Dreadnoughtus

The researchers who described Dreadnoughtus schrani estimated its weight using Equation 1 of Campione and Evans (2012),[4] which allows the body mass of a quadrupedal animal to be estimated based only on the circumference of the humerus and femur. Using this scaling equation, they concluded that the Dreadnoughtus type specimen weighed about 59.3 tonnes (58.4 long tons; 65.4 short tons).[3] By comparison, this would mean D. schrani weighed more than eight and a half times as much as a male African elephant and even exceeded the Boeing 737-900 airliner by several tons.[5] This very large mass estimate was quickly criticized. On SV-POW web blog, sauropod researcher Matt Wedel used volumetric models, based on the published figures, that yielded estimates between 36–40 tonnes (35–39 long tons; 40–44 short tons),[6] or even as low as approximately 30 tonnes (30 long tons; 33 short tons), based on a 20% shorter torso.[7] Researcher Gregory S Paul posted a response to Lacovara et al., pointing out that the error margins using equations based on limb bones are large; using the same equation the Dreadnoughtus type specimen could have been anywhere between 44–74 tonnes (43–73 long tons; 49–82 short tons). Using volumetric techniques based on a more accurate skeletal restoration, Paul estimated as low as 26 tonnes (26 long tons; 29 short tons).[8]

A formal re-evaluation of the animal's weight was published in June 2015. In it, a research team led by Karl T. Bates compared the simple scaling equation results with results found using a volume-based digital model with various amounts of soft tissue and "empty space" for the respiratory system. They found that any model using the scale-based weight estimate would have meant the animal had an impossible amount of bulk (fat, skin, muscle, etc.) layered onto its skeleton. They compared their D. schrani volumetric model to those of other sauropods with more complete skeletons and better understood mass estimates to conclude that the D. schrani type specimen must have weighed in the range of 22.1–38.2 tonnes (21.8–37.6 long tons; 24.4–42.1 short tons).[9] Lacovara disputes the methods used by Bates et al., arguing that the new study treats Dreadnoughtus as an exception to well-established mass estimate methods proven on living animals, and that the limb bones would be unnecessarily large if the new mass estimates were correct.[10][11] In 2019 Gregory S. Paul estimated the weight of Dreadnoughtus type specimen at 31 tonnes (34 short tons) , slightly higher than his previous estimation.[12] In 2020 Dreadnoughtus was estimated at weighing 49,000 kilograms, 108,027 pounds [13]

Size of the type specimen compared with a human

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.[3] At 1.74 m, its scapula is longer than any other known titanosaur shoulder blade.[3] Its ilium, the top bone of the pelvis, is also larger than any other, measuring 1.31 m in length.[3] The forearm is longer than any previously known from a titanosaur, and it is only shorter than the long forearms of brachiosaurids, which had a more inclined body posture.[3] Only Paralititan[15] 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.[3] The current estimate for the mass of the type specimen is 49,000 kilograms, 108,027 pounds. [16]

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:

• 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

The completeness of D. schrani compared with other extremely massive (over 40 metric tons) sauropods is as follows:[17]

Thus, the skeleton of D. schrani is substantially more complete than those of all other extremely massive (>40 metric tons) dinosaurs.[3]

Life restoration

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,[18][19] 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.[3] This and the fact that the head of the femur was not turned in towards the body as in saltasaurids[18] 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.[20]

Although the forelimbs of D. schrani are longer than in any other previously known titanosaur, they are not significantly longer than the hind limbs.[3] Therefore, Lacovara et al. (2014) reconstructed its neck to have been held more horizontally, rather than anteriorly inclined in the manner of Brachiosaurus.[21]

Composite image of the fossil caudal (tail) vertebrae

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.[3]

Just like modern archosaurs with tails (crocodilians, for example),[22] D. 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.[3]

The shoulder girdle and forelimb of D. 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.[3]

The holotype specimen was likely not fully grown when it died. The histology of the holotype humerus, which shows a lack of an external fundamental system (an outer layer of bone found only in fully-grown vertebrates) and abundant fast-deposited or still-growing woven tissue in the primary fibrolamellar bone of the outer bone cortex, led Lacovara et al. (2014) to determine that the specimen was still growing when it died.[3][24] It remains unknown how large this individual would have grown had it not died when it did.