Savannasaurus is a genus of titanosaurian sauropod dinosaur from the Late Cretaceous Winton Formation of Queensland, Australia. It contains one species, Savannasaurus elliottorum, named in 2016 by Stephen Poropat and colleagues. The holotype and only known specimen, originally nicknamed "Wade", is the most complete specimen of an Australian sauropod, and is held at the Australian Age of Dinosaurs museum. Dinosaurs known from contemporary rocks include its close relative Diamantinasaurus and the theropod Australovenator; associated teeth suggest that Australovenator may have fed on the holotype specimen.
At long, Savannasaurus was a medium-sized titanosaur. It is notable for its wide hips, which would have been over wide at their widest points. This would have distributed its body weight more evenly, along with a robust (upper arm bones) and possibly also the tall (ankle bone). Combined with a flexible vertebral column, these traits would have made Savannasaurus better at navigating the muddy ground of the floodplains that it lived on. Other titanosaur lineages also show some of these traits, which might have been independently acquired from similar environmental pressures.
Various traits suggest that Savannasaurus was an early-diverging (or basal) member of the Titanosauria, but not a member of the more specialized group Lithostrotia. Among these are the absence of in its vertebrae, which was one of the flexibility-enhancing traits that may have appeared in multiple titanosaur lineages. Palaeobiogeographic analyses suggest that the ancestors of Savannasaurus and Diamantinasaurus were from either Asia or one of the southern Gondwanan continents; regardless, they likely migrated to Australia through Antarctica around 105–100 million years ago.
Discovery and naming
In March 2005, the first specimen of Savannasaurus was discovered by David Elliot, founder of Australian Age of Dinosaurs (AODF), along with his wife Judy and their children. The specimen, now catalogued as AODF 660, was found on the Belmont sheep station, northeast of Winton, Queensland; the site was given the number AODF 82, and is informally called the "Ho-Hum site". Excavations by the Queensland Museum and hundreds of volunteers unearthed the specimen in July and September of that year. A siltstone concretion containing the specimen was split along gypsum-filled fractures using jackhammers and chisels, with the pieces being marked to facilitate reassembly. Preparation of the specimen took around a decade, with air scribes and micro-jacks being used. Adhesives, such as superglue and araldite, were then used to put the pieces back together. The marks allowed the orientation of the specimen as it was found to be reconstructed, which informed a three-dimensional site map constructed with photogrammetry.
AODF 660 consists of around 40 bones: one (neck), the third through tenth (trunk), at least four (hip), and at least five (tail) ; cervical and (fragmentary) dorsal ribs; portions of the shoulder girdle, including a fragmentary , a left , and both sternal plates; parts of the forelimbs, including portions of both , a fragmentary , the left , and hand bones (the left first to fifth , the right fourth metacarpal, and two ); the fused hip bones, namely both the left and the right and ; foot bones (the left and right third ); and other fragments. One of the foot bones was originally thought to have belonged to a theropod, while the metacarpals were initially also interpreted as having come from the opposite hands. The specimen is the most complete sauropod specimen known from Australia, and represents approximately 20% to 25% of the animal.
The bones were preserved over an area of less than . While the dorsal vertebrae were not articulated, they were preserved in sequence in front of the hip bones, allowing for their identification. The ribs on the left side appear to have been crushed prior to fossilization, before all of them were fragmented further. The fifth dorsal vertebra and both humeri show signs of being trampled by other dinosaurs before fossilization, which may also explain the scattered bones; however, the bones were less scattered than other dinosaur specimens from contemporary rocks. A third excavation in September 2006 discovered no additional remains, but a partial from a caudal vertebra was found on the surface in 2013.
Having been nicknamed "Wade", AODF 660 was formally described and named in 2016 by Stephen Poropat and colleagues in a paper that also described a skull belonging to Diamantinasaurus. They identified AODF 660 as the holotype of a new genus and species, Savannasaurus elliottorum; the generic name of Savannasaurus, from Taíno zavana (the root of the word "savanna"), refers to the environment in which it was found, while the specific name elliottorum honours the Elliott family and their contributions to Australian palaeontology. The preliminary anatomical description of AODF 660 in this paper was supplemented in 2020 by another paper from many of the same authors, which provided a full description of its osteology.
Description
Savannasaurus was a medium-sized titanosaur that measured about in length, with a weight of around and a shoulder height of around .
Vertebral column As with most other members of the Titanosauriformes, the vertebrae of Savannasaurus were camellate, or by small holes created by air sacs, and they lacked that allowed neighbouring vertebrae to interlock. The cervical and dorsal vertebrae of Savannasaurus were all , or having a centrum (vertebral body) that is convex in front and concave behind. All other members of Eusauropoda have opisthocoelous cervicals, and all other members of Macronaria have opisthocoelous dorsals. Unusually, the front articulating surface of one of the caudal centra was undulating, being concave in the upper half and convex in the lower half. Poropat and colleagues suggested this as a distinguishing characteristic of Savannasaurus. The other caudals were , or having centra that are concave on both ends, which was also rare among titanosaurs save for Baotianmansaurus and Dongyangosaurus.
The only known cervical vertebra of Savannasaurus is considered to have come from the rear of the neck due to its proportions. Unlike most titanosaurs but similar to the "ancestral" (plesiomorphic) condition among sauropods, the vertebra had a subtle keel running longitudinally along its bottom surface; Mendozasaurus, Overosaurus, Austroposeidon, and Rapetosaurus also have such keels. There was a well-developed longitudinal , or excavation, along the side of the vertebra that extends for nearly its entire length, unlike in many titanosaurs where it was faint or absent altogether. In contrast to more specialized titanosaurs like the Saltasauridae, the (one of the rib articulations) was restricted to the front half of the centrum. Associated cervical rib fragments suggested that they were relatively long, extending for the length of at least three centra.
Like many titanosaurs, the articulating surfaces of the dorsal centra in Savannasaurus were shorter vertically than they were wide. The dorsals have weakly-developed ridges bordering the sides of their bottom faces, as in Diamantinasaurus and Opisthocoelicaudia (where they were better developed), but Savannasaurus lacks the keel on the bottom of the centra as in these species. Like most other members of the Somphospondyli, the sides of the centra bore teardrop-shaped pneumatic holes. The at the front of the centra known as the were connected by a sheet of bone (), the transprezygapophyseal lamina; in the front dorsals, this sheet was V-shaped as in most titanosaurs, but it was basically flat in the rear dorsals unlike most other titanosaurs. Notably, the laminae in the dorsals of Savannasaurus were bilaterally asymmetric; the left side of the vertebrae would have been better reinforced by extra laminae. Also unlike Opisthocoelicaudia, the dorsal neural spines on top of the centra increased in height towards the rear dorsals, were not split into two, and were taller than their corresponding articular surfaces.
In addition to the undulating articulating surface in one of the front caudal centra (caudal "A"), the two front caudals (caudals "A" and "B") of Savannasaurus have another distinguishing characteristic. Like the non-titanosaur somphospondylan Padillasaurus, the front caudal centra had shallow pneumatic excavations on the sides surrounding small holes, whereas most members of the Somphospondyli have only the holes and not the excavations. This characteristic had previously been regarded as a distinguishing characteristic of the non-somphospondylan brachiosaurids. Two other caudal vertebrae (caudals "C" and "D") are known from further back in the tail. The centra of caudals B, C, and D were about the same length, suggesting that centrum length did not vary considerably. However, both articulating surfaces were wider than tall in caudal C, while the front one was taller than wide in caudal D, suggesting that it was further forward than caudal D. Like other titanosauriforms, the processes that enclose the neural canal—the pedicles—in caudals C and D were two-thirds the length of the centra, and were shifted forwards relative to the midline.
Shoulders and forelimbs Two distinguishing characteristics of Savannasaurus are found in the sternal plates. First, they were D-shaped with straight outer margins when viewed from the bottom, instead of being kidney-shaped as in other titanosaurs. Second, they lacked the long ridges that were present on the bottom surfaces among most members of Neosauropoda. In general, the bottom surfaces were devoid of the grooves and pits on the top surfaces which are indicative of cartilage coverings. However, like most other titanosaurs, the sternal plates would have been at least 65% as long as the humeri (in Savannasaurus, the ratio was approximately 71%). The coracoid of Savannasaurus was oval-shaped in side view, unlike the characteristically quadrangular coracoids of saltasaurids, and the glenoid fossa on it (which articulated with the humerus) was much larger than that of Diamantinasaurus. Unlike most titanosauriforms save for Daxiatitan, Ligabuesaurus, and Sauroposeidon, the articulation with the scapula at the back of the bone was taller than the entire bone was long front to back.
Like Diamantinasaurus, Opisthocoelicaudia, and saltasaurids, the humerus of Savannasaurus was robust. The radius was twisted about its axis, with the long axes of the two ends being oriented in different planes, seen otherwise only in Huabeisaurus, Epachthosaurus, and Rapetosaurus. The maximum width of its top end (33% of the bone's length) and the presence of a strong ridge at the rear outer corner (the interosseous ridge) were more typical. Like all neosauropods, the hand digits of Savannasaurus were arranged like a horseshoe. Like Diamantinasaurus and Wintonotitan, the third metacarpals were the longest, followed by the second, first, fourth, and fifth. The third metacarpal was 49% the length of the radius, which was short for a titanosaur, but was at least 45% as in all macronarians. A distinguishing feature is that the bottom of the fourth metacarpal was hourglass-shaped, not trapezoidal or hexagonal. In life, the metacarpals would have been bound at the top and splayed at the bottom. The presence of phalanges is unusual; all titanosaurs but Savannasaurus and Diamantinasaurus had hands formed only by metacarpals. All digits but the first likely had phalanges, although their articulating surfaces on the bottoms of the metacarpals did not extend to the front as would be expected.
Pelvis and hindlimbs