| Dreadnoughtus | |
|---|---|
.jpg) | |
| Skeletal restoration showing known elements | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | †Sauropodomorpha |
| Clade: | †Sauropoda |
| Clade: | †Macronaria |
| Clade: | †Titanosauria |
| Family: | †Saltasauridae |
| Subfamily: | †Opisthocoelicaudiinae |
| Genus: | †Dreadnoughtus Lacovaraet al., 2014 |
| Species: | †D. schrani |
| Binomial name | |
| †Dreadnoughtus schrani Lacovaraet al., 2014 | |
Dreadnoughtus is agenus oftitanosauriansauropoddinosaur containing a singlespecies,Dreadnoughtus schrani. It is known from two partial skeletons discovered inUpper Cretaceous (Campanian toMaastrichtian, approximately 76–70 million years ago) rocks of theCerro Fortaleza Formation inSanta Cruz Province, Argentina. It is one of the largest terrestrial vertebrates known, with the immature type specimen measuring 26 metres (85 ft) in total body length and weighing 48–49 metric tons (53–54 short tons) (the greatest mass of any land animal that can be calculated with reasonable certainty).
Dreadnoughtus is known from more complete skeletons than any other gigantic titanosaurian.Drexel University paleontologistKenneth Lacovara, who discovered the genus, chose the nameDreadnoughtus, which means "fears nothing", stating "I think it's time the herbivores get their due for being the toughest creatures in an environment."[1] Specifically, the name was inspired by thedreadnought, an extremely influential early 20th-century battleship type, known for revolutionarily outclassing (and thus supposedly never needing to fear) the smaller, weaker battleships that came before.[1]
American palaeontologist Kenneth Lacovara[2] discovered the remains in theCerro Fortaleza Formation inSanta 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 fouraustral 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 atDrexel University, theAcademy of Natural Sciences of Drexel University and theCarnegie Museum of Natural History.Dreadnoughtus schrani fossils were returned to their permanent repository at the Museo Padre Molina inRio Gallegos, Argentina in March 2015.[3]
The bones of bothDreadnoughtus specimens were scanned with a NextEngine3D laser scanner.[4] Using the softwareAutodesk 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 Lacovaraet al. (2014) to study the heavy fossils ofDreadnoughtus schrani in a way that was safe for the fossils and enhanced virtual and long-distance collaboration.
Theholotype specimen, MPM-PV 1156, consists of a partial skeleton, somewhat preserved in its original layout, that comprises: amaxilla (jaw) fragment; a tooth; aposteriorcervical vertebra;cervical ribs; multipledorsal vertebrae anddorsal ribs; thesacrum; 32caudal vertebrae and 18haemal arches (bones from the tail) that include a sequence of 17anterior and middlecaudal vertebrae and their corresponding haemal arches found in their original layout; the leftpectoral girdle andforelimb minus thefront foot; bothsternal plates; allpelvic elements; the left hind limb lacking ahind foot and righttibia;metatarsals I and II; and oneclaw fromdigit I.
Theparatype, MPM-PV 3546, consists of a partially articulatedpostcranial skeleton of a slightly smaller individual whose remains were discovered in the same location as the holotype. It includes a partial anterior cervical vertebra, multipledorsal vertebrae and ribs, the sacrum, sevencaudal vertebrae and five haemal arches, a nearly complete pelvis, and the leftfemur.[4]
According to the research team that discovered thetaxon, including notably Jason Schein, the genus nameDreadnoughtus "alludes to the gigantic body size of the taxon (which presumably rendered healthy adult individuals nearly impervious to attack)" and to the two Argentinedreadnoughts that served in the first half of the twentieth century,Rivadavia andMoreno. Thus, the genus name also honors the country in whichDreadnoughtus 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.[4]
The researchers who describedDreadnoughtus schrani estimated its weight using Equation 1 of Campione and Evans (2012),[5] 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 theDreadnoughtus type specimen weighed about 59.3 tonnes (58.4 long tons; 65.4 short tons).[4] By comparison, this would meanD. schrani weighed more than eight and a half times as much as a maleAfrican elephant and even exceeded theBoeing 737-900 airliner by several tons.[6] 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),[7] or even as low as approximately 30 tonnes (30 long tons; 33 short tons), based on a 20% shorter torso.[8] ResearcherGregory S Paul posted a response to Lacovaraet al., pointing out that the error margins using equations based on limb bones are large; using the same equation theDreadnoughtus 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).[9] Benson et al. suggested a maximum body mass of 59 metric tons (65 short tons),[10][11] but these estimates were questioned due to a very large error range and lack of precision.[12]
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 theirD. schrani volumetric model to those of other sauropods with more complete skeletons and better understood mass estimates to conclude that theD. 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).[13] Ullmann and Lacovara disputes the methods used by Bateset al., arguing that the new study treatsDreadnoughtus 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.[14][15][16] In 2019, considering the argument of Ullmann and Lacovara (2016),Gregory S. Paul moderated his mass estimate ofDreadnoughtus type specimen at 31 metric tons (34 short tons), slightly higher than his previous estimation; he even noted that the holotype may have been heavier a tonne or so.[17] In 2020, two studies estimated the mass ofDreadnoughtus much higher at 48 and 49 metric tons (53 and 54 short tons).[12][18]
The discovery ofDreadnoughtus schrani provides insight into the size and anatomy of giant titanosaurian sauropods, especially of the limbs and theshoulder andhip girdles. The majority ofD. 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.
| Dimension | Metric | Imperial |
|---|---|---|
| Maximum mass | 49,000kg[18] | 108,027lb |
| Total length | 26m | 85ft |
| Head and neck length | 12.2m | 40ft |
| Neck-only length | 11.3m | 37ft |
| Torso and hip length | 5.1m | 17ft |
| Tail length | 8.7m | 29ft |
| Shoulder height | 6m | 20ft |
Estimates based on measurements of the known parts of the skeleton suggest that the only known individual ofDreadnoughtus schrani was approximately 26 metres (85 ft) long and stood about 2 stories tall.[4] At 1.74 m, its scapula is longer than any other known titanosaur shoulder blade.[4] Itsilium, the top bone of the pelvis, is also larger than any other, measuring 1.31 m in length.[4] The forearm is longer than any previously known from a titanosaur, and it is only shorter than the long forearms ofbrachiosaurids, which had a more inclined body posture.[4] OnlyParalititan[19] preserves a longerhumerus (upper arm bone). Although each species likely had slightly different body proportions, these measurements demonstrate the massive nature ofDreadnoughtus schrani.[4] The estimated mass of the type specimen is about 48–49 metric tons (53–54 short tons).[12][18]
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 forDreadnoughtus schrani are as follows:
The completeness ofD. schrani compared with other extremely massive (over 40 metric tons) sauropods is as follows:[20]
| Sauropod | Skeletal Completeness Total | Mirrored Postcranial Completeness (i.e. types of bones) |
|---|---|---|
| Dreadnoughtus schrani | 45.5% | 70.4% |
| Turiasaurus riodevensis | 44.1% | 45.8% |
| Futalognkosaurus dukei | 15.2% | 26.8% |
| Paralititan stromeri | 7.8% | 12.7% |
| Argentinosaurus huinculensis | 5.1% | 9.2% |
| Antarctosaurus giganteus | 2.3% | 3.5% |
| Puertasaurus reuili | 1.6% | 2.8% |
Thus, the skeleton ofD. schrani is substantially more complete than those of all other extremely massive (>40 metric tons) dinosaurs.[4]
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 ofD. schrani is more completely preserved than other titanosaurs.[21]
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,[22][23] with their limbs held more widely than their ancestors and contemporaneous counterparts. The stance ofDreadnoughtus schrani was clearly wide-gauge, but not to the degree ofsaltasaurids because thefemoral condyles are perpendicular to its shaft rather thanbeveled.[4] This and the fact that the head of the femur was not turned in towards the body as in saltasaurids[22] support the phylogenetic conclusion thatDreadnoughtus was not a saltasaurid. The animal's broad sternal bones also demonstrate a widepectoral girdle, giving it a broad-shouldered, broad-chested appearance. Paleontologist Kenneth Lacovara compared the animal's gait to anImperial Walker.[24]
Although the forelimbs ofD. schrani are longer than in any other previously known titanosaur, they are not significantly longer than the hind limbs.[4] Therefore, Lacovaraet al. (2014) reconstructed its neck to have been held more horizontally, rather than anteriorly inclined in the manner ofBrachiosaurus.[25]
.jpg)
The tail ofDreadnoughtus 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 theneural spines are extensively subdivided into cavities caused by contact withair sacs (part of the dinosaur'srespiratory 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.[4]
Just like modern archosaurs with tails (crocodilians, for example),[26]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. InDreadnoughtus schrani the bottom stem of the "Y" is broadly expanded, likely for the attachment of muscles.[4]
The shoulder girdle and forelimb ofD. 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.[4]
Based on acladistic analysis,Dreadnoughtus schrani appears to be a "derived"basal titanosaur that is not quite alithostrotian.[4] Lacovaraet al. (2014) note that because of the wide array of relatively "advanced" and "primitive" features in the skeleton ofDreadnoughtus 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 thatDreadnoughtus possessed many of the characteristics of lithostrotians (in particular, it shares a number of traits withAeolosaurus andGondwanatitan), which collectively may indicate that it is actually a lithostrotian closely related toAeolosauridae. While no new phylogenetic analysis was conducted, they suggested that future cladistic analyses should investigate the relationships betweenDreadnoughtus,Aeolosaurus, andGondwanatitan.[14]
The holotype specimen was likely not fully grown when it died. Thehistology of the holotype humerus, which shows a lack of anexternal fundamental system (an outer layer of bone found only in fully-grownvertebrates) and abundant fast-deposited or still-growingwoven tissue in the primaryfibrolamellar bone of the outer bonecortex, led Lacovaraet al. (2014) to determine that the specimen was still growing when it died.[4][27][14]
Based on thesedimentary deposits at the site, the twoDreadnoughtus schrani specimens appear to have been buried quickly during afluvialavulsion event, or break in alevee resulting in a flood. This event generated aliquefactedcrevasse splay deposit which entombed the two dinosaurs. Thus, rapid and relatively deep burial of theDreadnoughtus type specimen accounts for its extraordinary completeness. Numerous smalltheropod teeth found amongst the bones are likely evidence ofscavenging, most likely bymegaraptorans,[4] perhapsOrkoraptor.
