Anatomy and systematics of the diplodocoidAmphicoelias altussupports high sauropod dinosaur diversity in the Upper Jurassic Morrison Formation of the USA

Abstract

Sauropod dinosaurs were an abundant and diverse component of the Upper Jurassic Morrison Formation of the USA, with 24 currently recognized species. However, some authors consider this high diversity to have been ecologically unviable and the validity of some species has been questioned, with suggestions that they represent growth series (ontogimorphs) of other species. Under this scenario, high sauropod diversity in the Late Jurassic of North America is greatly overestimated. One putative ontogimorph is the enigmatic diplodocoid Amphicoelias altus , which has been suggested to be synonymous with Diplodocus . Given that Amphicoelias was named first, it has priority and thus Diplodocus would become its junior synonym. Here, we provide a detailed re-description of A. altus in which we restrict it to the holotype individual and support its validity, based on three autapomorphies. Constraint analyses demonstrate that its phylogenetic position within Diplodocoidea is labile, but it seems unlikely that Amphicoelias is synonymous with Diplodocus . As such, our re-evaluation also leads us to retain Diplodocus as a distinct genus. There is no evidence to support the view that any of the currently recognized Morrison sauropod species are ontogimorphs. Available data indicate that sauropod anatomy did not dramatically alter once individuals approached maturity. Furthermore, subadult sauropod individuals are not prone to stemward slippage in phylogenetic analyses, casting doubt on the possibility that their taxonomic affinities are substantially misinterpreted. An anatomical feature can have both an ontogenetic and phylogenetic signature, but the former does not outweigh the latter when other characters overwhelmingly support the affinities of a taxon. Many Morrison Formation sauropods were spatio-temporally and/or ecologically separated from one another. Combined with the biases that cloud our reading of the fossil record, we contend that the number of sauropod dinosaur species in the Morrison Formation is currently likely to be underestimated, not overestimated.

Functional morphology of the Triassic apex predator Saurosuchus galilei (Pseudosuchia: Loricata) and convergence with a post‐Triassic theropod dinosaur

“…In the case of Saurosuchus , for example, the only other similarly sized terrestrial taxon (i.e., >250 kg) from the same biozones of the Ischigualasto Formation is the dicynodont therapsid Ischigualastia jenseni (Cox, 1962; Martínez et al, 2013). In contrast, Allosaurus with an estimated length of around 7.5 m is contemporaneous with several sauropod species from the Morrison Formation that attained sizes of 20–25 m (Mannion et al, 2021; Paul, 2016; although Allosaurus more likely predated upon smaller juveniles; Paul, 1988). It is therefore possible that basal loricatans did not need a strong bite typical of similarly sized theropods (or of modern pseudosuchians) to procure and process prey much smaller than themselves.…”

Sauropod dinosaur teeth from the lower Upper Cretaceous Winton Formation of Queensland, Australia and the global record of early titanosauriforms

“…North America. Despite their dominance in the Upper Jurassic Morrison Formation [244,245], neither camarasaurids nor diplodocoids are known from the Cretaceous of North America [246]. The only sauropod group that clearly straddles the Jurassic/Cretaceous boundary in North America is Brachiosauridae, with Brachiosaurus present in the Late Jurassic [247][248][249][250], and Cedarosaurus known from the Valanginian [251,252].…”

Fossils of the oldest diplodocoid dinosaur suggest India was a major centre for neosauropod radiation

“…In lateral view, the anterior and posterior borders of the neural spine remain subparallel (Fig. 4 K,L), unlike the sub-triangular spinal lateral profile characteristic of many titanosauriforms 34 , 37 . The neural spine bears a sharp prespinal lamina placed along the longitudinal midline on the anterior surface (Fig.…”