Introducing Vulcain

Report on a Large Sauropod Specimen From the Morrison Formation

The Late Jurassic Morrison Formation from the Midwest of the United States is famously known for its huge sauropod diversity (Tschopp et al. 2015, 2019). Until recent fifteen genera of this dinosaur group have been identified as valid taxa. These include the diplodocoids Amphicoelias, Apatosaurus, Barosaurus, Brontosaurus, Diplodocus, Dyslocosaurus, Haplocanthosaurus, Galeamopus, Kaatedocus, Smitanosaurus, Supersaurus, Suuwassea and the macronarians Brachiosaurus, Camarasaurus and Dystrophaeus. Within Diplodocoidea the genera Apatosaurus and Brontosaurus, which are characterized by a more massive body plan, form the clade Apatosaurinae including five species: A. ajax, A. louisae, B. excelsus, B. parvus and B. yahnahpin (Tschopp et al. 2015). Apatosaurus and Brontosaurus are one of the most abundant vertebra taxa in the Morrison Formation, being recorded in over 16% of the known localities (Foster 2003). However, most specimens have only been about 50% complete at best.

The new sauropod specimen was found in 2018 in Johnson County, Wyoming, about 100 km east of Thermopolis (Fig. 1) and excavated in the years 2018 to 2021. The specimen was shipped to France and prepared at the workshop of Nicolas Tourment in Saint-Étienne-les-Orgues. The scientific examination was done at the March 5th to 8th 2024.

General Preservation

The specimen represents an associated skeleton that was found isolated (Fig. 2) and not in a mass accumulation as typical for many dinosaur sites from the Morrison Formation (see e.g., Cleveland Lloyd Dinosaur Quarry, Dinosaur National Monument, Howe Quarry). Below the sacrum of the sauropod of one or two incomplete mid-size theropods were found.

Most skull bones identified come from the right side, allowing to reconstruct most of the cranium (Fig. 3, 6). However, as the bones were already integrated in a skull reconstruction, the actual conditions/ completeness of the bones could not be evaluated in detail. An additional bone fragment found in one of the boxes may represent the buccal margin of the left maxilla (Fig. 6B). Elements of the mandible and braincase are not present. The completeness of presacral axial column is about 96% only missing on anterior dorsal (probably D2). Only a few cervical ribs are prepared so far, but the dorsal ribcage is fairly complete (about 70%). The pectoral girdle includes one possible sternal element, the interclavicle, both scapulae, one coracoid and several sternal ribs (which were originally identified as gastralia). Only the left forelimb is preserved, including several manual bones. The sacrum and pelvis are complete, although the blade of the right ilium is largely missing. The hindlimbs are almost complete, missing the right astragalus and some pedal elements. The tail is missing most of the tail end, but the anterior portion until caudal no. 26 is complete (Fig. 4). In sum, these specimens contain bones from all body regions including skull material and can be seen as one of the most complete apatosaurine specimens from the Morrison Formation (Fig. 5, Table 1), comparable to CM 3018 (type specimen of A. louisae (Gilmore 1936 ; Berman & McIntosh 1978) and “Einstein” (Galiano & Albersdörfer 2010), but more complete than UW 15556 (types of B. parvus) and YPM 1980 (types of B. excelsus) and the A. cf. ajax specimen (NSMT-PV 20375) (Upchurch et al. 2004 ; Galiano & Albersdörfer 2010).

Taxonomic Identification

Based on its gross morphology, this specimen can be assigned to the Diplodocidae, but shows more similarities with Apatosaurinae than Diplodocinae (Table A1, A2). The specimen shows affinities of both Apatosaurus and Brontosaurus (Table A3, A6). However, on the species level, it matches better with Apatosaurus than Brontosaurus, in particular A. ajax. However, only two out of four diagnostic characters could be evaluated (Table A4) on the basis of the material available. The specimen also shares some diagnostic characters of A. louisae (Table A5). However, due to a mismatch with one third of the characters defining A. louisae, a referral to this species is not justified. The specimen does not match with the three Brontosaurus species (Table A7-A9). Therefore, it can be classified as Apatosaurus cf. ajax for the moment. However, the mix of diagnostic characters sharing with A. ajax, A. louisae but also with the genus Brontosaurus could indicate that this specimen may represents an intermediate species between A. ajax and A. louisae. This has to be studied in more detail using the specimen-based phylogenetic approach by Tschopp et al. (2015).

Brief Anatomical Overview

The skeletal anatomy of Apatosaurus was described by Gilmore (1936) and Upchurch et al. (2004) in great detail. Skull material is usually rare among sauropod fossils and only a handful of specimens referred to Apatosaurus and Brontosaurus include skull remains (e.g. BYU 17096, YPM 1840, YPM 1860 and “Einstein”), of which TATE 099 and CM 11162 are fairly complete (Berman & McIntosh 1978 ; Peterson et al. 2022). The skull remains of the specimen are missing the braincase and the lower jaw, but allow to reconstruct most of the right half of the upper skull.

The cervical series is complete, including the atlas and axis (Fig. 7A). The following cervicals usually contain of the centrum and the base of the neural arch. The neural spines are usually incomplete, missing the distal parts. Some vertebrae are dorsoventrally compressed. Due to the fine lamination of the bones, which is related to the pneumatic chambers inside the vertebrae, some bone walls are punctuated and need to be restored. Only few remains of cervical rips are preserved. The dorsal series is almost complete, missing only D2. Although suffering from multiple breakages, the vertebrae could be largely restored. Multiple dorsal ribs are preserved, but have not been prepared, yet. The sacrum is more or less complete, having remains of the right ilium from the acetabular region still attached (Fig. 7B). However, it suffered from some deformation, creating a left-right asymmetry in the orientation of the sacral ribs. The anterior portion of the tail including the chevrons is well preserved. Only the most anterior caudals show some restoration in the neural arch.

Both scapula blades are preserved, missing the acromion. The left scapula also misses its distal end. As typical for Apatosaurus, the ventral margin of the scapula blade is straight and the acromion is situated in the anterior third of the scapula. Only the right coracoid is preserved, but its anterodorsal margin is incomplete. Importantly, the pectoral girdle also preserves the interclavicle, one sternal plate and several sternal ribs (labelled as gastralia) (see Tschopp & Mateus 2013) (Fig. 7C, E, F). Only the left forelimb is preserved, including one large carpal bone (Fig. 7G) and several manual bones, including Mc I. The restoration is minimal and affects primarily the joint regions of the long bones.

The left ilium is complete with a nicely preserved pubic peduncle, showing only some minor damage along the margins (Fig. 8A). The right pubis is more or less complete, showing only some minor damage in the region of the ischial peduncle (Fig. 8B). In the left pubis, the ischial peduncle is broken off, but the rest of the bone is complete. The right ischium is fully complete, but the midshaft of the left one need to be restored. Both hindlimb are present. The left hindlimb is better preserved, showing only minor restoration (Fig. 8C, D). The left tibia articulates still with the astragalus (Fig. 8D). The right femur is missing the midsection of the shaft and the right fibular, the proximal joint is partly reconstructed. The specimen also preserves pedal elements, including both Mt I and II and multiple phalanges and unguals.

Size and Age Estimation

Based on the limb measurements of Apatosaurinae (incl., humerus, ulna, radius, femur, tibia and fibula) provided by Wilhite (2003), the specimen plots around the median values of the sample, indicating an average size for an adult Apatosaurus (Fig. 9).

Based on the circumferences of humerus and femur, a body mass of 20 to 25 tons is estimated, using three different equations (Table 2). In combination with histological data from Apatosaurus (Lehman & Woodward 2008), this body mass indicates that the specimen was at least 46 years old (see Griebeler et al. 2013), representing an adult individuum. However, an exact age estimation is only possible via histological thin sections from long bones or rib heads (Curry 1999 ; Waskow & Sander 2014), which on the basis of the bone structure further allows to verify, when the specimen became sexually mature and if the specimen already reached its final body size (Klein & Sander 2008). In rare cases, the bone structure of the long bones could also indicate the sex of the specimen, if medullary bone was present (Canoville et al. 2020).

Pathologies

This specimen bears a prominent pathology within the caudal series, which represents one of the largest documented in sauropod dinosaurs. The caudals 10 to 12 and 13 to 14 are strongly fused with each other (Fig. 10). The external surface of these pathologic vertebrae is characterized by amorphic reactive bone growth that covers the centra, neural arches and chevrons.

The pathologic bone is organized in multiple parallel running horizontal bands, which most likely represent ossified tendons. In caudal 13 and 14, the neural spines are not affected by the reactive bone growth and the pathology is stronger developed on left side of the two vertebrae. Some further caudal vertebrae also bear little signs of amorphic bone growth on the centrum, which is probably associated with the main pathology described above. Tail pathologies are quite common in sauropods, but are mainly documented for Titanosaur (Cruzado-Caballero et al. 2023). However, they are also identified in multiple Morrison sauropods, including Apatosaurus (Rothschild & Berman 1991). The most common pathologies are infections, spondyloarthropathy and diffuse idiopathic skeletal hyperostosis (Cruzado-Caballero et al. 2023). The external morphology of the pathology in the specimen resembles those of specimen CPPLIP-1020 (two fused caudals with ossified tendons) belonging to the titanosaur genus Uberabatitan (Martinelli et al. 2015). Based on the CT data, the pathology of CPPLIP-1020 is much more complex internally and the final diagnosis is a spondyloarthropathy. However, if this diagnosis can be also applied to this specimen cannot be evaluated here, as it requires a proper CT survey to study the internal morphology of the pathology. According to Rothschild (1987), a diffuse idiopathic skeletal hyperostosis is also likely.