Daspletosaurus wilsoni Warshaw &amp; Fowler 2022, sp. nov.
Creators: name: Warshaw, Elías A.
name: Fowler, Denver W.
Publisher: name: Zenodo
Year: 2022
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Reptilia
Dinosauria
Tyrannosauridae
Daspletosaurus
Daspletosaurus wilsoni

description: <i>D. wilsoni sp. nov.</i> Etymology <i>wilsoni</i>, Latinization of “Wilson,” after John Wilson, the discoverer of the holotype specimen. Holotype BDM 107, preserving a partial disarticulated skull and postcranium, including both premaxillae, a right maxilla, jugal, lacrimal, quadrate, quadratojugal, dentary, and splenial, and a left postorbital and squamosal. Also preserved are partial cervical, sacral, and caudal series, a rib, a chevron, and a first metatarsal. Cranial bones are very finely preserved, with intricate and detailed surface textures especially on the maxilla and postorbital, with teeth preserved in the maxilla, dentary, and one premaxilla. The sacral and caudal centra are preserved in a heavy and hard concretion and are not yet prepared. The holotype specimen is stored in the collections of the Badlands Dinosaur Museum (BDM) in Dickinson, North Dakota. Geological Setting BDM 107 was recovered from the site “Jack’ s B2,” discovered in 2017 by John Wilson in exposures of the Judith River Formation near Glasgow (Valley County, MT, USA). This is significantly further east than classic ‘Judith’ localities (Fig. 1), and is sedimentologically atypical, representing distal floodplain and delta sediments deposited during the maximum Campanian regression of the Western Interior Seaway. Here, the Judith River Formation is up to ~48 m thick, with the “Jack’ s B2” site occurring ~30 m below the contact with the overlying Bearpaw Shale. Precise stratigraphic placement of this easternmost Judith is currently unclear, although an age of ~76.5 Ma seems most likely, which would correlate in time with the lower to middle part of the Dinosaur Park Formation, Alberta (<i>Eberth, Currie &amp; Koppelhus, 2005</i>; <i>Fowler, 2017</i>). A youngest age limit of 75.64 Ma (<i>Ogg &amp; Hinnov, 2012</i>) is delineated by ammonites tentatively identified as <i>Didymoceras stevensoni</i> (J. Slattery, 2020, personal communication) collected by BDM from local outcrops of the overlying Bearpaw Shale (although these were not at the base of the Bearpaw, so older ammonite specimens may be encountered during future prospecting). At present, more precise stratigraphic position can be inferred from the timing of the maximum regression of the Western Interior Seaway during the Campanian (correlated with the R8 regression of <i>Kauffman (1977)</i> and <i>Rogers et al. (2016)</i>). In Alberta and Saskatchewan, the Foremost, Oldman, and Dinosaur Park formations represent early to late subcycles (respectively) of the R8 regression, and of these, the Foremost (~80.5–79.5 Ma) and lower Oldman (~79.5–79.0 Ma; and regional equivalents) are restricted to the west (Alberta and west central Montana), and did not extend as far east as Saskatchewan or our study area in eastern Montana (<i>Eberth, Currie &amp; Koppelhus, 2005</i>). During late R8, the upper Oldman (~77.5–77.0 Ma) and Dinosaur Park (~76.9–76.0 Ma) Formations were deposited much further to the east, with the lowermost Dinosaur Park recording the R8 maximum regression at ~76.9–76.4 Ma (<i>Eberth, Currie &amp; Koppelhus, 2005</i>; <i>Fowler, 2017</i>). This correlates well with the Judith River Formation of Montana, where <i>Rogers et al. (2016)</i> show the maximum regression of R8 occurring shortly before 76.2 Ma, based on radiometric dates acquired either side of the mid-Judith discontinuity. As such, it seems likely that the study section corresponds in age to the lower to middle part of the Dinosaur Park Formation (although not necessarily lithostratigraphically correlated). A radiometric analysis of a newly discovered volcanic ash is currently underway, and it is hoped that this will provide definitive stratigraphic placement. Regardless of the precise age of BDM 107, it can be expected to lie intermediate stratigraphically between <i>D. torosus</i> (known from the upper Oldman Formation, ~77.0 Ma; <i>Paulina Carabajal et al., 2021</i>) and <i>D. horneri</i> (known from the Two Medicine Formation, ~75.0 Ma; <i>Carr et al., 2017</i>). Diagnosis <i>D. wilsoni</i> can be assigned to <i>Daspletosaurus</i> based on the following characteristics: extremely coarse subcutaneous surface of the maxilla with no elevated ridges or corresponding fossae (<i>Carr et al., 2017</i>; <i>Voris et al., 2020</i>); cornual process of the postorbital approaching the laterotemporal fenestra (<i>Carr et al., 2017</i>); dorsal postorbital process of the squamosal terminating caudal to the rostral margin of the laterotemporal fenestra (<i>Carr et al., 2017</i>; <i>Voris et al., 2019</i>); and extremely coarse symphyseal surface of the dentary (<i>Voris et al., 2020</i>). <i>D. wilsoni</i> possesses a single autapomorphy: a rostrocaudally elongate and dorsoventrally narrow mylohyoid foramen of the splenial (this foramen is much deeper in other <i>Daspletosaurus</i>, <i>Carr et al., 2017</i>; see below), and can additionally be diagnosed by a unique combination of ancestral and derived <i>Daspletosaurus</i> characteristics. <i>D. wilsoni</i> and <i>D. torosus</i> share a pneumatic inflation of the lacrimal reaching the medial edge of the bone (this inflation does not reach the medial edge of the bone in the holotype of <i>D. horneri</i>, but this may represent an allometric, ontogenetic, or taphonomic bias; E. A. Warshaw, 2022, unpublished data; <i>Carr et al., 2017</i>), cornual process of the postorbital approaching the laterotemporal fenestra (this process terminates much more rostrally relative to the fenestra in <i>D. horneri</i>, <i>contra Carr et al., 2017</i>; see below), cornual process of the postorbital subdivided into two distinct processes (this subdivision is absent in <i>D. horneri</i>, E. A. Warshaw, 2022, personal observations; see Description); prefrontal oriented rostromedially (determined from the angle of the prefrontal articular surface on the lacrimal of the holotype of <i>D. wilsoni</i>, which does not preserve a prefrontal; the prefrontal of <i>D. horneri</i> is oriented mediolaterally), pneumatic excavation of the squamosal that does not undercut its rostromedial margin (entire margin undercut in <i>D. horneri</i>; <i>Carr et al., 2017</i>), and quadratojugal lacking a pneumatic foramen in its lateral surface (although the presence of this foramen is highly intraspecifically variable in both <i>D. horneri</i> and <i>Tyrannosaurus</i>, such that further discoveries of <i>D. wilsoni</i> individuals may reveal its presence in this taxon; <i>Carr et al., 2017</i>; <i>Carr, 2020</i>). <i>D. horneri</i> and <i>D. wilsoni</i> share, to the exclusion of <i>D. torosus</i>, a premaxillary tooth row oriented entirely mediolaterally, such that all but one premaxillary tooth is concealed in lateral view (rostromedial orientation in <i>D. torosus</i> and less derived tyrannosaurids), antorbital fossa of the maxilla terminating at the rostral limit of the external antorbital fenestra (this fossa extends ahead of this boundary onto the subcutaneous surface of the maxilla in <i>D. torosus</i> and less derived tyrannosaurids; <i>Carr et al., 2017</i>; E. A. Warshaw, 2022, unpublished data), rostrodorsal ala of the lacrimal inflated (uninflated in <i>D. torosus</i> and less derived tyrannosaurids), ventral ramus of the lacrimal longer than the rostral ramus (determined largely by the height of the postorbital bar in the reconstructed skull, given that the ventral ramus is largely unpreserved in the holotype of <i>D. wilsoni</i>; the rostral ramus of the lacrimal is longer than the ventral ramus in <i>D. torosus</i>; <i>Carr et al., 2017</i>), short cornual process of the lacrimal (tall in <i>D. torosus</i>, although this process is taller in <i>D. wilsoni</i> than <i>D. horneri</i> and may best be described as intermediate between the previously named species of this genus; <i>Carr et al., 2017</i>), and dorsal quadrate contact of the quadratojugal visible in lateral view (concealed in <i>D. torosus</i> and less derived tyrannosaurids). Description Given the wealth of detailed osteologies describing tyrannosaurine specimens (<i>e.g</i>., <i>Carr (1999)</i>; <i>Brochu (2003)</i> and <i>Hurum &amp; Sabath (2003)</i>), our description of the holotype of <i>D. wilsoni</i> places heavy emphasis on characteristics (or combinations of characteristics) unique to this specimen, as well as those that are otherwise taxonomically or phylogenetically informative within Tyrannosaurinae, so as to avoid the reiteration of plesiomorphic tyrannosaurine morphologies (or synapomorphies of <i>Daspletosaurus</i>) already described by previous authors (<i>e.g</i>., <i>Carr et al. (2017)</i>; <i>Voris et al. (2019)</i> and <i>Voris et al. (2020)</i>). <i>Ontogenetic Stage of BDM 107</i> In order to facilitate comparison with other tyrannosaurine individuals of equivalent ontogenetic stages (and in doing so, to avoid the misattribution of a phylogenetic signal to ontogenetically derived characteristics), brief comment is warranted on the ontogenetic stage represented by BDM 107; two lines of evidence suggest that this specimen is of advanced ontogenetic age. Firstly, BDM 107 is among the largest known <i>Daspletosaurus</i> individuals (articulated skull length 105 cm; <i>D. torosus</i> holotype CMN 8506 skull length 104 cm, <i>Voris et al., 2019</i>; <i>D. horneri</i> holotype MOR 590 skull length 89.5 cm, <i>Carr et al., 2017</i>). Although <i>Carr (2020)</i> criticized the use of size as an indicator of ontogenetic status in <i>Tyrannosaurus</i>, this criticism was based on the absence of a correlation between size and maturity among adult individuals; all the largest specimens of this genus were unambiguously recovered as adult by <i>Carr’ s (2020)</i> analysis (<i>i.e</i>., within the final stages of ontogenetic development), such that this feature remains ontogenetically informative in distinguishing adults from juveniles and subadults. Secondly, BDM 107 displays several morphologies known otherwise to characterize mature tyrannosaurines, including a deeply scalloped maxilla-nasal suture (<i>Carr &amp; Williamson, 2004</i>; <i>Carr, 2020</i>), a maxillary fenestra positioned rostrally within the antorbital fossa (<i>Carr, 2020</i>), a cornual process of the lacrimal inflated and positioned dorsal to the ventral ramus (<i>Carr, 1999</i>; <i>Currie, 2003</i>; <i>Carr, 2020</i>), and a grossly exaggerated cornual process of the postorbital (<i>Carr, 1999</i>; <i>Currie, 2003</i>; <i>Voris et al., 2019</i>; <i>Carr, 2020</i>). The totality of this evidence supports an adult ontogenetic stage or later for BDM 107 (adult <i>sensu Carr, 2020</i>; ontogenetic Stage 4 <i>sensu Carr, 1999</i>); this hypothesis may be tested in future work through histological analysis and/ or comparison with further discoveries of <i>D. wilsoni</i> individuals of different ontogenetic stages, both of which lie outside of the scope of the present study. <i>Premaxilla</i> The premaxillae of <i>D. wilsoni</i> are similar to those of <i>D. horneri</i> (<i>Carr et al., 2017</i>, Fig. 1; Fig. 2), <i>Tarbosaurus</i> (<i>Hurum &amp; Sabath, 2003</i>, Fig. 3), and <i>Tyrannosaurus</i> (<i>Brochu, 2003</i>, Fig. 4) in that the alveolar row is oriented largely mediolaterally, such that the rostrum of the skull is broad and the labial surfaces of the premaxillary teeth face rostrally. In <i>Tyrannosaurus</i> and similarly derived tyrannosaurines (<i>Tarbosaurus</i> and <i>D. horneri</i>), the premaxillary teeth largely overlap each other in lateral view such that only the distalmost tooth is clearly visible; the same would be true of the holotype of <i>D. wilsoni</i>, were more than a single premaxillary tooth preserved within its socket. Conversely, the premaxillary tooth row of <i>D. torosus</i> and less derived tyrannosauroids is oriented rostromedially, such that multiple teeth are clearly visible in lateral view (<i>Voris et al., 2019</i>, Fig. 6). Although previous authors have regarded a mediolaterally oriented premaxillary tooth row as a synapomorphy of Tyrannosauridae or more inclusive groups (<i>e.g</i>., <i>Carr et al., 2017</i>: character 15), this is in error; mature specimens of <i>Gorgosaurus</i> (UALVP 10, <i>Voris et al., 2022</i>, Fig. 1; AMNH 5458, <i>Matthew &amp; Brown, 1923</i>, Fig. 2) and <i>Qianzhousaurus</i> (GM F10004, <i>Foster et al., 2021</i>, Fig. 2), have rostromedially oriented premaxillary tooth rows such that in specimens with preserved teeth, all premaxillary teeth are visible in lateral view (although all tyrannosaurids do have premaxillary tooth rows oriented more medially than basal tyrannosauroids; this is the phylogenetic signal recorded in character 15 of <i>Carr et al. (2017)</i>). Comparison with other tyrannosaurids is hampered by the absence of preserved premaxillae and/or published descriptions of this element for several species (<i>e.g</i>., <i>Thanatotheristes</i>, <i>Voris et al., 2020</i>; <i>Dynamoterror</i>, <i>McDonald, Wolfe &amp; Dooley, 2018</i>; <i>Nanuqsaurus</i>, <i>Fiorillo &amp; Tykoski, 2014</i>; <i>Lythronax</i> and <i>Teratophoneus</i>, <i>Loewen et al., 2013</i>, for which all published specimens lack premaxillae); however, <i>D. wilsoni</i> and more derived tyrannosaurines (<i>D. horneri</i>, <i>Tarbosaurus</i>, <i>Tyrannosaurus</i>) represent the greatest exaggeration of the medial inclination of the premaxillary tooth row among tyrannosaurids for which comparative material is available (although this condition, with only one clearly visible premaxillary tooth in lateral view, is present in at least one <i>Gorgosaurus</i>: TCMI 2001.89.1, <i>Voris et al., 2022</i>, Fig. 10). <i>D. torosus</i> is intermediate between the (presumably) ancestral rostromedial orientation and the mediolateral condition of later <i>Daspletosaurus</i> species; two to three premaxillary teeth are visible in lateral view in the holotype specimen, CMN 8506 (<i>Carr &amp; Williamson, 2004</i>, Fig. 6; <i>Voris et al., 2019</i>, Fig. 6). It should be noted that the orientation of the premaxillary tooth row is not necessarily equivalent to the orientation of the premaxillae themselves. In <i>Tyrannosaurus</i> AMNH 5027, for example, the premaxillae appear to be rostromedially oriented in dorsal view (<i>Carr &amp; Williamson, 2004</i>, Fig. 7); however, the premaxillary alveoli are mediolaterally arranged when viewed ventrally (E. A. Warshaw, 2022, personal observations; <i>Osborn, 1912</i>, Fig. 5A; <i>Molnar, 1991</i>, Fig. 9A). The taxonomic utility of this character is a hypothesis that will require further testing as individuals of <i>D. wilsoni</i> and other tyrannosaurids with preserved premaxillae are discovered; notably, two specimens previously referred to <i>D. torosus</i> display the derived condition (mediolateral orientation), sharing it with <i>D. wilsoni</i> and more derived tyrannosaurines: FMNH PR308 (<i>Matthew &amp; Brown, 1923</i>, Fig. 5; <i>Carr, 1999</i>, Fig. 1) and TMP 2001.36.1 (<i>Voris et al., 2019</i>, Fig. 6). If these individuals were to represent <i>D. torosus</i>, the distinction between this species and <i>D. wilsoni</i> in the orientation of the premaxillary tooth row would be heavily undermined; however, both of these specimens have previously been noted as belonging to a novel taxon from the Dinosaur Park Formation (FMNH PR308, <i>Currie, 2003</i>; TMP 2001.36.1, <i>Paulina Carabajal et al., 2021</i>). Therefore, although relevant comparisons will be made with these specimens hereafter, they will be considered separately from <i>D. torosus</i> (and will be referred to below as the Dinosaur Park taxon). A precise taxonomic designation for these specimens is reserved for future work in accordance with comments by previous authors (<i>Currie, 2003</i>; <i>Paulina Carabajal et al., 2021</i>). There is a small (~2 cm diameter) indentation in the nasal process of the right premaxilla of BDM 107; this is most likely pathological, as it is irregular in form and not present on the left premaxilla. <i>Maxilla</i> As in other <i>Daspletosaurus</i>, the subcutaneous surface of the maxilla in <i>D. wilsoni</i> is densely covered in anastomosing sulci extending from neurovascular foramina (<i>Carr et al., 2017</i>; <i>Voris et al., 2020</i>; Fig. 3). The degree of sculpturing of this surface in BDM 107 is similar to CMN 8506 (<i>D. torosus</i>), although in the former, there is no smooth region rostral to the external antorbital fenestra indicating a rostral continuation of the antorbital fossa as <i>D. torosus</i> and alioramins (<i>Carr et al., 2017</i>). As in <i>Thanatotheristes</i> and other <i>Daspletosaurus</i> species, the shallow excavations that characterize the maxillae of the most derived tyrannosaurines (<i>Zhuchengtyrannus, Tyrannosaurus</i>, <i>Tarbosaurus</i>; <i>Hone et al., 2011</i>; <i>Voris et al., 2020</i>) are absent from the holotype maxilla of <i>D. wilsoni</i>. Also absent are the textural ridges present on the maxillae of <i>Zhuchengtyrannus</i> (<i>Hone et al., 2011</i>), <i>Tarbosaurus</i>, <i>Tyrannosaurus</i>, and <i>Thanatotheristes</i> (<i>Voris et al., 2020</i>), but not any <i>Daspletosaurus</i> species. The rostral end of the maxilla of BDM 107 is bowed subtly medially towards its contact with the premaxilla and nasal; this may be a structural consequence of the greater medial inclination of the premaxillary tooth row (see above), as a similar condition characterizes <i>D. horneri</i> (MOR 590, E. A. Warshaw, 2022, personal observations), <i>Tarbosaurus</i> (<i>Hurum &amp; Sabath, 2003</i>, Fig. 15), and <i>Tyrannosaurus</i> (MOR 008, MOR 980, E. A. Warshaw, 2022, personal observations). Tyrannosaurids with more rostromedially inclined premaxillary tooth rows lack this bowing (<i>e.g</i>., <i>D. torosus</i> CMN 8506, J. T. Voris, 2022, personal communication). The maxilla of BDM 107 is irregular relative to other species of <i>Daspletosaurus</i> in that it is proportionally elongate, being 64.1 cm in length and 24.8 cm in height (ratio of length to height = 2.6). This bone is 58.6 cm long rostrocaudally and 27.5 cm tall dorsoventrally in the holotype of <i>D. horneri</i> (ratio of length to height = 2.1; MOR 590, <i>Carr et al., 2017</i>). Given the broad range of variation in the proportions of this element in other tyrannosaurine species for which larger sample sizes are known (<i>e.g</i>., <i>Tyrannosaurus</i>; <i>Carpenter, 1990</i>; <i>Paul, Persons &amp; Van Raalte, 2022</i>; E. A. Warshaw, 2022, personal observations), this characteristic was not included as an autapomorphy of <i>D. wilsoni</i>. Consistency in this trait across further discoveries of <i>D. wilsoni</i> individuals may require a reevaluation of the taxonomic utility of this character. <i>D. wilsoni</i> possesses 15 maxillary alveoli, as in other species of <i>Daspletosaurus</i> (<i>Carr et al., 2017</i>). The 13 th alveolus bears a swollen abscess in BDM 107, and the 15 th maxillary tooth conceals a small replacement tooth within its root that is visible in medial (lingual) view. In general, the maxillary teeth are similar to those of other tyrannosaurid species in being labiolingually broad, although not to the degree present in more derived tyrannosaurines (<i>e.g</i>., <i>Tyrannosaurus</i> and <i>Tarbosaurus</i>), in which the labiolingual width of the maxillary teeth is subequal to their mesiodistal length (<i>Carr et al., 2017</i>). The first maxillary alveolus is not small and also bears an incrassate tooth (<i>i.e</i>., it does not bear a d-shaped crown similar to those present in the premaxillae, as in <i>Gorgosaurus</i>; <i>Currie, 2003</i>; <i>Voris et al., 2022</i>).&lt;
description: Published as part of <i>Warshaw, Elías A. &amp; Fowler, Denver W., 2022, A transitional species of Daspletosaurus Russell, 1970 from the Judith River Formation of eastern Montana, pp. 1-29 in PeerJ (e 14461) (e 14461) 10</i> on pages 2-19, DOI: 10.7717/peerj.14461, http://zenodo.org/record/7372806
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