An articulated cervical series of Alamosaurus sanjuanensis Gilmore, 1922 (Dinosauria, Sauropoda) from Texas: new perspective on the relationships of North America's last giant sauropod

Abstract

The sauropod dinosaur Alamosaurus sanjuanensis Gilmore, 1922 has been known from Maastrichtian deposits of southwestern North America for nearly a century. Alamosaurus is the youngest sauropod taxon known in North America. Originally described from an isolated scapula and ischium from New Mexico, more of Alamosaurus was revealed by an incomplete skeleton from southern Utah. Additional referred specimens from western Texas provided the first few examples of cervical, dorsal and sacral vertebrae known for the taxon, but these came from relatively small and immature individuals. Here we describe an articulated series of cervical vertebrae of a large, mature titanosaur from Big Bend National Park, Texas, and provide evidence that the specimen can be referred to A. sanjuanensis. The specimen represents the first articulated cervical vertebral series described for the taxon, which clarifies aspects of cervical vertebral anatomy and provides at least one new diagnostic character for Alamosaurus. Many previous cladistic analyses found Alamosaurus to be a saltasaurid titanosaur, sometimes closely related to the Asian taxon Opisthocoelicaudia skarzynskii. We present cladistic analyses incorporating new data from this and other specimens from Big Bend National Park. The first places Alamosaurus as a lithostrotian titanosaur outside Saltasauridae. The second analysis, with greater focus on South American titanosaurs, finds Alamosaurus allied to Lognkosauria, a clade of South American titanosaurs notable for giant size and exceptionally robust necks. This relationship may be more congruent with the fossil record than hypotheses of phylogeny that would require saltasaurid titanosaurs to inhabit northern Laramidia, Beringia and Central Asia through the late Campanian and Maastrichtian while leaving no record of their presence.

A new giant titanosaur sheds light on body mass evolution among sauropod dinosaurs

“…The recently described giant titanosaur from the Late Cretaceous of western Argentina, Notocolossus, was suggested to be the sister taxon of Dreadnoughtus and closely related to Lithostrotia [16]. Our analysis places Notocolossus as the sister taxon of Lognkosauria, whereas Dreadnoughtus is recovered as closely related to Lithostrotia, as in previous studies [12,46]. The monophyly of Notocolossus plus Lognkosauria is supported by a single synapomorphy: the presence of reduced spinoprezygapophyseal laminae in anterior caudal vertebrae (ch.…”

“…Our analysis reveals a single major event of body mass increase at the base of the clade formed by Notocolossus and Lognkosauria, implying a threefold body mass increase (38-60 tonnes) with respect to the body mass reconstruction of most nodes within Titanosauria (12-20 tonnes; figure 3). The acquisition of gigantism in this clade was not the only marked increase in body mass, as two other titanosaur lineages (Dreadnoughtus and Alamosaurus [12,46]; figure 3) independently acquired large body masses, although without reaching to the extreme gigantism recorded in Lognkosauria (e.g. Patagotitan).…”

“…In the present analysis, Alamosaurus, from the Maastrichtian of North America, is recovered as a derived lithostrotian. This taxon was recently recovered as closely related to Lognkosauria [46]. Nevertheless, in our data set (which includes character scorings based on [46]) this position is highly suboptimal, requiring 36 extra steps to place Alamosaurus as the sister taxon to Lognkosauria.…”

Stratigraphy and depositional history of the Tornillo Group (Upper Cretaceous–Eocene) of West Texas

“…Remains of sauropod dinosaurs are very common throughout the formation, and most or all of the specimens collected in the upper part of the formation are attributable to Alamosaurus sanjuanensis (e.g., TMM 41541 and 45891; Lawson, 1972;Lehman and Coulson, 2002;Woodward and Lehman, 2009;Fronimos and Lehman, 2014;Tykoski and Fiorillo, 2016). Sauropod specimens from the lower part of the formation (e.g., TMM 40597) are less diagnostic but may represent a different species (Wick and Lehman, 2014).…”

Postcranial osteology and phylogenetic relationships of the Early Cretaceous titanosaur Tapuiasaurus macedoi Zaher et al. 2011

“…Nonetheless, although Tapuiasaurus has been incorporated into numerous recent phylogenetic analyses (e.g. Zaher et al 2011, Carballido & Sander 2014, Gorscak et al 2014, Lacovara et al 2014, Poropat et al 2015, Bandeira et al 2016, González Riga et al 2016, Martínez et al 2016, Wilson et al 2016, Averianov & Skutschas 2017, Tykoski & Fiorillo 2017, Carballido et al 2017, Gorscak et al 2017, Averianov & Efimov 2018, González Riga et al 2018, Sallam et al 2018, its postcranial skeleton has not been previously fully described and illustrated in the literature.…”