Phylogenetic analysis

We conducted a phylogenetic analysis to assess the affinities of Dreadnoughtus schrani within Titanosauria. We added the new Patagonian taxon to a recently-published matrix of 70 sauropod taxa and 341 morphological characters57 that contains a broad diversity of titanosaurian and non-titanosaurian titanosauriform genera, including the gigantic form Argentinosaurus. We also added Futalognkosaurus to the matrix (the scoring of which was based on the description by Calvo et al.3 and is presented in the Supplementary Information) to more thoroughly investigate relationships between Dreadnoughtus and other giant titanosaurians. We analysed the augmented matrix using the methods outlined in Carballido and Sander57. Specifically, the multistate characters 12, 58, 95, 96, 102, 106, 108, 115, 116, 119, 120, 154, 164, 213, 216, 232, 233, 234, 235, 256, 267, 298, 299 and 301 were treated as ordered and the matrix was subjected to a heuristic search in TNT (Tree Analysis Using New Technology) v. 1.158 (1,000 replicates of Wagner trees, random addition sequence, tree bisection reconnection branch swapping algorithm, ten trees saved per replicate). The initial trial yielded ten most parsimonious trees of 1,028 steps, the strict consensus of which recovered numerous taxa traditionally considered to be basal titanosauriforms as members of Titanosauria and provided little resolution within Lithostrotia (Supplementary Fig. 19).

Based on numerous aspects of its morphology, the titanosaurian affinities of Dreadnoughtus are not in doubt; the aim of our analysis was therefore to ascertain the position of the new taxon within Titanosauria. Consequently, in an attempt to better resolve titanosaurian interrelationships, we inspected the matrix using the program TAXEQ3 (Safe Taxonomic Reduction Based on Taxonomic Equivalence59). The TAXEQ3 examination did not find taxa that could be safely deleted via safe taxonomic reduction, but did underscore the high proportion of missing data in the matrix. Although Dreadnoughtus lacked only 42.5% of the available character information, 47 (of 72 total) taxa lacked more than 50% of these data. We reanalysed the matrix after pruning 18 of the 20 fragmentary and unstable taxa pruned by Carballido and Sander57, retaining Andesaurus (to define the node-based clade Titanosauria) and Argentinosaurus (another giant titanosaurian). This second iteration recovered 30 most parsimonious trees of 943 steps. The strict consensus of these trees (Fig. 3, Supplementary Fig. 20; Consistency Index = 0.42, Retention Index = 0.76, Rescaled Consistency Index = 0.32) yields considerably greater resolution within Titanosauria and posits Dreadnoughtus as a non-lithostrotian titanosaur more derived than Andesaurus, Epachthosaurus, Argentinosaurus and lognkosaurians (i.e., Futalognkosaurus, Mendozasaurus) but less derived than all other Campanian–Maastrichtian titanosaurs.

Phylogenetic character states of Dreadnoughtus schrani

Scores for Dreadnoughtus schrani for the 341 morphological characters employed by Carballido and Sander57 are as follows:

?????????? ?????????? ?????????? ?????????? ?????????? ?????????? ?????????? ?????????? ?????????? ?????????? 1231?202?? 1110-30102 ??0?110001 00-?10102(01) 1121-11111 1200101011 (01)111101111 000211??11 31?????013 ?030000001 00000-1003 10(12)000100? ??0-111011 ?0?0011001 1100011101 0110120111 1001101??? ????????11 1101101000 0110110011 2111?001?1 1111010110 0??11101?? ????1?11?? ?

Body dimensions

Among many other elements, the Dreadnoughtus schrani holotype (MPM-PV 1156) includes the complete left humerus and femur. Because the minimum midshaft circumferences of these bones have been found to scale with body mass in quadrupedal terrestrial tetrapods13,48,49, the preservation of these elements in MPM-PV 1156 permits an estimate of the mass of this specimen. The midshaft circumference of the humerus is 785 mm and that of the femur is 910 mm. Using the scaling equation recently proposed by Campione and Evans49 (logBM = 2.749 * logC H+F − 1.104, where BM is body mass and C H+F is combined humeral and femoral circumference), these values yield an estimated body mass of 59,291 kg (~59.3 metric tons, or 65.4 short tons) for this Dreadnoughtus individual. By comparison, the estimated masses of other medium- and large-bodied titanosaur specimens for which humeral and femoral shaft circumferences have been published are as follows: Opisthocoelicaudia ZPAL MgD-I/4835, 25,418 kg; Alamosaurus TMM 41541-160, 35,164 kg; and Elaltitan PVL 46288, 42,798 kg. Additionally, Benson et al.13 provide an estimate of 38,139 kg for Futalognkosaurus3, based on an undescribed humerus and femur. Large individuals of the diplodocid Diplodocus (USNM 10865) and the brachiosaurid Giraffatitan (HMN SII) have been estimated at 14,813 and 34,003 kg, respectively, using the same equation13,49. It therefore appears that the Dreadnoughtus schrani holotype was considerably more massive than most other titanosaurian specimens and indeed, most other sauropods.

Due primarily to uncertainty in the morphology of the cervical series, it is currently not possible to definitively establish the total body length of Dreadnoughtus. Nevertheless, the elongate nature of the two preserved cervical vertebrae implies that the taxon had a long neck, more similar to those of titanosaurs such as Futalognkosaurus3 and Rapetosaurus18 than to comparatively short-necked forms such as Isisaurus38 and Mendozasaurus62. The centrum of the complete posterior cervical vertebra of MPM-PV 1156 is 113 cm in anteroposterior length. Assuming that the neck proportions of Dreadnoughtus were similar to those of Futalognkosaurus3,63 and that this complete Dreadnoughtus cervical vertebra corresponds to the ninth vertebra in the series, this yields a neck length of approximately 11.3 m for MPM-PV 1156.

The holotype of Tapuiasaurus (MZSP-PV 807) is the only adult titanosaurian specimen known to preserve a complete skull associated with appendicular elements41. Using the radius length of MPM-PV 1156 (95 cm) and scaling up from the relative lengths of the skull and radius in Tapuiasaurus, we estimate the length of the mostly unknown skull of this Dreadnoughtus specimen at 88 cm. Thus, the total combined length of the skull and neck of MPM-PV 1156 is estimated at roughly 12.2 m.

The remainder of the axial skeleton is well-represented in Dreadnoughtus, permitting more precise estimates of its length. The majority of titanosaurs for which the dorsal series is completely known (e.g., Futalognkosaurus, Rapetosaurus, Trigonosaurus, Overosaurus) possess ten vertebrae in this region3,18,39,64; with 11 dorsal vertebrae, Opisthocoelicaudia is the only known exception in this regard35. We therefore assume that ten dorsal vertebrae were present in Dreadnoughtus. The only described titanosaurian specimen with a completely preserved caudal series is the holotype of Opisthocoelicaudia (ZPAL MgD-I/48); this includes 34 vertebrae35. Nevertheless, evidence indicates that the tail may have been longer in some other titanosaurian taxa. For example, exceptional specimens of Epachthosaurus (UNPSJB-PV 920) and Alamosaurus (USNM 15560) preserve the first 30 and 29 caudal vertebrae, respectively, all of which are strongly procoelous61,65. In both of these caudal sequences, the posterior-most preserved vertebra is clearly not the terminal caudal vertebra; moreover, Wilson et al.66 demonstrated that the posterior-most ~seven to eight caudal vertebrae of at least some titanosaurs were biconvex. Furthermore, an as-yet undescribed titanosaurian skeleton from northern Patagonia is reputed to include approximately 65 caudal vertebrae67. Taking all of the above into account, we provisionally estimate that the complete caudal series of Dreadnoughtus would have included approximately 40 vertebrae. Digital reconstruction of the dorsal, sacral and caudal vertebral sequences that incorporates these assumptions (Supplementary Fig. 9) yields a combined length of 13.8 m for these parts of the axial skeleton. When added to the skull and neck length estimated above, this yields an approximate total body length of 26 m for MPM-PV 1156.

Calculating completeness vs. other giant titanosaurs

We quantified the skeletal completeness of Dreadnoughtus and other enormous titanosaurs using the following method. (We chose to devise an un-weighted metric that counts the presence of each element equally. See Mannion and Upchurch68 for an alternative, weighted method.) First, we used material of the most completely known titanosaurs of any size (e.g., Epachthosaurus, Nemegtosaurus, Opisthocoelicaudia, Overosaurus, Rapetosaurus, Saltasaurus, Tapuiasaurus, Trigonosaurus) to estimate the total number of bones in a representative titanosaurian skeleton at 256 (Supplementary Table 2). We then counted the total number of non-duplicated elements preserved between both known Dreadnoughtus specimens (MPM-PV 1156 and MPM-PV 3546) at 116 and used the literature to tally the elements preserved for five other giant titanosaurs: ‘Antarctosaurus’ giganteus (six bones)8, Argentinosaurus (13 bones)6, Futalognkosaurus (39 bones)3, Paralititan (20 bones)7,69 and Puertasaurus (four bones)9. These numbers were then divided by 256 to yield completeness percentages for each taxon (Supplementary Table 2).

Because the six titanosaurs under consideration are represented exclusively (‘A.’ giganteus, Argentinosaurus, Futalognkosaurus, Paralititan, Puertasaurus) or almost exclusively (Dreadnoughtus) by postcranial bones, we then explored the postcranial completeness of each taxon. We calculated the total number of postcranial elements expected in a representative titanosaur and then counted all such bones for each taxon in question. We divided the elements preserved by the elements expected to produce percentages of postcranial completeness for each titanosaur (Supplementary Table 2).

Finally, because, from a morphological standpoint, the preservation of one bilaterally symmetrical element is effectively as informative as is the preservation of both, we calculated what we term the ‘mirrored postcranial completeness’ of each of these giant titanosaurs (Supplementary Table 2). When determining mirrored postcranial completeness, bilateral elements are treated as equivalents; in other words, if at least one of a pair of bilaterally symmetrical bones is preserved in a given taxon, that element is counted as fully preserved in that taxon. (As an example, in mirrored postcranial completeness, the humerus of Dreadnoughtus is treated as completely represented, even though only the left humerus of this titanosaur is actually preserved.)

Data archiving

Data reported in this paper are available as Supplementary Information. Specimens MPM-PV 1156 and MPM-PV 3546 are permanently reposited at the Museo Padre Molina in Río Gallegos, Santa Cruz Province, Argentina.