MPC-D 102/110 was collected from either Bugiin Tsav or Guriliin Tsav in the north-western part of the Nemegt Basin. Geochemical fingerprinting on MPC-D 102/110 strongly suggests that the specimens are from the Nemegt Formation, specifically the Nemegt locality, although the Bugiin Tsav and Guriliin Tsav localities were not included in that sample (Fanti et al. 2017). Another line of evidence is an abandoned ankylosaur skeleton re-discovered in 2018 at Guriliin Tsav, poached using similar methods and materials as used to collect MPC-D 102/110 (Fig. 2C, D). Both field jackets are made of thin pre-prepared blaster bandages with blue fibres, and yellow plastic bags were used as a separating layer. As these features of the specimens are unusual, this suggests, minimally, that the same group of poachers collected both specimens. Lastly, legitimately collected specimens of Oksoko avarsan are known from Bugiin Tsav (MPC-D 100/33) and Guriliin Tsav (MPC-D 102/12), showing that this taxon was present in the area. Specimens of Oksoko avarsan and other oviraptorosaurs housed in the collections of the CMN, MPC, ROM, TMP, and UALVP were examined firsthand and measured using digital calipers (±0.1 mm) or a fabric measuring tape (± 1 mm). Information about other specimens was taken from the literature. The specimens were photographed using a Nikon D5000, Nikon D7200, or Nikon COOLPIX AW120 camera with a variety of lenses. CT scans of MPC-D 102/110 were performed at The National Museum of Natural History and Science in Tokyo, Japan.Despite minimal matrix adhered to the specimen, the scans suffered from severe beam hardening artefacts and ghosting, such that the borders of many elements at the surface are difficult to discern. Contrast within the block is relatively poor, especially within the endocranial cavities, which cannot be segmented. Contrast surrounding each cranium is particularly low, and thus details of their morphology cannot be ascertained from the CT scans. However, many of the more robust postcranial bones, including the vertebrae and limbs, can be adequately distinguished from the matrix and each other, and thus are useful for visualizing which parts of the individuals are preserved and how they are arranged, although their morphologies cannot be reconstructed. These images verify the arrangement of the specimens, their positions with respect to each other, and the associations between skulls and postcrania (Fig. 5). This latter aspect of the arrangement of the skeletons was initially ambiguous because of the unusual positions of the skulls appressed to the sternae. However, CT images show that the cervical vertebrae of each specimen are curled to form a spiral, confirming the ownership of each skull, as interpreted by Funston et al. (2020). D E SCR I P T I O N Between the six skeletons known, every skeletal element of Oksoko avarsan is represented (Figs 3, 4). Each of these is exquisitely preserved and has suffered minimal post-mortem scavenging or weathering, and moderate to no crushing. These specimens reveal an oviraptorid with a distinctive cranial crest, short forelimbs with only two functional digits, elongate hindlimbs, and a relatively short tail (Fig. 3). MPC-D 102/110.a is the most complete skeleton, including a complete skull and skeleton, missing only the distal caudal vertebrae. MPC-D 102/110.b preserves a slightly disarticulated skull, parts of the vertebral column, a forelimb, the pelvis, and the hindlimbs. MPC-D 102/110.c preserves a partial ilium, some vertebrae, and a complete tibiotarsus that was revealed by CT scans. Thus, the individuals in MPC-D 102/110 (Figs 3, 5) provide excellent representation of the skeleton of Oksoko avarsan. Nonetheless, articulation of these specimen means that some elements are not visible in all views, and so some bones are better represented by the completely prepared specimens: MPC-D 100/33, MPC-D 102/11.a, MPC-D 102/12, and the fully prepared manus of MPC-D 102/110.a. Cranial skeleton The skulls of MPC-D 102/110.a and MPC-D 102/110.b are complete (Figs 6, 7), but are crushed mediolaterally. The posterior portion of the skull of MPC-D 102/110.b, including the braincase and suspensorium, is disarticulated slightly from the anterior part of the skull and rotated so that it is oriented perpendicular to the remainder of the cranium (Fig. 6). The skull of MPC-D 102/11.a is relatively complete, but is missing most of the anterior parts of the face, palate, and mandible (Figs 8, 9). The left side of the skull is roughly articulated and is well preserved. The right and posterior sides have been crushed and lie on a single plane parallel to the left side of the skull. Three extra skull bones from a second individual (MPC-D 102/11.b) are preserved alongside the more complete skull of MPC-D 102/11.a: the postorbital, quadratojugal, and quadrate (Fig. 10). The other specimens (MPC-D 100/33 and MPC-D 102/12) lack cranial elements. Premaxilla The premaxilla (Figs 6, 7) is completely preserved in MPC-D 102/110.a and MPC-D 102/110.b, but only a small portion is present in MPC-D 102/11.a (Fig. 8). It is tall dorsoventrally and constricted anteroposteriorly. Dorsally, it is divided by the naris into two processes: the nasal process extending dorsally and the subnarial process directed posterodorsally. The nasal process is much narrower than the subnarial process in lateral view, unlike in Citipati osmolskae Clark et al. 2001, Khaan mckennai Clark et al. 2001, Nemegtomaia barsboldi (Lü et al. 2004), and Rinchenia mongoliensis (Barsbold 1986), where these processes are subequal in width (Barsbold 1986, Clark et al. 2001, 2002, Lü et al. 2004, Balanoff and Norell 2012, Funston et al. 2018). In contrast, the nasal process is wider than the subnarial process in Banji long Xu and Han 2010, Huanansaurus ganzhouensis Lü et al. 2015, and Tongtianlong limosus Lü et al. 2016. The nasal process of Oksoko curves posterodorsally so that it forms a small part of the continuous semicircular crest with the nasals, frontals, and parietals, but not to the same extent as the anteroposteriorly broad premaxilla of Tongtianlong limosus. In Oksoko avarsan, the nasal process extends dorsally to the ventral third of the naris, whereas in Banji long and Rinchenia mongoliensis the premaxilla extends far above the naris (Xu and Han 2010, Funston et al. 2018). The subnarial process of the premaxilla is broad and tapers posteriorly. Posteriorly, it separates the lacrimal and nasal anteriorly and prevents the maxilla from contacting the nasal on the lateral surface of the skull. The lateral surface of the body of the premaxilla is pierced by multiple small foramina. Ventral to the small, oval naris, there is a lateral depression in the premaxilla similar to that of Citipati osmoslkae (Clark et al. 2001), but shallower than the prominent fossa in Banji long (Xu and Han 2010). The occlusal margin of the premaxilla has at least two denticulations, but this area is broken in both individuals and there may have been more. The occlusal edge of the premaxilla is relatively longer anteroposteriorly than in Rinchenia mongoliensis or Corythoraptor jacobsi Lü et al. 2017, more comparable to Citipati osmolskae or Nemegtomaia barsboldi. The palatal surface of the premaxilla cannot be seen on any of the specimens. Maxilla The maxilla (Figs 6, 7) is missing in MPC-D 102/11.a and poorly preserved in both individuals of MPC-D 102/110, but best observed in MPC-D 102/110.b (Fig. 6B). The antorbital fossa is small and the antorbital fenestra is divided in two by a dorsally expanding strut of bone (Fig. 6B), as in most oviraptorids. The jugal process is relatively short and extends only partway under the orbit. The labial–buccal transition on the lateral side of the maxilla is marked by a ridge, ventral to which there is a pronounced lateral tubercle, as in Rinchenia mongoliensis (Funston et al. 2018). The presence of longitudinal palatal ridges on the maxilla, as exhibited in Citipati, cannot be determined because of overlying matrix. Likewise, the maxillovomeral tubercle (=palatal ‘tooth’), which is present in all oviraptorids, is obscured by matrix in MPC-D 102/110, so the contribution of the maxilla cannot be determined. Nasals The fused nasals (Figs 6–9, 11) are complete but crushed in both individuals of MPC-D 102/110. Only the posterolateral wings of the nasals are preserved in MPC-D 102/11.a (Figs 8, 9, 11). In this individual, the nasals are fused along the midline, but posteriorly a suture is still visible (Fig. 11). Like Rinchenia mongoliensis and Corythoraptor jacobsi, the great size of the nasals is mostly due to expansion of the lateral descending processes. Similar to Citipati osmolskae (Clark et al. 2001) and Khaan mckennai (Balanoff and Norell 2012), the posterodorsal part of the premaxilla excludes the maxilla from contributing to the naris and from contacting the nasal on the lateral surface of the skull. The elliptical naris is displaced

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