Dreadnoughtus

Dreadnoughtus is a genus of giant titanosaurian sauropod dinosaur which contains a single species, Dreadnoughtus schrani. D. schrani is known from two partial skeletons discovered in Upper Cretaceous (Campanian-Maastrichtian; 84–66 Ma) rocks of the Cerro Fortaleza Formation in Santa Cruz Province, Argentina. It is one of the largest of all known terrestrial vertebrates, possessing the greatest mass of any land animal that can be calculated with reasonable certainty. Dreadnoughtus schrani is known from a more complete skeleton than any other gigantic titanosaurian.

Etymology
"Dreadnought" is an English term for the predominant type of battleship in the early 20th century, deriving from the words "dread" (fear) + "nought" (nothing). According to the research team that discovered the taxon, the genus name Dreadnoughtus "alludes to the gigantic body size of the taxon (which presumably rendered healthy adult individuals nearly impervious to attack)" and to the two Argentine dreadnoughts that served in the first half of the twentieth century, Rivadavia and Moreno. Thus, the genus name also honors the country in which Dreadnoughtus schrani was discovered. The name of the type species, schrani, was given in recognition of the American entrepreneur Adam Schran for his financial support of the project.

Description
The discovery of Dreadnoughtus schrani provides insight into the size and anatomy of giant titanosaurian dinosaurs, especially of the limbs and the shoulder and hip girdles. The majority of Dreadnoughtus schrani bones are very well preserved. There is minimal deformation, especially in the limb bones. Fine features, such as locations of muscle attachment, are frequently clearly visible.

Specimens
The holotype specimen, MPM-PV 1156, consists of a partial skeleton, somewhat preserved in its original layout, that comprises: a maxilla (jaw) fragment; a tooth; a posterior cervical vertebra; cervical ribs; multiple dorsal vertebrae and dorsal ribs; the sacrum; 32 caudal vertebrae and 18 haemal arches (bones from the tail) that include a sequence of 17 anterior and middle caudal vertebrae and their corresponding haemal arches found in their original layout; the left pectoral girdle and forelimb minus the front foot; both sternal plates; all pelvic elements; the left hind limb lacking a hind foot and right tibia; metatarsals I and II; and one claw from digit I.

The paratype, MPM-PV 3546, consists of a partially articulated postcranial skeleton of a slightly smaller individual whose remains were discovered in the same location as the holotype. It includes a partial anterior cervical vertebra, multiple dorsal vertebrae and ribs, the sacrum, seven caudal vertebrae and five haemal arches, a nearly complete pelvis, and the left femur.

Size
Estimates based on measurements of the known parts of the skeleton suggest that the only known individual of Dreadnoughtus schrani was approximately 26 metres (85 ft) long and stood about 2 stories tall.[1] At 1.74 m, its scapula is longer than any other known titanosaur shoulder blade.[1] Its ilium, the top bone of the pelvis, is also larger than any other, measuring 1.31 m in length.[1] The forearm of is longer than any previously known from a titanosaur, sand it is only shorter than the long forearms of brachiosaurids, which had a more inclined body posture.[1] Only Paralititan[3] preserves a longer humerus (upper arm bone). Although each species likely had slightly different body proportions, these measurements demonstrate the massive nature of Dreadnoughtus schrani.[1] The current estimate for the mass of the type specimen, created using a 3D skeleton and volume estimation of mass method, derives a range of 22.1–38.2 tonnes (21.8–37.6 long tons; 24.4–42.1 short tons).

Completeness
Completeness may be assessed in different ways. Sauropod dinosaur skeletons are often recovered with little to no skull material, so completeness is often looked at in terms of postcranial completeness (i.e., the completeness of the skeleton excluding the skull). Completeness may also be assessed in terms of the numbers of bones versus the types of bones. The most important metric for understanding the anatomy of a fossil animal is the types of bones. The completeness statistics for Dreadnoughtus schrani are as follows: The completeness of Dreadnoughtus schrani compared with other extremely massive (over 40 metric tons) sauropods is as follows: Thus, the skeleton of Dreadnoughtus schrani is substantially more complete than those of all other extremely massive (>40 metric tons) dinosaurs.
 * 116 bones out of ~256 in the entire skeleton (including the skull) = 45.3% complete
 * 115 bones out of ~196 in the skeleton (excluding the skull) = 58.7% complete
 * 100 types of bones out of ~142 types in the skeleton (excluding the skull) = 70.4% complete

Posture
All titanosaurs had what is called wide-gauge posture, a relative term to describe a stance in which the feet fell apart from the body midline. More derived titanosaurs had a greater degree of wide-gauge posture,[5][6] with their limbs held more widely than their ancestors and contemporaneous counterparts. The stance of Dreadnoughtus schrani was clearly wide-gauge, but not to the degree of saltasaurids because the femoral condyles are perpendicular to its shaft rather than beveled.[1] This and the fact that the head of the femur was not turned in towards the body as in saltasaurids[5] support the phylogenetic conclusion that Dreadnoughtus was not a saltasaurid. The animal’s broad sternal bones also demonstrate a wide pectoral girdle, giving it a broad-shouldered, broad-chested appearance. Paleontologist Kenneth Lacovara compared the animal's gait to an Imperial Walker's.[7]

Although the forelimbs of Dreadnoughtus schrani are longer than in any other previously known titanosaur, they are not significantly longer than the hind limbs.[1] Therefore, Lacovara et al. (2014) reconstructed its neck to have been held more horizontally, rather than anteriorly inclined in the manner of Brachiosaurus.

Distinctive features
The tail of Dreadnoughtus schrani has several characteristic features included in the diagnosis of the species. The first vertebra of the tail has a ridge on its ventral surface called a keel. In the first third of the tail, the bases of the neural spines are extensively subdivided into cavities caused by contact with air sacs (part of the dinosaur’s respiratory system). In addition, the anterior and posterior boundaries of these neural spines have distinct ridges (pre- and postspinal laminae) connecting them to the pre- and postzygapophyses (the articulation points of the neural arches). In the middle of its tail, the vertebrae have a triangular process that extends over the centrum towards each preceding vertebra.[1]

Just like modern archosaurs with tails (for example crocodilians,[9] Dreadnoughtus schrani had bones below the vertebrae called chevrons or haemel arches. These bones connect with the ventral surface of the vertebrae and are “Y” shaped when viewed anteriorly. In Dreadnoughtus schrani the bottom stem of the “Y” is broadly expanded, likely for the attachment of muscles.[1]

The shoulder girdle and forelimb of Dreadnoughtus schrani also exhibit unique features. An oblique ridge crosses the interior face of the scapular blade, extending from the top side near the far end of the blade to the bottom side near the base of the scapular blade. Finally, each end of the radius exhibits a unique form: the top, or proximal end, has a distinct concave embayment on its posterior face while the bottom, or distal end, is nearly square in shape instead of broadly expanded.