Dreadnoughtus schrani – A Gigantic Titanosaurian Sauropod Dinosaur, Southern Patagonia, Argentina

Australia: The Land Where Time Began

Dreadnoughtus schrani – A Gigantic Titanosaurian Sauropod Dinosaur, Southern Patagonia, Argentina

The most diverse and large-bodied, abundant, herbivores in the southern continents over the last 30 million years of the Cretaceous were the Titanosaurian sauropod dinosaurs. According to the authors¹ there are several of the species of titanosaur that are regarded ae the most massive land animal so far discovered, though almost all of these enormous animals are known only from fossils that are very incomplete, which is a problem for gaining an understanding of the anatomy. In this paper the authors¹ describe a very large new titanosaur, Dreadnoughtus schrani, (MPM-PV 1156) discovered in sediments of Cretaceous age in southern Patagonia, Argentina. This is the most complete remains found so far of a titanosaur with about 70 % of the post-cranial skeleton found, plus some craniodental remains, which provides a new insight into the morphology and evolutionary history of these huge animals. In spite of the huge mass of 59.3 metric tons this animal is shown by the histology of its bones to have been still growing at the time of its death.

Discussion

The authors suggest Dreadnoughtus is a member of Titanosauria that is intermediate phylogenetically between the basal representatives of the clade such as Andesaurus and Lithostrotia, which is a relatively derived Titanosaurian subclade. Argentinosaurus and Futalognkosaurus, also very large forms, were recovered as non-lithostrotian titanosaurs. The authors¹ suggest phylogenetic analysis of Titanosauria is necessary to clarify the relationship of these taxa.

Though Dreadnoughtus is among the largest known titanosaurs it is not possible at present to establish conclusively its size relative to those of most other very large members of the clade, such as ‘Antarctosaurus’giganteus, Argentinosaurus, Paralititan, Puertasaurus, as the remains of the latter forms are too fragmentary. Elements of MPM-PV 1156 are, however, of comparable linear dimensions to their counterparts in other very large titanosaurs. An equation for calculating body mass in quadrupedal tetrapods that is based on humeral and femoral circumference (13,48,49) that has recently been refined, yields an estimate of 59.3 metric tons for this particular individual, which is close to twice the values that are previously been estimated for Giraffatitan (34.0 metric tons), and about 4 times that estimated for the diplodocid Diplodocus (14,8 metric tons), by the use of the same method. When the Dreadnoughtus skeleton was reconstructed with the cervical series that were mostly missing being restored after that of Futalognkosaurus it resulted in a body length of about 26 m.

In spite of the remarkable size of this type specimen of Dreadnoughtus, multiple lines of evidence indicate that this individual was not osteologically mature at the time of death. The scapula had not fused to the coracoid, and the posteromedial margin of the coracoid foramen abuts the scapula; both of these morphologies have been cited as osteological indicators of immaturity in sauropods. Also, though the cortical bone in the humerus has undergone extensive secondary remodelling as indicated by histological analysis, the element has still retained a thick layer of vascularised fibrolamellar bone between the inner cortex, that has been remodelled, and the periphery of the periosteal surface. It also lacks the arrested growth lines or an external fundamental system (Schroeter, Boles & Lacovara, 2011), which may indicate osteological immaturity (Klein & Sander, 2008), though it has been proposed (Company, J., 2011) that this is a synapomorphy of Titanosauria, and not an indicator of the ontogenetic stage. The dorsal ribs display a similar pattern, which have been remodelled extensively, though they still retain fibrolamellar bone tissue towards the periosteal surface. It has been suggested by recent analyses (Stein et al., 2010; Klein et al., 2012) that extensive remodelled tissues remain after the attainment of sexual maturity but prior to reaching maximum size. The presence of fibrolamellar bone in the outer cortex of MPM-PV 1156 humerus that is unremodelled suggests this massive Dreadnoughtus individual was still growing at the time of its death.

Dreadnoughtus schrani is the most completely know giant titanosaur. Metrics of completeness indicate that this new taxon from Patagonia has preserved approximately 45.3% of the bones that would be expected in a complete titanosaurian skeleton, and, depending on how the bones are counted, up to 70.4 % of the postcranial elements. Compared to the skeletons of other titanosaurs that have been found these same values are, Futalognkosaurus 15.2% and 26.8%, Paralititan 7.8% and 12.7%, Argentinosaurus 5.1% and 9.2%, ‘A.’ giganteus 2.3% and 3.5%, Puertasaurus 1.6% and 2.8%. The skeleton of Dreadnoughtus is also among the most anatomically informative of the titanosaurs of any body size. This new taxon can be evaluated definitively for 57.5 % of morphological characters included in the data matrix that has recently been published (Carballido &Sander, 2014). Also, Dreadnoughtus is dramatically more informative than the others that were included in the phylogenetic analysis of the authors¹ Futalognkosaurus and Argentinosaurus, which scored for only 18.5 % and 12.6 % of the characters that were available, respectively. Therefore important new osteological data is offered by Dreadnoughtus for future investigations of the anatomy, biomechanics, and evolution of the most massive land animals that have ever existed.