Dreadnoughtus

''Dreadnoughtus'' is a genus of titanosaurian sauropod dinosaur containing a single species, ''Dreadnoughtus schrani''. ''D. schrani'' is known from two partial skeletons discovered in Upper Cretaceous (Campanian to Maastrichtian; approximately 76–70 Ma) rocks of the Cerro Fortaleza Formation in Santa Cruz Province, Argentina. It is one of the largest terrestrial vertebrates known, with the immature type specimen reaching in total body length and possessing the greatest mass of any land animal that can be calculated with reasonable certainty. ''D. schrani'' is known from more complete skeletons than any other gigantic titanosaurian.

Drexel University paleontologist Kenneth Lacovara, who discovered the species, chose the name ''Dreadnoughtus'', which means “fears nothing", stating “I think it’s time the herbivores get their due for being the toughest creatures in an environment."

Discovery and study

American palaeontologist Kenneth Lacovara discovered the remains in the Cerro Fortaleza Formation in Santa Cruz Province, Patagonia, Argentina in 2005. Due to the large size of the bones and the remote location where they were found, it took his team four austral summers to fully excavate the remains. Mules, ropes and many team members were needed to finally get the field-jacketed bones to a truck.

In 2009, the fossils were transported to Philadelphia via an ocean freighter for preparation and study. Fossil preparation and analysis occurred at Drexel University, the Academy of Natural Sciences of Drexel University and the Carnegie Museum of Natural History. ''Dreadnoughtus schrani'' fossils were returned to their permanent repository at the Museo Padre Molina in Rio Gallegos, Argentina in March 2015.

The bones of both ''Dreadnoughtus'' specimens were scanned with a NextEngine 3D laser scanner. Using the software Autodesk Maya, the scans of each bone were positioned in 3D space to create a digital articulated skeleton, which was then converted into 3D PDF files using the software GeoMagic. The high fidelity of these scans allowed Lacovara ''et al.'' (2014) to study the heavy fossils of ''Dreadnoughtus schrani'' in a way that was safe for the fossils and enhanced virtual and long-distance collaboration.

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.

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.

Controversy over the mass/weight

The researchers who described ''Dreadnoughtus schrani'' estimated its weight using Equation 1 of Campione and Evans (2012), 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 . 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. 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 , or even as low as approximately , based on a 20% shorter torso. 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 . Using volumetric techniques based on a more accurate skeletal restoration, Paul estimated as low as . Benson et al. suggested a maximum body mass of , but these estimates were questioned due to a very large error range and lack of precision.

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 . Ullmann and 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. In 2019, considering the argument of Ullmann and Lacovara (2016), Gregory S. Paul moderated his mass estimate of ''Dreadnoughtus'' type specimen at , slightly higher than his previous estimation; he even noted that the holotype may have been heavier a tonne or so. In 2020, two studies estimated the mass of ''Dreadnoughtus'' much higher at , but Campione & Evans (2020) acknowledged the low estimate by Paul (2019) as appropriate.

Description

The discovery of ''Dreadnoughtus schrani'' provides insight into the size and anatomy of giant titanosaurian sauropods, especially of the limbs and the shoulder and hip girdles. The majority of ''D. 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. ''Dreadnoughtus'' also has an unusually long neck for its body size, making up almost half of the animal's length.

Size

Estimates based on measurements of the known parts of the skeleton suggest that the only known individual of ''Dreadnoughtus schrani'' was approximately long and stood about 2 stories tall. At 1.74 m, its scapula is longer than any other known titanosaur shoulder blade. Its ilium, the top bone of the pelvis, is also larger than any other, measuring 1.31 m in length. 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. Only ''Paralititan'' 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''. The estimates for the mass of the type specimen without controversy range between .

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:

* 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:

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

In 2022, Schroeter and her colleagues discovered soft tissues and collagens from the holotype specimen. They noted the possibility that the individual, to which the holotype specimen belongs, may have been trapped in a rapid burial event; this may explain why the holotype of ''D. schrani'' is more completely preserved than other titanosaurs.

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, 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. This and the fact that the head of the femur was not turned in towards the body as in saltasaurids 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.

Although the forelimbs of ''D. schrani'' are longer than in any other previously known titanosaur, they are not significantly longer than the hind limbs. 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.

Just like modern archosaurs with tails (crocodilians, for example), ''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.

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.

Classification

Based on a cladistic analysis, ''Dreadnoughtus schrani'' appears to be a "derived" basal titanosaur that is not quite a lithostrotian. Lacovara ''et al.'' (2014) note that because of the wide array of relatively "advanced" and "primitive" features in the skeleton of ''Dreadnoughtus schrani'' and the current instability of titanosaurian interrelationships, future analyses may find widely differing positions for it within Titanosauria.

However, in a subsequent analysis of its limb bones, Ullman & Lacovara found that ''Dreadnoughtus'' possessed many of the characteristics of lithostrotians (in particular, it shares a number of traits with ''Aeolosaurus'' and ''Gondwanatitan''), which collectively may indicate that it is actually a lithostrotian closely related to Aeolosauridae. While no new phylogenetic analysis was conducted, they suggested that future cladistic analyses should investigate the relationships between ''Dreadnoughtus'', ''Aeolosaurus'', and ''Gondwanatitan''.

Paleobiology

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. It remains unknown how large this individual would have grown had it not died when it did.

Taphonomy

Based on the sedimentary deposits at the site, the two ''Dreadnoughtus schrani'' specimens appear to have been buried quickly during a fluvial avulsion event, or break in a levee resulting in a flood. This event generated a liquefacted crevasse splay deposit which entombed the two dinosaurs. Thus, rapid and relatively deep burial of the ''Dreadnoughtus'' type specimen accounts for its extraordinary completeness. Numerous small theropod teeth found amongst the bones are likely evidence of scavenging, most likely by megaraptorans, perhaps ''Orkoraptor''.

References

External links

3D PDF files of the entire skeleton and selected bones
are published in the supplementary section of Lacovara ''et al.'' (2014). In Acrobat, the viewer can zoom in and out on the skeleton, rotate the view, and turn individual bones on and off.
*
Dreadnoughtus: A New Dinosaur Discovery
' on YouTube

{{Taxonbar, from=Q17581455

Lithostrotians
Late Cretaceous dinosaurs of South America
Cretaceous Argentina
Fossils of Argentina
Fossil taxa described in 2014
Taxa named by Kenneth Lacovara
Taxa named by Matt Lamanna
Taxa named by Fernando Novas