Sulcimentisauria

Sulcimentisauria gen. et sp. indet. (Fig. 4) Referred material: UWGM 7550, the distal end of a left humerus and the proximal end of a right femur, UWGM 7407. Locality and horizon: The Garrett’s Surprise locality from the purple unit of the Popo Agie Formation (see further details above). Remarks: The silesaurid affinities of UWGM 7407 are readily determined by the following combination of features: a notch ventral to the anteromedial tuber; a subtriangular proximal outline due to a prominent anterolateral tuber and incipient posteromedial tuber; and a transverse groove on the proximal surface of the femur (Martz and Small 2019). The silesaurid affinities of the distal humerus UWGM 7550 are less certain, but it compares favourably with the morphology of the Moroccan silesaurid Diodorus scytobrachion Kammerer et al., 2012. We will refer to the combined OTU of UWGM 7407 and UWGM 7550 simply as UWGM 7407, unless specifically referring to the femur or humerus, then the element will be included and the catalogue number in parentheses. The combined character states found in the femur (UWGM 7407) are also present in Di. scytobrachion, Sacisaurus agudoensis Ferigolo and Langer, 2007, and Silesaurus opolensis Dzik, 2003; unfortunately the characters that differentiate these taxa are damaged, obscured, or entirely missing in UWGM 7407 (Fig. 4). Description: The humerus (UWGM 7550) is fragmentary with only the distal end preserved and significant fracturing through the shaft. The anatomical orientation of the distal end of this silesaurid humerus is complicated by the presence of torsion in the humerus of Kwanasaurus williamparkeri Martz and Small, 2019 and lack thereof in other sulcimentisaurian silesaurids (Dzik 2003, Kammerer et al. 2012) and the non-sulcimentisaurian silesaurid Asilisaurus kongwe Nesbitt et al., 2010, so all interpretations of orientation assume no humeral torsion as is plesiomorphic for silesaurids. The only character relevant to the character matrix determined from the humerus is that the distal condyles are relatively narrow relative to the humeral shaft, as is typical for sulcimentisaurian silesaurids (e.g. Dzik 2003, Kammerer et al. 2012), in contrast to the broader distal humerus of non-sulcimentisaurian silesaurids (Ezcurra et al. 2020b, Nesbitt et al. 2020a). The radial and ulnar condyles both form rounded, subtriangular projections with a notch between them, as opposed to the more well-developed condyles of non-sulcimentisaurian silesaurids (Ezcurra et al. 2020b, Nesbitt et al. 2020a). The ulnar condyle is slightly larger than the radial condyle and has a medial margin that is more posteriorly projected. On both the anterior and posterior face of the humerus above the condyles, there is a fossa that extends about 1 cm on to the humeral shaft. The proximal outline of the femur (UWGM 7407) is subtriangular due to the presence of a prominent anterolateral and anteromedial tuber and significant reduction of the posteromedial tuber and straight margins between these features. A straight, transverse groove is present along the proximal surface. A notch is present distal to the head of the femur, which is ubiquitous among silesaurids (e.g. Martz and Small 2019). The dorsolateral trochanter is broken and displaced across the anterolateral face, but what is preserved is ridge-like. The broken dorsolateral trochanter and surrounding matrix obscure a clear view of the anterior trochanter (= ‘lesser trochanter’), but what is visible is mound-like. However, it cannot be determined if the anterior trochanter was small and confluent with the femoral shaft, as in non-sulcimentisaurian silesaurids, or if it was large and offset from the shaft by a cleft, as in sulcimentisaurs, some early core ornithischians, and some neotheropods. The ‘greater trochanter’ is prominent and rounded, and the fossa trochanterica (= facies articularis antitrochanterica) is level with the ‘greater trochanter’. The femoral shaft curves slightly posteriorly, and the femur is broken above the fourth trochanter.