A gigantic new dinosaur from Argentina and the evolution of the sauropod hind foot

Abstract

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.

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.)

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).

Figure 3. Comparison of anterior caudal vertebrae of Notocolossus gonzalezparejasi.

Photographs (a,b,e,f) and interpretive drawings (c,d,g,h) of the anterior caudal vertebra of the holotype (UNCUYO-LD 301) (a,c,e,g) and the first five anterior caudal vertebrae of the referred specimen (UNCUYO-LD 302) (b,d,f,h) in dorsal (a–d) and left lateral (e–h) views (e and g reversed). Abbreviations, avr, anteroventral ridge; ns, neural spine; poz, postzygapophysis; prz, prezygapophysis; tp, transverse process. Scale bar, 10 cm.

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).

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.: 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 Zhang45); (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. Institutional abbreviations see Supplementary Information.