Ardetosaurus viator Linden, Tschopp, Sookias, Wallaard, Holwerda & Schulp, 2024, gen. et sp. nov.

Ardetosaurus viator gen. et sp. nov. Holotype. MAB011899: two cervical vertebrae, 10 dorsal vertebrae, sacrum, five caudal vertebrae, eight dorsal ribs, two chevrons, a left coracoid, a left ilium, both pubes, both ischia, a left femur, a left tibia, and a partial left fibula. Diagnosis. Ardetosaurus viator is diagnosed by the combination of the following autapomorphies: 1) the presence of distinct, paired accessory laminae in the spinoprezygapophyseal fossae (SPRF) in the posterior cervical and anterior dorsal vertebrae, 2) anteroventrally bifurcating anterior centrodiapophyseal laminae (ACDLs) in the anterior dorsal vertebrae, 3) the presence of centropostzygapophyseal lamina fossae (CPOL-f) in the second dorsal vertebra, 4) a vertebral height/centrum length ratio of <2.5 of the posterior dorsal vertebrae, and 5) reduced or absent centroprezygapophyseal laminae (CPRLs) in the anterior-most caudal vertebrae. Ardetosaurus viator differs from all other diplodocines by having unbifurcated CPRLs in the posterior cervical vertebrae and a highly elliptical femoral cross-section. Ardetosaurus viator differs from Amphicoelias Cope, 1878, in lacking the rounded, lateral projections of the neural spine tip and the thin neural spine base in the dorsal vertebrae; from Barosaurus Marsh, 1890, by having tall cervical neural spines, single midline keels, narrower prezygapophyseal rami in the cervical vertebrae, ten dorsal vertebrae, the presence of infradiapophyseal foramina in the dorsal vertebrae, unbifurcated caudal neural spines and a posterodorsally expanded distal end of the ischia; from Diplodocus by having more elongated posterior cervical vertebrae, postzygapophyses that terminate in front of the cotyle edge, the presence of a prespinal lamina (PRSL) and laterally inclined spinoprezygapophyseal laminae (SPRLs) in the posterior cervical vertebrae, the absence of the midline cleft in the dorsal vertebrae and the presence of lateral projections on caudal neural spines tips; from Galeamopus Tschopp et al., 2015a, by having posteriorly projecting interpostzygapophyseal laminae (TPOL) in the posterior cervical vertebrae, strongly opisthocoelous anterior dorsal vertebrae, and the absence of a second cnemial crest in the tibia; from Kaatedocus in lacking the rugose tuberosities and transverse sulci posterior to the prezygapophyses, by having a wider gap between the metapophyses, and postzygapophyses which terminate in front of the rim of the cotyle of the cervical vertebrae; from Leinkupal in having procoelous-distoplatyan caudal vertebrae without distinct pleurocoels and with less developed transverse processes; from Tornieria in lacking strongly procoelous anterior caudal vertebrae, having a mildly concave ischial acetabular surface, having elongate lateral fossae on the ischial shaft and a more transversely expanded distal end of the ischia; and from Supersaurus Jensen, 1985 by being much smaller (S. vivianae), the presence of bifurcated neural spines in the cervical vertebrae (S. lourinhanensis), the absence of distinct grooves posterolateral to the parapophyses, and paired fossae lateral to the midline keel in the cervical vertebrae, anteriorly inclined neural spines, horizontal transverse processes, and less persistent bifurcation along the series in the dorsal vertebrae, as well as lacking pneumatic foramina and oblique ridges on the thoracic rib heads (both species). Etymology. ‘Ardeto’ is an inflection of Latin ārdēre, meaning ‘to burn.’ It refers to the history of some of the elements, which were either fully destroyed in a fire, or still show burn scars from the fire. ‘saurus,’ Latinized form of the Greek σαῦρος (saúros), meaning lizard or reptile. ‘viator’ is Latin for traveler, referring to the journey of the specimen from the USA, via Switzerland and Germany, to the Netherlands. Locality and horizon. Ardetosaurus viator comes from the Howe-Stephens Quarry of northern Wyoming, USA. The quarry is dated, based on magnetostratigraphy and correlation with other sections in the Morrison basin (Maidment and Muxworthy, 2019; Maidment, personal communication, 2022) at 150.44 to 149.21 million years old, placing it in the Kimmeridgian Stage of the Upper Jurassic. DESCRIPTION OF MAB011899 Terminology Terminology from Wilson (1999) and Wilson (2012) is used for the vertebral laminae, Wilson et al. (2011) for the vertebral fossae, and Wilson (2011) for the sacrum. For the definitions of the positional terms for the vertebrae, table 3 from Tschopp et al. (2015a) is followed. Following Tschopp et al., (2015a), Wilson (2002), and Upchurch (1998) and many other authors, ‘anterior’ and ‘posterior’ is preferred over ‘cranial’ and ‘caudal’. As suggested by Tschopp and Mateus (2013), interpre- and interpostzygapophyseal lamina is here preferred over the terms intrapre- and intrapostzygapophyseal lamina of Wilson (1999), as both laminae are positioned in between their respective zygapophyses. Cervical ribs are fused to their respective vertebrae, whereby the tuberculum fuses with the diapophysis and the capitulum fuses with the parapophysis. This creates a structure known as the ansa costotransversaria in birds, but in non-avian dinosaurs, such as sauropods, it is referred to simply as the ‘cervical rib loop’ (Taylor and Wedel, 2013). AXIAL SKELETON The Cervico-Dorsal Transition and Presacral Neural Spine Bifurcation Patterns Identifying the first dorsal vertebra is important, because the precise location of the cervico-dorsal transition (Tschopp and Mateus, 2017) or junction (Taylor, 2022) has implications for the biomechanics of the animal, as well as for phylogenetic analyses and morphological comparisons. Tschopp and Mateus (2017) discuss several characteristics that could be informative regarding the transition in diplodocids, whereas Taylor (2022) broadened the discussion towards all sauropods. In MAB011899, 16 presacral vertebrae were recorded in the quarry map, which were articulated with the sacrum. Twelve of these vertebrae are still preserved today. The elongation of the anterior-most, preserved presacral vertebrae and the presence of cervical ribs clearly demonstrate that they are cervical vertebrae (Figure 5). Based on photographs taken during preparation (Figure 5C and 5D), the two cervical vertebrae and the 10 other, articulated presacral vertebrae, were separated from each other by one additional vertebra, which also bore cervical ribs. The anterior-most element of the 10 articulated presacral vertebrae is identified as the first dorsal vertebra based on a combination of the following characteristics: 1) the assumed general vertebral count for diplodocids (n=25, 15 CV, 10 DV; Hatcher, 1901; Huene, 1929; see Tschopp and Mateus, 2017; Taylor, 2022), thus the position within the axial column as seen in Figure 2; 2) the lack of a fused rib, whereas more anterior elements clearly have fused cervical ribs; 3) the location of the parapophysis ventral to the pleurocoel, which follows a similar, gradual transition as in Diplodocus carnegii (Hatcher, 1901); and 4) the distinct shortening from DV2 to DV3 as also occurs in Diplodocus and Barosaurus (Hatcher, 1901; McIntosh, 2005). Hence, assuming a total of 25 presacral vertebrae, we interpret the preserved presacral vertebrae to represent CV13 and 14, and subsequently DV1 to DV10. Although CV15 is currently missing, neural spine bifurcation is relatively clear for all vertebrae. Following the terminology of Wedel and Taylor (2013), the neural spines of CV13 through DV4 (including CV15) are all deeply bifid, with bifurcation deeper than half of the neural spine length. Shallowing of this bifurcation occurs rapidly, as DV5 is shallowly bifid, DV6 is notched/unsplit (see description of DV6), and DV7-9 and all sacral vertebrae are unsplit, and do no longer show evidence of dorsal midline indentation (no neural spine tip is preserved in DV10). From the cervical to the anterior dorsal vertebrae, the transverse distance increases between the metapophyses, but is never as short as in Suuwassea (Harris and Dodson, 2004). Cervical Vertebrae (Figures 5–7, Table 1) Preservation and orientation. The cervico-dorsal transitional vertebra is missing, as well as the three vertebrae anterior to CV13. There are photographs of CV15; this vertebra will be briefly described based on those photographs. Both CV13 and CV14 are well-preserved, although both vertebrae were subject to transverse compression and shearing, especially the centra. Additionally, all cervical rib loops are now missing from both vertebrae, including the accompanying cervical ribs. However, left lateral photographs (see Figure 5) exist that show the vertebrae with preserved ribs; these have been used to describe and score these parts of the vertebrae. The vertebrae are described with the long axis of the centra parallel to the horizontal. Centrum morphology. Both cervical vertebral centra are strongly opisthocoelous. In posterior view, the articular facet has an oval shape, which is higher than wide. Foramina present in the pleurocoels indicate that pneumatic diverticula along the neck invaded the vertebral centra. Because of compression and shear, the condyle and the cotyle of CV13 have a flattened oval shape, but they were probably subcircular in outline originally, as in Diplodocus, Kaatedocus and Galeamopus (Hatcher, 1901; Tschopp and Mateus, 2013, 2017). The condyle is pronounced, rugose, and bordered by a ridge, which is preserved on the left lateral and dorsal sides of the condyle. The lateral sides of the centra are strongly pneumatized. In CV13, a large pleurocoel, with multiple subfossae, is present on the left lateral surface, extending from the anterior border of the parapophysis to the beginning of the last quarter of the centrum. The pleurocoel is oval, longer anteroposteriorly than dorsoventrally. The anterior-most subfossa is subtriangular and contains two smaller subfossae. The lamina dividing the first and se