Savannasaurus

Map of the Belmont sheep station and outcrops of the Winton Formation (yellow-green) near Winton

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, 60 kilometres (37 mi) northeast of Winton, Queensland; the site was given the number AODF 82, and is informally called the "Ho-Hum site".[1] 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,[2] 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.[3]

AODF 660 consists of around 40 bones: one cervical (neck), the third through tenth dorsal (trunk), at least four sacral (hip), and at least five caudal (tail) vertebrae; cervical and (fragmentary) dorsal ribs; portions of the shoulder girdle, including a fragmentary scapula, a left coracoid, and both sternal plates; parts of the forelimbs, including portions of both humeri, a fragmentary ulna, the left radius, and hand bones (the left first to fifth metacarpals, the right fourth metacarpal, and two phalanges); the fused hip bones, namely both the left and the right pubis and ischium; foot bones (the left astragalus and right third metatarsal); and other fragments.[1] One of the foot bones was originally thought to have belonged to a theropod,[4] while the metacarpals were initially also interpreted as having come from the opposite hands.[3] The specimen is the most complete sauropod specimen known from Australia, and represents approximately 20% to 25% of the animal.[2]

Orientation of the holotype specimen as it was found

The bones were preserved over an area of less than 20 m² (220 sq ft). 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 centrum from a caudal vertebra was found on the surface in 2013.[3]

Having been nicknamed "Wade",[2] 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 elliotorum; 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 elliotorum honours the Elliott family and their contributions to Australian palaeontology.[1] 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.[3]

Size comparison

Savannasaurus was a medium-sized titanosaur that measured about 15 metres (49 ft) in length, with a weight of around 20 tonnes (22 short tons) and a shoulder height of around 3 metres (9.8 ft).[4]

Skeletal reconstructions of Diamantinasaurus, a close relative and contemporary of Savannasaurus

For their 2016 description, Poropat and colleagues conducted a phylogenetic analysis to test the affinities of Savannasaurus and the new skull specimen of Diamantinasaurus. They added these specimens to a dataset created by Philip Mannion (one of the co-authors of the description of Savannasaurus) and colleagues for the 2013 redescription of Lusotitan.[6] The analysis placed Savannasaurus and Diamantinasaurus together in a clade (unified group) close to the root of Titanosauria, outside the derived (specialized) group Lithostrotia.[1] Similar results were recovered by Rafael Royo-Torres and colleagues in their 2017 description of Soriatitan;[7] Mannion and colleagues in their 2017 description of Vouivria;[8] Alexander Averianov and Vladimir Efimov in their 2018 description of Volgatitan;[9] Bernardo Gonzàlez Riga and colleagues in their 2018 redescription of Mendozasaurus;[10] and Pedro Mocho and colleagues in their 2019 descriptions of an indeterminate lithostrotian[11] and Oceanotitan.[12]

In 2020, Poropat and colleagues noted a number of features that support a position for Savannasaurus outside Lithostrotia: the horizontal transprezygapophyseal laminae, the lack of constriction in the vertebrae of the sacrum, the amphicoelous caudal centra, the pneumatic excavations on the front caudals, the D-shaped sternal plates, the presence of phalanges, and the lack of a longitudinal ridge on the side of the pubis. At the same time, the lack of hyposphene-hypantrum articulations in the dorsal and caudal vertebrae provides evidence against a more basal (non-specialized or rootward) position, given their presence in many non-titanosaur titanosauriforms and rarity among titanosaurs.[6] Some titanosaurs also had these articulations; the basal Andesaurus and the lithostrotian Epachthosaurus had them in the dorsals, while the lithostrotians Epachthosaurus, Opisthocoelicaudia, Volgatitan, and Malawisaurus had them in the caudals. However, Poropat and colleagues recognized the possibility that they could have been acquired independently multiple times among titanosaurs.[3]

Dorsal vertebrae and ribs of Baotianmansaurus, found by some analyses as a close relative of Savannasaurus

Nevertheless, other phylogenetic analyses based on Mannion and colleagues' 2013 dataset have found different results. Royo-Torres and colleagues repeated their 2017 analysis after removing a titanosauriform from the Cloverly Formation, Chubutisaurus, and Angolatitan; they found instead that Savannasaurus and Diamantinasaurus were successively basal to a clade consisting of Baotianmansaurus and Dongyanosaurus.[7] Savannasaurus shared amphicoelous caudal centra with both genera, various features of vertebral laminae with Baotianmansaurus, and unkeeled dorsal centra and the absent longitudinal ridge on the side of the pubis with Dongyanosaurus. However, unlike Savannasaurus, Baotianmansaurus had hyposphene-hypantrum articulations, and both genera had bifid dorsal neural spines (i.e., split into two).[3]

Similarly, for a 2019 paper naming the new clade Colossosauria, Gonzàlez Riga and colleagues extended their 2018 phylogenetic analysis and found an unresolved polytomy of Savannasaurus, Baotianmansaurus, Dongyanosaurus, and a clade of more derived titanosaurs. Diamantinasaurus was placed in a more derived position as the sister group of Lithostrotia.[13] Also in 2019, Julian Silva Jr. and colleagues extended the 2018 analysis of Gonzàlez Riga and colleagues for their redescription of Uberabatitan. They found Savannasaurus outside the Titanosauria in a polytomy with Euhelopus, Erketu, Qiaowanlong, a clade of Tangvayosaurus and Phuwiangosaurus, and more derived somphospondyls; they also found Diamantinasaurus and Baotianmansaurus to form a clade inside the Saltasauridae.[14] Poropat and colleagues considered their results unusual in 2020.[3] In their 2019 redescription of Jiangshanosaurus and Dongyanosaurus, Mannion and colleagues found both Savannasaurus and Diamantinasaurus inside Saltasauridae by equally weighting characteristics in their dataset, but using implied weighting eliminated this result.[15]

The phylogenetic tree from the analysis of Poropat and colleagues in 2016 is reproduced below, at left, with emphasis on relevant clades.[1] An alternative tree by Royo-Torres and colleagues in 2017, from the analysis that removed the Cloverly titanosauriform, Chubutisaurus, and Angolatitan, is also shown at right.[7]

Palaeobiogeography

As the southern supercontinent of Gondwana broke up during the Cretaceous period, dinosaur faunas became more diverse.[16] Conventional hypotheses suggest that the faunas from the Gondwanan continents (including Australia) would be closely related,[17] but mid-Cretaceous Australian dinosaurs have been found to be more closely related to their Laurasian (northern) counterparts in spite of the fact that Laurasia and Gondwana had already been separated by the Tethys Ocean for a significant span of time.[18][19] In 2016, Poropat and colleagues tested this hypothesis by performing a palaeobiogeographic analysis using the results of their phylogenetic analysis. Using the modern geography of the continents, they found that the ancestors of the Australian sauropods Savannasaurus, Diamantinasaurus, and Wintonotitan were likely Asian. Incorporating Cretaceous geography suggested ancestral ranges also spanning the Gondwanan continents of South America, Africa, and Indo-Madagascar.[1]

Time-calibrated phylogeny of Poropat and colleagues, colour-coded with the continents where genera lived (light blue = North America; light green = Europe; red = Asia; dark blue = South America; yellow = Africa; purple = India; dark green = Australia)

Combining their palaeobiogeographic analysis with time-calibrated phylogenies based on fossil age estimates, Poropat and colleagues inferred that somphospondylans and titanosaurs had attained a wide distribution across the world by the Barremian epoch, 131 to 125 million years ago, if not earlier. However, they ultimately diverged into a number of lineages ultimately with limited geographical ranges; they attributed this endemism to regional extinction events. They estimated that the Savannasaurus+Diamantinasaurus and Wintonotitan lineages had separately reached Australia no later than the late Albian epoch, 105 to 100 million years ago, but the exact timing depended on the models that they used. The oldest Australian titanosauriforms, Austrosaurus and the "Hughenden sauropod", date to this interval.[1][20]

Although they acknowledged the possibility of sampling biases, Poropat and colleagues considered a late Albian dispersal likely because of the most probable dispersal route, which would have entailed beginning from South America and crossing through Antarctica to southeastern Australia. In the Aptian to early Albian epochs, this region of Australia would have been situated at a latitude of 70°S, with a cool, temperate climate.[21] No sauropods have been found from these latitudes and environments, suggesting that they were adapted to warm climates and avoided these regions. Thus, they would likely have been unable to disperse across Antarctica until the late Albian, when global warming led to a smoother temperature gradient.[22][23] Poropat and colleagues suggested that climate preferences could also explain the Laurasian affinities of Australian dinosaurs.[1]

In 2020, Tai Kubo applied network analysis to a phylogenetic "supertree" of Australian dinosaurs. He found a strong connection between Australian dinosaur faunas and other Gondwanan dinosaurs, and identified Gondwanan Cretaceous dinosaurs as a distinct community. Thus, he suggested that most previous results identifying affinities between Australian and Laurasian dinosaurs were caused by insufficient sampling of Gondwanan dinosaurs and the phylogenetic instability of Australian dinosaurs. For instance, he pointed out that the megaraptoran Australovenator was thought to be an offshoot of an Asian lineage until more Gondwanan megaraptorans were found. While he suggested that the same was true of these sauropods, he also recognized the possibility that they had a different biogeographical history than smaller dinosaurs.[24]

Skeleton of Opisthocoelicaudia from the front, showing the barrel-like body and wide-gauge stance that it shared with Savannasaurus

Like all titanosaurs, Savannasaurus was likely a "wide-gauge" sauropod, meaning that it would have stood and walked with its feet far apart from the midline.[25] In 2020, Poropat and colleagues pointed to the width of its sternal plates and sacrum as evidence for a wide-gauge stance. The robust humerus of Savannasaurus was also considered a likely adaptation to a wide-gauge stance, which would have been convergently acquired with saltasaurines. They also suggested that the unusual form of the astragalus was also an adaptation to weight-bearing, but the absence of other hindlimb bones made this unclear. Yet, Poropat and colleagues also noted traits that would have increased the flexibility of the vertebral column, namely the absence of hyposphene-hypantrum articulations and the presence of prominent excavations (the centroprezygapophyseal fossae) below the transprezygapophyseal laminae. Both of these were shared with Diamantinasaurus, but were otherwise rare among sauropods.[1][3]

Poropat and colleagues noted that both Savannasaurus and Diamantinasaurus would have lived on a floodplain with clay-rich soil and high rainfall (see § Palaeoecology), which would imply that the ground would occasionally become muddy and hazardous for large sauropods. They considered it possible that environmental pressures would have selected for individuals capable of navigating muddy ground, and that Savannasaurus might have spent more time near water than other sauropods. Flexible vertebrae (to escape bogs), barrel-like bodies similar to those of hippopotami, wide-gauge stance, and robust forelimbs (all to distribute its body weight) would all have been helpful for such a niche. They postulated that saltasaurines might have been subject to similar selective pressures, and so the "typical" titanosaur bodyplan may have been acquired convergently multiple times. While the same environment could also have selected for longer necks to provide easier access to water, Poropat and colleagues noted that Savannasaurus and Diamantinasaurus likely had relatively short necks, though overlapping cervical ribs might have improved stability in Savannasaurus.[3]

Selected plant fossils from the Winton Formation

The Belmont sheep station is part of the fossil-bearing rock units in the upper Winton Formation, which has been dated to around or just after the boundary between the Cenomanian and Turonian epochs of the Cretaceous—about 93.9 million years ago—based on uranium-lead dating of detritial zircons.[26] It is the geologically youngest rock unit of the Mesozoic era in the Eromanga Basin. Six major facies, or distinct rock types, are found in the Winton Formation: two different types of sandstone-siltstone combinations (associated with meandering rivers, crevasse splays, and levees), mudstones (associated with oxbow lakes and ponds), sandy siltstones to silty mudstones (associated with tidal flats), plant-rich mudstone and coal (associated with swampy ground), intraformational conglomerates (associated with flooding). These rocks suggest that Savannasaurus lived on a freshwater floodplain covered in broad, meandering river channels with low-energy waterflow, which was subject to occasional flooding.[27][28]

During the Cenomanian and Turonian, the Winton area had a latitude of around 50°S. Leaf fossils show that the climate was warm, with a mean annual temperature of 16 °C (61 °F), and wet, with a mean annual precipitation of over 1,300 millimetres (51 in). Although the weather was seasonal, there was no extreme cycle between the wet season and dry season as is the case today. The growing season would have been eight to nine months long. However, weather patterns also appeared to have been cyclical on a multi-year basis, similar to the modern Pacific decadal oscillation; this may also have influenced the sporadic flooding. Conifers and angiosperms were the most common plants in the Winton Formation; conifers included the cypress Austrosequoia, the araucarias Araucaria and Emwadea, and the podocarp Protophyllocladoxylon, while angiosperms included Lovellea and various unnamed forms. Other plants included the ferns Phyllopteroides and Tempskya; the liverwort Marchantites; the horsetail Equisetites; the bennettitaleans Otozamites and Ptilophyllum; and the ginkgoalean Ginkgo.[3][28]

Australovenator fed on Savannasaurus and other sauropods

Savannasaurus lived alongside a diverse vertebrate fauna in the upper Winton Formation.[26] Contemporary dinosaurs included the sauropods Diamantinasaurus matildae and Wintonotitan wattsi, the megaraptoran theropod Australovenator wintonensis, and indeterminate ankylosaurians and hypsilophodonts. A tooth belonging to Australovenator was found near the Savannasaurus holotype, suggesting that its feeding was responsible for the specimen's disarticulation. Other archosaurs included the pterosaur Ferrodraco lentoniT[29] and the crocodilian Isisfordia duncani, along with other undescribed pterosaurs and crocodilians. Additional vertebrates known from the Winton Formation are chelid turtles; an indeterminate varanoid lizard;[30] the lungfish Metaceratodus ellioti and M. wollastoni; and ray-finned fish including Cladocyclus geddesi.[31] Invertebrates were dominated by the bivalves Alathyria jaqueti, Hyridella goondiwindiensis, Megalovirgus wintonensis, and Prohyria macmichaeli, but also included the gastropod Melanoides, dragonflies, scorpionflies, and oribatid mites known from trace fossils found in silicified wood.[3][28][32]

• 2016 in paleontology