A gigantic, exceptionally complete titanosaurian sauropod dinosaur from southern Patagonia, Argentina

Abstract

Titanosaurian sauropod dinosaurs were the most diverse and abundant large-bodied herbivores in the southern continents during the final 30 million years of the Mesozoic Era. Several titanosaur species are regarded as the most massive land-living animals yet discovered; nevertheless, nearly all of these giant titanosaurs are known only from very incomplete fossils, hindering a detailed understanding of their anatomy. Here we describe a new and gigantic titanosaur, Dreadnoughtus schrani, from Upper Cretaceous sediments in southern Patagonia, Argentina. Represented by approximately 70% of the postcranial skeleton, plus craniodental remains, Dreadnoughtus is the most complete giant titanosaur yet discovered, and provides new insight into the morphology and evolutionary history of these colossal animals. Furthermore, despite its estimated mass of about 59.3 metric tons, the bone histology of the Dreadnoughtus type specimen reveals that this individual was still growing at the time of death.

Figure 1. Axial skeletal anatomy of the gigantic titanosaur Dreadnoughtus schrani.

Posterior (~9^th) cervical vertebra in (A) left lateral, (B) right lateral, (C) posterior, and (D) ventral views. (E) Anterior (~4^th) dorsal vertebra in right lateral view. Middle (~6^th) dorsal vertebra in (F) left lateral and (G) anterior views. (H) Posterior (~7^th) dorsal vertebra in right lateral view. Posterior (~8^th) dorsal vertebra in (I) left lateral and (J) posterior views. (K) Partial sacrum in ventral view. (L) Biconvex first caudal vertebra in posterior view. (M) First 32 caudal vertebrae and 18 haemal arches in left lateral view (positions of first 21 caudal vertebrae and haemal arches 4 to 18 known with certainty). All depicted elements definitively pertain to the holotype (MPM-PV 1156), with the possible exceptions of the dorsal vertebrae in (E) and (H) to (J), which belong to either the holotype or the paratype (MPM-PV 3546). Abbreviations: acpl, anterior centroparapophyseal lamina; apcdl, accessory posterior centrodiapophyseal lamina; a-spdl, anterior ramus of spinodiapophyseal lamina; cdf, centrodiapophyseal fossa; cpol, centropostzygapophyseal lamina; dp, diapophysis; nc, neural canal; ns, neural spine; pacdf, parapophyseal centrodiapophyseal fossa; pcdl, posterior centrodiapophyseal lamina; pcdl-f posterior centrodiapophyseal fossa, pcpl, posterior centroparapophyseal lamina; pe, paddle-shaped distal expansion; pocdf, postzygapophyseal centrodiapophyseal fossa; podl, postzygodiapophyseal lamina; posdf, postzygapophyseal spinodiapophyseal fossa; poz, postzygapophysis; pp, parapophysis; ppdl, paradiapophyseal lamina; prdl, prezygodiapophyseal lamina; prsdf, prezygapophyseal spinodiapophyseal fossa; prsl, prespinal lamina; prz, prezygapophysis; p-spdl, posterior ramus of spinodiapophyseal lamina; s6, sixth sacral vertebra; sdf, spinodiapophyseal fossa; spdl, spinodiapophyseal lamina; spof, spinopostzygapophyseal fossa; spol, spinopostzygapophyseal lamina; sprl, spinoprezygapophyseal lamina; sr, sacral rib; sy, sacricostal yoke; tp, transverse process; tpol, intrapostzygapophyseal lamina; tprl, intraprezygapophyseal lamina. Scale bars equal 50 cm.

Figure 2. Reconstruction, appendicular skeletal anatomy, and bone histology of Dreadnoughtus schrani.

(A) Reconstructed skeleton and body silhouette in left lateral view with preserved elements in white. (B) Left scapula and coracoid in lateral view. (C) Sternal plates in ventral view. (D) Left forelimb (metacarpus reconstructed) in anterior view. (E) Left pelvis (ilium partially reconstructed) in lateral view. (F) Left hind limb in anterior view (metatarsus and pes partially reconstructed and reversed from right). (G) Transverse ground thin section of humeral shaft, showing heavy secondary remodelling (arrow indicates extent of dense osteon formation), a thick layer of well-vascularized fibrolamellar bone, and a lack of lines of arrested growth or an external fundamental system. Abbreviations: acet, acetabulum; acf, acromial fossa; acp, acromial process; acr, acromial ridge; ast, astragalus; cc, cnemial crest; cof, coracoid foramen; cor, coracoid; dpc, deltopectoral crest; fem, femur; fhd, femoral head; fib, fibula; flb, fibrolamellar bone; gl, glenoid; hum, humerus; il, ilium; ilp, iliac peduncle; isc, ischium; isp, ischial peduncle; lt, lateral trochanter; mtI, metatarsal I; mtII, metatarsal II; of, obturator foramen; pop, postacetabular process; prp, preacetabular process; pu, pedal ungual; pub, pubis; pup, pubic peduncle; rac, radial condyle; rad, radius; sc, scapula; scb, scapular blade; sr, secondary remodelling; tib, tibia; tpp, tuberosity on preacetabular process; ul, ulna; ulc, ulnar condyle. Scale bars equal 1 m in (A) to (F) and 1 mm in (G). (Skeletal reconstruction by L. Wright, with G. Schultz.)

Figure 3. Time-calibrated hypothesis of phylogenetic relationships of Dreadnoughtus schrani (Consistency Index = 0.42, Retention Index = 0.76, Rescaled Consistency Index = 0.32) with relevant clades labelled.

Geologic timescale after Walker et al.; stratigraphic ranges (indicated by thick black bars) for most taxa follow Wilson, D'Emic, and Mannion et al.. In instances where these sources disagree on the age of a particular taxon, we follow the most recent source that includes that taxon. Additional age sources are as follows: Dreadnoughtus; Epachthosaurus; Futalognkosaurus; Mendozasaurus; Neuquensaurus. Numbers beside nodes are Bremer decay indices. Abbreviations: Br, Brachiosauridae; Lo, Lognkosauria; Mya, million years ago. See Supplementary Fig. 20 for an unabridged version of this tree that presents the entire topology and also includes bootstrap and jack-knife support values.