Wednesday, January 20, 2016

[Paleontology • 2016] Notocolossus gonzalezparejasi • A Gigantic New Dinosaur from Argentina and the Evolution of the Sauropod Hind Foot

Notocolossus gonzalezparejasi 
Riga, Lamanna, David, Calvo & Coria, 2016

Titanosauria is an exceptionally diverse, globally-distributed clade of sauropod dinosaurs that includes the largest known land animals. Knowledge of titanosaurian pedal structure is critical to understanding the stance and locomotion of these enormous herbivores and, by extension, gigantic terrestrial vertebrates as a whole. However, completely preserved pedes are extremely rare among Titanosauria, especially as regards the truly giant members of the group. Here we describe Notocolossus gonzalezparejasi gen. et sp. nov. from the Upper Cretaceous of Mendoza Province, Argentina. With a powerfully-constructed humerus 1.76 m in length, Notocolossus is one of the largest known dinosaurs. Furthermore, the complete pes of the new taxon exhibits a strikingly compact, homogeneous metatarsus—seemingly adapted for bearing extraordinary weight—and truncated unguals, morphologies that are otherwise unknown in Sauropoda. The pes underwent a near-progressive reduction in the number of phalanges along the line to derived titanosaurs, eventually resulting in the reduced hind foot of these sauropods.

Systematic palaeontology
Dinosauria Owen, 1842
Saurischia Seeley, 1887

Sauropoda Marsh, 1878

Titanosauriformes Salgado, Coria, and Calvo, 1997
Somphospondyli Wilson and Sereno, 1998

Titanosauria Bonaparte and Coria, 1993
Lithostrotia Upchurch, Barrett, and Dodson, 2004

Notocolossus gonzalezparejasi gen. et sp. nov.

Etymology: From the Greek notos (southern) and the Latin colossus, in reference to the gigantic size and Gondwanan provenance of the new taxon. Species name honours Dr. Jorge González Parejas, who has collaborated and provided legal guidance on the research, protection, and preservation of dinosaur fossils from Mendoza Province for nearly two decades. In so doing, he has advised researchers on the creation of a natural park that serves to protect dinosaur footprints in Mendoza.

Figure 2: Vertebral morphology of Notocolossus gonzalezparejasi.
Anterior (second or third) dorsal vertebra of the holotype (UNCUYO-LD 301) in (a) anterior and (b) left anterolateral views. Anterior caudal vertebra of the holotype (UNCUYO-LD 301) in (c) anterior, (d) posterior, and (e) right lateral views. Anterior caudal vertebra of the referred specimen (UNCUYO-LD 302) in (f) anterior, (g) posterior, and (h) left lateral views.
Abbreviations: al1, ‘accessory’ lamina 1; al2, ‘accessory’ lamina 2; cd, condyle; ct, cotyle; dp, diapophysis; nc, neural canal; ns, neural spine; pacdf, parapophyseal centrodiapophyseal fossa; posl, postspinal lamina; poz, postzygapophysis; pp, parapophysis; ppdl, paradiapophyseal lamina; prdl, prezygodiapophyseal lamina; prpl, prezygoparapophyseal lamina; prsl, prespinal lamina; prz, prezygapophysis; spdl, spinodiapophyseal lamina; spol, spinopostzygapophyseal lamina; sprl, spinoprezygapophyseal lamina; tp, transverse process; tpol, intrapostzygapophyseal lamina; tprl, intraprezygapophyseal lamina; vasl, ‘V-shaped’ anterior spinal lamina. Scale bars, 20 cm (a,b), 10 cm (c–h).

Holotype: UNCUYO-LD 301, an associated partial skeleton of a very large individual consisting of an anterior dorsal vertebra, an anterior caudal vertebra, the right humerus, and the proximal end of the left pubis (Figs 1b, 2a–e, 3a,c,e,g and 4a–c; Supplementary Figs S1, S3). We consider these elements to represent a single titanosaurian individual because they were found within an area of 8 m by 8 m at the same stratigraphic level and are of the appropriate size and morphology to have been derived from a single skeleton.

Figure 4: Appendicular skeletal morphology of Notocolossus gonzalezparejasi.
(a) Right humerus of the holotype (UNCUYO-LD 301) in anterior view. Proximal end of the left pubis of the holotype (UNCUYO-LD 301) in lateral (b) and proximal (c) views. Right tarsus and pes of the referred specimen (UNCUYO-LD 302) in (d) proximal (articulated, metatarsus only, dorsal [=anterior] to top), (e) dorsomedial (articulated), and (f) dorsal (disarticulated) views.
Abbreviations: I–V, metatarsal/digit number; 1–2, phalanx number; ast, astragalus; cbf, coracobrachialis fossa; dpc, deltopectoral crest; hh, humeral head; ilped, iliac peduncle; of, obturator foramen; plp, proximolateral process; pmp, proximomedial process; rac, radial condyle; ulc, ulnar condyle. Scale bars, 20 cm (a–c), 10 cm (d–f).

Type locality and horizon: Cerro Guillermo, Malargüe Department, southern-most Mendoza Province, Argentina (Fig. 1a; coordinates on file at UNCUYO-LD). The holotype and referred specimen were collected 403 m apart in the basal-most bed of the Upper Cretaceous (upper Coniacian–lower Santonian, ~86 Ma) Plottier Formation of the Neuquén Group (see Supplementary Information for details).

Figure 5: Hypothesized phylogenetic position of Notocolossus gonzalezparejasi and pedal evolution of Sauropoda.
(a) Time-calibrated hypothesis of phylogenetic relationships of Notocolossus with relevant clades labelled. Depicted topology is that of the single most parsimonious tree of 720 steps in length (Consistency Index = 0.52; Retention Index = 0.65). Stratigraphic ranges (indicated by coloured bars) for most taxa follow Lacovara et al.4: fig. 3 and references therein. Additional age sources are as follows: Apatosaurus, Cedarosaurus, Diamantinasaurus, Diplodocus, Europasaurus, Ligabuesaurus, Neuquensaurus, Omeisaurus, Saltasaurus, Shunosaurus, Trigonosaurus, Venenosaurus, Wintonotitan. Stratigraphic ranges are colour-coded to also indicate geographic provenance of each taxon: Africa (excluding Madagascar), light blue; Asia (excluding India), red; Australia, purple; Europe, light green; India, dark green; Madagascar, dark blue; North America, yellow; South America, orange. (b–h) Drawings of articulated or closely associated sauropod right pedes in dorsal (=anterior) view, with respective pedal phalangeal formulae and total number of phalanges per pes provided (the latter in parentheses). (b) Shunosaurus (ZDM T5402, reversed and redrawn from Zhang); (c) Apatosaurus (CM 89); (d) Camarasaurus (USNM 13786); (e) Cedarosaurus (FMNH PR 977, reversed from D’Emic32); (f) Epachthosaurus (UNPSJB-PV 920, redrawn and modified from Martínez et al.22); (g) Notocolossus; (h) Opisthocoelicaudia (ZPAL MgD-I-48). Note near-progressive decrease in total number of pedal phalanges and trend toward phalangeal reduction on pedal digits II–V throughout sauropod evolutionary history (culminating in phalangeal formula of 2-2-2-1-0 [seven total phalanges per pes] in the latest Cretaceous derived titanosaur Opisthocoelicaudia). Abbreviation: Mya, million years ago.

Figure 1: Geographic provenance and speculative reconstruction of the gigantic titanosaurian sauropod dinosaur Notocolossus gonzalezparejasi gen. et sp. nov.
(a) Type locality of Notocolossus (indicated by star) in southern-most Mendoza Province, Argentina. (b) Reconstructed skeleton and body silhouette in right lateral view, with preserved elements of the holotype (UNCUYO-LD 301) in light green and those of the referred specimen (UNCUYO-LD 302) in orange. Scale bar, 1 m. 
(All images were hand drawn by the senior author [B.J.G.R.] and subsequently edited using Adobe Illustrator software.) DOI: 10.1038/srep19165

Bernardo J. González Riga, Matthew C. Lamanna, Leonardo D. Ortiz David, Jorge O. Calvo and Juan P. Coria. 2016. A Gigantic New Dinosaur from Argentina and the Evolution of the Sauropod Hind Foot. Scientific Reports 6, Article number: 19165. DOI: 10.1038/srep19165