Thanatotheristes Voris & Therrien & Zelenitsky & Brown 2020

<i>Thanatotheristes degrootorum</i> gen. et. sp. nov. ZOOBANK ID: urn:lsid:zoobank.org:act: 581C0612-B697-4D01- 915C-E3E937D3667A <i>Etymology.</i> From <i>Thanatos</i>, the Greek god of and embodiment of death, and <i>theristes</i> (Greek), one who reaps or harvests, a reaper. The specific name, <i>degrootorum</i>, is in honor of John and Sandra De Groot of Hays, Alberta, who discovered the holotype specimen and have been supportive of paleontological research in the area. <i>Holotype.</i> TMP 2010.5.7 (Fig. 2), a partial skull from a large subadult individual (estimated skull length <b>¼</b> ~ 800 mm, based on frontal width, metrics from Currie, 2003a) that includes the right maxilla, right jugal, right postorbital, right surangular, right quadrate, right laterosphenoid, left frontal, and both dentaries. Immature bone grain (most apparent within the antorbital fossa of the maxilla) indicates the specimen was not osteologically mature at the time of death (Carr, 1999), but the well-developed nature of ornamentation on the maxilla suggests the individual was probably sexually mature (Knell et al., 2013; Hone et al., 2016). <i>Type locality and horizon.</i> TMP 2010.5.7 was found at the base of a steep cliff on the eastern shore of the Bow River approximately 0.5 km upstream of the confluence with the Oldman River (i.e., The Forks) in Alberta, Canada (Fig. 3A). The cliff is approximately 11 m in height and consists of exposures of the Taber Coal Zone, below the Herronton Sandstone, of the Foremost Formation (Fig. 3B). The exact stratigraphic provenance of the specimen within the Taber Coal Zone is unknown, as the skull was found scattered along the shoreline over a lateral extent of ~ 10 m, but the specimen likely derived from the lower half of the cliff due to a small-scale landslide (Fig. 3C). <i>Referred specimen.</i> TMP 2018.16.1, a partial right maxilla from a subadult individual. TMP 2018.16.1 was found along Twelve Mile Coulee, approximately 10 km northeast of Hays, Alberta, stratigraphically high within the Herronton Sandstone of the Foremost Formation. <i>Diagnosis.</i> Tyrannosaurine tyrannosaurid diagnosed by the following autapomorphies: single row of evenly-spaced dorsoventrally-oriented ridges on the subcutaneous surface of the maxilla ventral and anteroventral to the antorbital fossa; rounded and inflated orbital margin of the jugal; sagittal crest on the frontal extends anterior to the supratemporal ridge as a broad and rounded ridge; lacrimal contact surface on the frontal extends anteromedially at ~60 relative to interfrontal suture; prefrontal with two posteriorly-projecting prongs articulating with the frontal on ventral surface of skull roof (long, medial, primary prong and shorter secondary, lateral prong). 5. Description <b>Maxilla</b> -The partial right maxilla of the holotype specimen, TMP 2010.5.7, consists of a complete anterior body and two fragments of the jugal ramus, one of which preserves the three posterior-most alveoli and the groove-like primary posterior foramen of the maxillary nerve and the second preserves a single alveolus (Fig. 4). The anterior body is deep, consistent with the proportions of other medium to large-sized tyrannosaurines. The anteroventral corner of the bone is slightly downturned and tapers into a shallow angle, as in <i>Daspletosaurus</i>. The referred maxilla (TMP 2018.16.1) consists of the subcutaneous surface ventral to the anterior half of the antorbital fossa (Fig. 5). The subcutaneous surface of both specimens is extremely rugose, particularly anteroventral to the antorbital fossa where it is incised by deep anastomosing sulci, as in <i>Daspletosaurus</i> (Carr et al., 2017). The subcutaneous texturing differs from that of <i>Daspletosaurus</i> and all other tyrannosaurids in the presence of a single row of prominent and evenly spaced dorsoventrally-oriented ridges located ventral and anteroventral to the antorbital fossa (Figs. 4 and 5). In <i>Tyrannosaurus</i>, <i>Tarbosaurus</i>, and <i>Zhuchengtyrannus</i>, the subcutaneous surface is ornamented with at least two rows of unevenly spaced and anastomosing ridges that circumferent shallow oval fossae (Hone et al., 2011; Carr et al., 2017); such fossae are absent in <i>Thanatotheristes</i>. The ventral margin of the promaxillary fenestra in TMP 2010.5.7 appears as a wide concavity in the far anterior corner of the antorbital fossa in lateral view (Fig. 4). The presence of a short base of the preantral strut, however, suggests the exposure of the promaxillary fenestra in lateral view is the result of taphonomic postburial deformation, and that the promaxillary fenestra would have been concealed in lateral view, as in derived tyrannosaurines (e.g., <i>Daspletosaurus</i>, <i>Tarbosaurus</i>, <i>Tyrannosaurus</i>; Brusatte and Carr, 2016; Carr et al., 2017). The ventral margin of the maxillary fenestra is broken in TMP 2010.5.7 but its location can be determined based on the thinness of the bone between two thickened areas representing the base of the interfenestral and preantral struts. The ventral margin of the maxillary fenestra is widely separated from the ventral margin of the antorbital fossa, as in albertosaurines, alioramins, <i>Teratophoneus</i>, and juvenile individuals of the genera <i>Daspletosaurus</i> and <i>Tyrannosaurus</i> (Carr, 1999; Carr et al., 2017). This contrasts with the condition seen in subadult and adult <i>Daspletosaurus</i> individuals, where the ventral margin of the maxillary fenestra is closer to the ventral margin of the antorbital fossa (Brusatte et al., 2012), and in subadult and adult individuals of more derived tyrannosaurines (<i>Tyrannosaurus</i>, <i>Tarbosaurus</i>, <i>Zhuchengtyrannus</i>), where the maxillary fenestra is in contact with the margin of the antorbital fossa (Carr et al., 2017). The neurovascular foramina of the maxilla are large in TMP 2010.5.7 and occur in two distinct rows: a dorsal circumfenestral row inset into an elongate fossa and a ventral alveolar row. Anteriorly, the circumfenestral foramina diverge from the linear arrangement with individual foramina becoming randomly dispersed. Several deep and anteroventrally-oriented neurovascular sulci originate from these anterior circumfenestral foramina. The posterior-most foramen of the maxilla exits the subcutaneous surface between the two posterior-most alveoli and continues posteriorly as an elongate neurovascular sulcus, as seen in <i>Lythronax</i>, <i>Daspletosaurus</i>, and <i>Tyrannosaurus</i>. In contrast, the foramen is located posterior to the posterior-most alveolus in albertosaurines. The jugal ramus of the maxilla in TMP 2010.5.7 is dorsoventrally constricted at the level of the two posterior alveoli, such that the posterior-most maxillary foramen lies near the dorsal margin of the subcutaneous surface (Fig. 4), as in <i>Daspletosaurus</i>. In mature specimens of all other tyrannosaurids, the jugal ramus gradually tapers posteriorly and the posterior-most maxillary foramen is widely separated from the dorsal margin of the subcutaneous surface. In total, the anterior body of the maxilla and posterior jugal ramus fragments of TMP 2010.5.7 preserve 12 teeth. However, as these fragments do not articulate with each other and are broadly separated, it is likely that the individual possessed at least 15 maxillary teeth, indicating the animal possessed a long snout (Fig. 4), as in <i>Daspletosaurus</i>. The base of the tooth crowns are marginally narrower than long (fore-aft base length <i>&gt;</i> crown width). The mesial carinae terminate distal to the base of the crown, which differs from that reported for specimens of <i>Daspletosaurus</i> in which the carinae extend to the base of the crown (Carr and Williamson, 2000). The subcutaneous surface of TMP 2010.5.7 is incised by a prominent anteroventral bite mark located immediately ventral to the antorbital fossa (Type 2: sensu Tanke and Currie, 1998). Orientation of the healed bone grain indicates the bite originated at the dorsal extent of the scar and was dragged downward to the alveolar margin at the anterior edge of the sixth alveolus (Fig. 4). Several possible long-healed Type 1 bite marks (sensu Tanke and Currie, 1998) adorn the subcutaneous surface more anteriorly. <b>Jugal</b> -The jugal of TMP 2010.5.7 is represented by a fragment of the suborbital region of the bone and a natural endocast of the jugal sinus (Fig. 6A and Fig. S3). Despite the fragmentary nature of this bone, the jugal exhibits several features that permit comparison with other tyrannosaurids. The orbital margin of the jugal is deeply concave and is inclined steeply anterodorsally towards the dorsoventrally-expanded maxillary ramus. The orbital margin is wide and has a rounded edge, resulting in an oval cross-section of the suborbital region (Fig. 6A). This morphology differs from that of most other tyrannosaurids (including <i>Daspletosaurus</i>), where the orbital margin has a tapered blade-like edge, resulting in a tear drop-shaped crosssection (Fig. 6B). In <i>Lythronax</i> (UMNH VP 20200) and some specimens of <i>Tyrannosaurus rex</i> (TMP 1981.6.1), the jugal suborbital margin is rounded but narrow, differing from the wider margin of <i>Thanatotheristes</i>. The jugal sinus is greatly inflated in <i>Thanatotheristes</i>, presumably related to the rounder cross-section of the element. The sinus is of consistent depth within the suborbital region, possessing roughly parallel dorsal and ventral margins (Fig. 6A), in contrast to <i>Alioramus</i> (Gold et al., 2013:fig. 5), <i>Gorgosaurus</i> (TMP 2000.12.11), and <i>Daspletosaurus</i> (Fig. 6B), where the suborbital region of the jugal sinus is dorsoventrally-constricted and hourglass-shaped in lateral view. The lateral surface of the jugal is inflated into a rounded bulge as in <i>Daspletosaurus</i> and most other tyrannosaurids, but unlike in <i>Gorgosaurus</i> and <i>Bistahieversor</i> where it is raised in

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