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Dreadnoughtus
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Dreadnoughtus
Temporal range: Late Cretaceous (Campanian–Maastrichtian), 76–70 Ma
Skeletal restoration showing known elements
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Dinosauria
Clade: Saurischia
Clade: †Sauropodomorpha
Clade: †Sauropoda
Clade: †Macronaria
Clade: †Titanosauria
Clade: †Lithostrotia
Genus: †Dreadnoughtus
Lacovara et al., 2014
Species:
†D. schrani
Binomial name
†Dreadnoughtus schrani
Lacovara et al., 2014

Dreadnoughtus is a genus of titanosaurian sauropod dinosaur containing a single species, Dreadnoughtus schrani. It is known from two partial skeletons discovered in Upper Cretaceous (Campanian to Maastrichtian, approximately 76–70 million years ago) 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 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 paleontologist Kenneth Lacovara, who discovered the genus, 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."[1] Specifically, the name was inspired by the dreadnought, 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]

Discovery and study

[edit]

American palaeontologist Kenneth Lacovara[2] 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.[3]

The bones of both Dreadnoughtus specimens were scanned with a NextEngine 3D laser scanner.[4] 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.

Lacovara (left) with fibula and humerus of Dreadnoughtus

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.[4]

According to the research team that discovered the taxon, including notably Jason Schein, 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.[4]

Controversy over the mass/weight

[edit]

The researchers who described Dreadnoughtus 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 the Dreadnoughtus type specimen weighed about 59.3 tonnes (58.4 long tons; 65.4 short tons).[4] 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.[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] 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).[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 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).[13] 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.[14][15][16] In 2019, considering the argument of Ullmann and Lacovara (2016), Gregory S. Paul moderated his mass estimate of Dreadnoughtus 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 of Dreadnoughtus much higher at 48 and 49 metric tons (53 and 54 short tons).[12][18]

Description

[edit]

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

[edit]
Approximate dimensions of Dreadnoughtus[4][13]
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
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.[4] At 1.74 m, its scapula is longer than any other known titanosaur shoulder blade.[4] Its ilium, 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 of brachiosaurids, which had a more inclined body posture.[4] Only Paralititan[19] 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.[4] The estimated mass of the type specimen is about 48–49 metric tons (53–54 short tons).[12][18]

Completeness

[edit]

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:[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 of D. 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 of D. schrani is more completely preserved than other titanosaurs.[21]

Posture

[edit]
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,[22][23] 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.[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 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.[24]

Although the forelimbs of D. schrani are longer than in any other previously known titanosaur, they are not significantly longer than the hind limbs.[4] Therefore, Lacovara et al. (2014) reconstructed its neck to have been held more horizontally, rather than anteriorly inclined in the manner of Brachiosaurus.[25]

Distinctive features

[edit]
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.[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. In Dreadnoughtus schrani the bottom stem of the "Y" is broadly expanded, likely for the attachment of muscles.[4]

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.[4]

Classification

[edit]

Based on a cladistic analysis, Dreadnoughtus schrani appears to be a "derived" basal titanosaur that is not quite a lithostrotian.[4] 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.

Macronaria

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.[14]

Paleobiology

[edit]

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.[4][27][14]

Taphonomy

[edit]

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,[4] perhaps Orkoraptor.

References

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Dreadnoughtus

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from Grokipedia
Dreadnoughtus schrani is a and species of gigantic titanosaurian sauropod that lived during the period, approximately 84 to 66 million years ago, in the forests and floodplains of what is now southern , . Known from two exceptionally complete partial skeletons representing about 45% of the overall anatomy (and up to 70% of postcranial elements), it measured roughly 26 meters (85 feet) in length and weighed an estimated 59.3 metric tons, ranking among the largest terrestrial vertebrates ever discovered with a reliably calculable body mass. The animal was still growing at the time of its death, suggesting even greater potential size in fully mature individuals. The fossils of D. schrani were first discovered in 2005 by a paleontological team led by Kenneth J. Lacovara along the east bank of the Río La Leona in Santa Cruz Province, within the Upper Cretaceous Cerro Fortaleza Formation. Excavations between 2005 and 2009 yielded the holotype specimen (an adult) and a paratype (a subadult), providing unprecedented insight into the anatomy of giant titanosaurs due to the skeletons' completeness—far surpassing that of other comparably massive sauropods like Argentinosaurus or Futalognkosaurus. The genus name Dreadnoughtus derives from the early 20th-century dreadnought battleships, symbolizing the dinosaur's immense size and presumed invulnerability to predation as an adult, while the species name honors the Schran family, who supported the research. The specific epithet reflects the donation that enabled detailed study and preservation of the specimens. Anatomically, D. schrani featured elongate, peg-like teeth suited for cropping , camellate (hollow, air-filled) e in the and back for lightweight support of its massive frame, and a distinctive biconvex first caudal with a ventral . Its limb bones, including robust humeri and femora, indicate powerful locomotion despite the enormous body mass, with the forelimbs nearly as long as the hindlimbs—a trait shared with other advanced titanosaurs. Paddle-shaped anterior haemal arches in the tail suggest adaptations for stability during movement. As a , it likely browsed on high in a warm, humid environment, contributing to our understanding of sauropod gigantism and the ecological role of titanosaurs in . The discovery has advanced reconstructions of titanosaurian , particularly the , through subsequent studies of its musculature and .

Discovery and naming

Initial discovery

In 2005, paleontologist Kenneth Lacovara and his team discovered the first fossils of Dreadnoughtus schrani during a field expedition in the , located on the east bank of the Río La Leona in Santa Cruz Province, southern , . The initial find included a large sauropod , which prompted further of the site, revealing two partial skeletons that would become the (specimen MPM-PV 1156) and (specimen MPM-PV 3546). The excavation spanned four field seasons from 2005 to 2009, involving an international team that included researchers from and the , such as Lucio M. Ibiricu and Matthew Lamanna. Over this period, more than 100 bone elements were carefully extracted from the Upper sediments, requiring meticulous planning to preserve the fragile fossils amid challenging Patagonian terrain. The specimens, representing approximately 45% completeness for the , were transported to in in 2009 under a formal for preparation and study. At Drexel, initial analysis incorporated advanced techniques, including 3D laser scanning of all bones using a NextEngine scanner to create digital models for virtual reconstruction and biomechanical assessment. These models facilitated detailed examination of muscle scars and skeletal articulation without risking damage to the originals. In March 2015, the fossils were repatriated to their permanent home at the Museo Padre Molina in Río Gallegos, , honoring the international collaboration.

Etymology and formal description

The genus name Dreadnoughtus derives from the term "dreadnought," meaning "fearing nothing," combined with "saurus" for , emphasizing the dinosaur's immense size and presumed invulnerability to predators; this choice also alludes to early 20th-century battleships, evoking a sense of formidable power. The epithet schrani honors Adam Schran for providing crucial logistical and research support during the project's development. Dreadnoughtus schrani was formally described and named in a 2014 paper by Kenneth J. Lacovara and colleagues, published in the open-access journal Scientific Reports. The description established D. schrani as the type species, based on the holotype specimen (MPM-PV 1156, a partial articulated skeleton representing about 45% of the overall skeleton and 70% of postcranial elements) and paratype (MPM-PV 3546), both recovered from the Upper Cretaceous Cerro Fortaleza Formation in southern Patagonia, Argentina. In the original publication, the taxon was classified as a titanosaurian sauropod within Titanosauriformes, positioned as a non-lithostrotian titanosaur more derived than Andesaurus but basal within the clade; the authors compared its anatomy to other South American titanosaurs such as Futalognkosaurus and Rapetosaurus to highlight shared derived traits like pneumatic vertebrae and robust limb elements.

Description

Size and mass estimates

The specimen of Dreadnoughtus schrani (MPM-PV 1156) measures approximately 26 meters in total length, with a estimated at around 6 meters based on limb proportions and skeletal reconstruction. This makes it one of the largest known sauropod dinosaurs, though the individual is considered immature and thus not fully grown. Initial mass estimates for the were derived using limb scaling equations, specifically the bivariate predictive model based on humeral and femoral midshaft circumferences (785 mm and 910 mm, respectively), yielding 59.3 metric tons. Subsequent volumetric reconstructions, which involve of the body outline around the articulated skeleton, have revised this downward; for example, a study using convex hulling and expansion factors produced masses ranging from 22 to 38 metric tons, assuming a body density of about 0.9–1.0 g/cm³. A 2019 scaling-based volumetric model, incorporating analogs scaled to the 1.525-meter and adjusted densities (0.6 g/cm³ for the and 0.9 g/cm³ for the body), estimated the mass at 31 metric tons. These methods highlight differences between limb scaling, which can overestimate mass if soft tissue expansion is not fully adjusted, and volumetric approaches, which better integrate skeletal completeness but vary with assumptions about posture and density. The original 59.3-ton estimate has been critiqued for requiring implausibly high body densities (up to 0.925 g/cm³) and excessive soft tissue bulk, issues addressed in peer-reviewed analyses favoring the lower range of 30–50 metric tons.
YearMethodEstimated Mass (metric tons)Key AssumptionsSource
2014Limb bone scaling (circumferences)59.3Standard without expansion adjustmentLacovara et al. (2014)
2015Volumetric ( + expansion)22–38 ~0.9–1.0 g/cm³; minimal to maximal Bates & Falkingham (2015)
2019Volumetric scaling ( model)31 0.6 g/cm³ (neck), 0.9 g/cm³ (body); immature scalingPaul (2019)

Skeletal completeness

The specimen of Dreadnoughtus schrani (MPM-PV 1156) represents a partial of an individual, comprising 116 elements (including a ) out of an estimated 256 total skeletal elements, yielding an overall completeness of 45.3%. Excluding the , it includes 115 elements out of approximately 196 expected postcranial bones, or 58.7% complete by individual count, and 70.4% of postcranial bone types (100 out of 142). This exceptional preservation allowed for detailed anatomical analysis and volumetric reconstructions of the animal's body form. The preserved elements of the encompass craniodental remains (a fragment and a ), one posterior cervical , seven dorsal vertebrae (with associated ), a complete (six fused vertebrae), 31 caudal vertebrae, 18 haemal arches (chevrons), the left pectoral girdle ( and ), , , , both sternal plates, a complete (, , and pubis), both femora, the left , right , right astragalus, and partial elements of the right pes (metatarsal III and phalanges). These include substantial portions of the , particularly the tail, as well as key appendicular components for understanding limb proportions. The specimen (MPM-PV 3546), from a smaller subadult individual, is less complete, consisting of approximately 30 bones and representing unique elements not fully preserved in the , such as a partial anterior cervical , additional dorsal and ribs, the , seven caudal vertebrae, five haemal arches, all pelvic elements, and the left . This specimen contributes complementary data, particularly to the anterior and , enhancing the overall understanding of intraspecific variation. Among giant titanosaurs, Dreadnoughtus stands out for its skeletal completeness, exceeding that of contemporaries like (15.2% overall, 26.8% postcranial types) and (5.1% overall, 9.2% postcranial types), which are known from far fewer elements. This high degree of preservation facilitates robust phylogenetic placements and biomechanical inferences, surpassing the fragmentary remains typical of most massive sauropods and enabling more accurate paleobiological interpretations.

Posture and locomotion

Dreadnoughtus exhibited a wide-gauge posture, characterized by limbs positioned farther apart from the body's midline than in narrower-gauged sauropods, inferred from the robust proportions of its limb bones and the laterally flared structure of its pelvis. This stance, typical among titanosaurs, enhanced stability for supporting the dinosaur's immense body mass during quadrupedal movement, as evidenced by the broad acetabular region and the medially beveled distal femoral condyles that promoted an abducted limb orientation. The forelimbs of Dreadnoughtus were nearly equal in length to the hindlimbs, with the measuring approximately 1.60 m and the 1.91 m, contributing to a columnar, upright posture without sprawling. This near-equality in limb lengths, combined with the straight shafts of the major long bones, allowed for efficient and minimal lateral sway during locomotion, aligning with the biomechanical demands of a gigantic quadruped. Locomotion in Dreadnoughtus was that of a slow quadrupedal walker, with estimated speeds of 5–7 km/h based on trackway analyses of similar titanosaurs and limb scaling models. The powerful , supported by 32 caudal vertebrae with flexible procoelous articulations and robust haemal arches, likely served as a to aid balance, as suggested by recent musculoskeletal reconstructions emphasizing its role in stabilizing the posterior during forward motion. A 2025 study on locomotion further indicates that such a muscular tail helped maintain postural equilibrium in large sauropods like Dreadnoughtus. The was held in a sub-horizontal position for ground-level browsing, facilitated by the elongated (e.g., a posterior cervical 1.13 m long), while the flexible caudal series permitted elevation to counterbalance shifts in head position. This configuration optimized energy-efficient without requiring extreme neck flexion or extension.

Distinctive anatomical features

Dreadnoughtus schrani possesses notable features in its vertebral column that distinguish it among titanosaurs, particularly in the structure and pneumaticity of the dorsal and caudal vertebrae. The dorsal vertebrae exhibit tall neural spines with subparallel lateral margins, which are posterodorsally oriented and contribute to the overall robust axial framework. These spines incorporate thin spinoprezygapophyseal laminae (SPRLs) that join the prezygapophyseal laminae (PRSLs) high on the spine, enhancing structural integrity. Pneumaticity in the vertebrae of D. schrani is extensive, with dorsal centra filled with camellate internal tissue and featuring large, undivided lateral pneumatic fossae that function as pleurocoels to reduce skeletal weight while maintaining strength. These fossae extend along nearly the entire length of the centrum in some posterior dorsals and are more anteriorly positioned in others, reflecting a high degree of invasion typical of advanced titanosaurs. also display camellate pneumaticity, though lacking pleurocoels in the centra, which supports efficient respiration and in this massive animal. The caudal vertebrae further highlight unique traits, including a biconvex first centrum with a prominent ventral , while subsequent centra are strongly procoelous and lack pneumatic fossae. Neural spines in anterior caudals bear hypertrophied prespinal and postspinal laminae, and the associated haemal arches (chevrons) possess deep articular facets and paddle-shaped distal expansions for robust articulation. These features indicate substantial tail musculature attachment points, providing strong support for the elongated . Middle caudal neural spines include triangular anterodorsal processes that extend beyond the anterior margin of the centrum, adding to the 's distinctive profile. Unlike some titanosaurs such as , which bear osteoderms forming dermal armor, no such structures have been preserved or identified in D. schrani specimens, suggesting minimal or absent armor in this . The high degree of skeletal completeness in the , representing over 70% of postcranial elements, facilitates precise characterization of these anatomical details.

Classification

Phylogenetic position

Dreadnoughtus schrani was recovered as a member of in the original phylogenetic analysis, positioned as a basal titanosaur outside both and Saltasauroidea. This placement situates it more derived than basal titanosaurs such as and Epachthosaurus but basal to more advanced – forms. The analysis, based on a modified character-taxon matrix yielding 30 most parsimonious trees, supported Dreadnoughtus's titanosaurian affinities through shared derived characters including procoelous caudal vertebrae and pneumatic dorsal vertebrae exhibiting camellate internal bone structure. In the resulting , Dreadnoughtus forms a to a comprising Andesaurus and Malawisaurus. A subsequent phylogenetic study incorporating additional titanosaurs recovered Dreadnoughtus as the sister taxon to Lithostrotia, suggesting a position closer to this derived clade than previously thought. This alternative placement is bolstered by vertebral metrics, such as a convex anterior articular surface on the first caudal vertebra and the absence of a ventral bulge on anterior caudal vertebrae. A 2021 Bayesian phylogenetic analysis further recovered Dreadnoughtus as a non-saltasaurid member within Lithostrotia, sister to a clade including Alamosaurus sanjuanensis and Baurutitan britoi. Later reconstructions of Dreadnoughtus's musculature have reinforced its moderately derived position within , consistent with proximity to lithostrotian-grade titanosaurs while lacking osteoderms typical of more nested saltasauroids.

Relationships to other titanosaurs

Dreadnoughtus schrani is classified as a lithostrotian titanosaur, positioned phylogenetically as more derived than basal titanosaurs such as , with which it shares features like small vascular foramina in anterior caudal centra indicative of vertebral pneumaticity. However, Dreadnoughtus exceeds in overall size and complexity of pneumatic structures, including deep subdivided pneumatocoels in caudal neural spines, marking its more advanced position within . In contrast to members of , such as mayorum, Dreadnoughtus exhibits more robust limb bones and differences in vertebral morphology, including biconvex first caudal centra versus the flat anterior surface seen in . Phylogenetic analyses recover Dreadnoughtus within , outside , which forms a separate non-lithostrotian characterized by elongated neural spines and highly developed spinodiapophyseal laminae. Dreadnoughtus shows some affinities to derived titanosaurs within , particularly opisthocoelicaudiines like , in features such as the coels on the first caudal neural spine and a shared pedal ungual flexor process. Nonetheless, it lacks defining saltasaurid traits, including proximolateral humeral expansion and armor osteoderms observed in , aligning it outside despite similarities in ventral keels of caudal centra to genera like Bonatitan reigi. From southern , Dreadnoughtus is coeval with other regional titanosaurs such as Rinconsaurus caudamirus during the –, contributing to evidence of a radiation of diverse titanosaur lineages in this area.

Growth and

Histological analysis of the specimen (MPM-PV 1156) of Dreadnoughtus schrani reveals that the individual was immature at the time of death, with ongoing rapid growth indicated by the presence of thick, well-vascularized fibrolamellar bone tissue in the and an absence of lines of arrested growth (LAGs). This microstructure, characterized by heavy secondary remodeling but no external fundamental system (EFS), confirms that the dinosaur had not yet reached skeletal maturity, as the EFS typically marks the cessation of significant longitudinal growth in vertebrates. Additional osteological evidence, such as the unfused and , further supports this subadult ontogenetic stage. A 2022 study confirmed these findings through detailed analysis of the , revealing preserved soft-tissue microstructures including vessels and osteocytes, as well as endogenous I, providing further evidence of rapid growth typical of titanosaurs. The growth dynamics of Dreadnoughtus align with those of other titanosaurs and sauropods more broadly, featuring high rates of bone deposition consistent with tachymetabolic physiology. Bone histology indicates maximum growth increments of approximately 1–2 metric tons per year during later , enabling the rapid attainment of supermassive body sizes through sustained, determinate growth patterns. This rate is derived from analyses of fibrolamellar apposition in multiple sauropod taxa, where vascularization and laminar organization reflect efficiencies comparable to those in modern endothermic like whales. Given the immaturity of the , mass estimates for Dreadnoughtus—such as the approximately 59-ton figure for this specimen—likely represent submaximal values, with adults potentially achieving 10–25% greater dimensions based on projected continued growth. These findings underscore how extended ontogenetic periods, combined with high annual mass gains, facilitated the evolution of extreme in titanosaurs like Dreadnoughtus, distinguishing them from smaller relatives through prolonged phases of rapid somatic expansion.

Diet and habitat

Dreadnoughtus schrani was an herbivorous sauropod, possessing elongate, peg-like teeth well-suited for cropping tough vegetation with little oral processing. Its exceptionally long , measuring around 11 meters, allowed it to browse high-canopy such as and ferns that dominated the landscape. This feeding strategy aligns with that of other titanosaurs, emphasizing efficient intake of fibrous foliage over ground-level . The species inhabited warm, humid forested floodplains in southern during the late to stages of the , approximately 84 to 66 million years ago. These environments, characterized by seasonal rivers and overbank deposits within the , supported diverse vegetation including a predominance of gymnosperms (75% of woody ) alongside emerging angiosperms. Behavioral inferences suggest Dreadnoughtus may have lived in small groups or solitarily, as evidenced by the close proximity (approximately 40 meters apart horizontally and 25 meters apart vertically) of the two known specimens—two immature individuals of different ontogenetic stages—discovered at the type locality. Its enormous size likely minimized predation risks from contemporary theropods, such as abelisaurids, with the robust, whip-like potentially serving as a defensive against threats. Ecologically, Dreadnoughtus filled the role of a dominant high-level herbivore, using its reach to access resources unavailable to smaller sauropods and thereby influencing competition for canopy vegetation in its floodplain habitat.

Paleoecology and taphonomy

Geological setting and environment

The Dreadnoughtus fossils were discovered in the Cerro Fortaleza Formation, a Late Cretaceous stratigraphic unit dated to the Campanian–Maastrichtian stages (approximately 76–70 Ma), exposed in the Río La Leona Valley of Santa Cruz Province, southern Patagonia, Argentina. This formation, part of the broader Austral Basin, consists primarily of fluvial and aeolian deposits, including sandstones, claystones, conglomerates, siltstones, and paleosols, representing a thickness of 350–500 m. These sediments were laid down by a meandering fluvial system with low to moderately sinuous channels flowing southwestward, interspersed with overbank and floodplain facies exhibiting fining-upward cycles, on the northeastern margin of the basin near the ancient Atlantic shoreline. The paleoenvironment of the encompassed fluvial-palustrine and settings, supporting woodland ecosystems along rivers and alluvial plains, with occasional lacustrine influences. The climate was humid with seasonal rainfall, characterized by warm-temperate to warm-wet conditions and a high , as evidenced by grey clays, beds, and growth rings in fossil wood. Diverse thrived in this landscape, dominated by gymnosperms (75–80% of assemblages), particularly araucarian such as Agathoxylon (attributed to ), alongside other conifer genera like Podocarpoxylon, Taxodioxylon, and Cupressinoxylon; angiosperms, including Hedycaryoxylon and Nothofagoxylon, comprised about 20–25% of the vegetation. Associated fauna in the formation included other titanosaurs such as , theropods comprising abelisaurids and megaraptorids (e.g., ), and crocodylomorphs like peirosaurids, reflecting a diverse community in this fluvial-dominated habitat. Regionally, sedimentation and faunal distribution were influenced by the , which drove eastward migration of the foredeep, basin subsidence, and regression during the –, as part of the ongoing breakup and tectonic evolution of the Austral Basin.

Preservation and burial processes

The exceptional preservation of Dreadnoughtus schrani specimens is attributed to rapid burial in splay deposits within a fluvial system of the . This taphonomic process involved a likely fluvial avulsion event, where a led to flooding and quick sedimentation, entombing the (MPM-PV 1156) and (MPM-PV 3546) in liquefied sands and muds. Such rapid burial minimized exposure to and activity post-mortem, although evidence of limited perimortem scavenging by theropods (e.g., tooth marks on vertebrae) indicates some initial disruption before entombment. The resulting and penecontemporaneous deformation preserved the skeletons or with minimal transport, contributing to their high degree of articulation. A 2022 geochemical and molecular of the revealed remarkable soft-tissue preservation, including vascular structures, osteocytes, and a fibrous matrix, alongside endogenous I confirmed through and enzyme-linked immunosorbent assay (). These findings suggest that anoxic to weakly oxidizing burial conditions limited microbial degradation, fostering exceptional mineralization where bones retained approximately 95% original composition with only minor and dolomite infilling. The low uptake of trace elements in the bone cortex further indicates negligible microbial alteration, preserving molecular integrity over ~70 million years. Diagenetic processes in D. schrani involved protracted infiltration of light rare earth element (LREE)-enriched groundwaters, leading to early replacement that reinforced durability without extensive recrystallization. This minimal late-stage alteration enhanced the fossils' resistance to mechanical damage during excavation, as evidenced by the bones' high structural integrity ( index of 5/5) and low . Overall, these perimortem and postmortem events explain the specimens' suitability for advanced paleobiological analyses.
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