The breakup of the Gondwanan Supercontintent into its constituent parts (South America, Antarctica, Africa, Madagascar, India, Australia, New Zealand and some smaller landmasses) during the Cretaceous played an important role in the distribution of terrestrial animals and plants in the Southern Hemisphere that can still be seen today. How this would have affected the distribution of large animals such as Dinosaurs is particularly interesting, but is hard to assess as of these continents only South America has a good terrestrial fossil record extending all the way through the Cretaceous. Northern and Southeast Africa, and eastern Australia have strata which produce numerous terrestrial fossils including Dinosaurs, but almost no such fossils are known from the Late Cretaceous here, while the reverse is true in India, Madagascar and Antarctica, with numerous Late Cretaceous Dinosaurs but very few Middle Cretaceous specimens.
In a paper published in the journal Scientific Reports on 20 October 2016, Stephen Poropat of the Department of Earth Sciences at Uppsala University and the Australian Age of Dinosaurs Museum of NaturalHistory, Philip Mannion of the Department of Earth Science and Engineering at Imperial College London, Paul Upchurch of the Department of Earth Sciences at University College London, ScottHocknull of Geosciences at the Queensland Museum, Benjamin Kear, also of the Department of Earth Sciences at Uppsala University and of the Museum of Evolution, also at Uppsala University, Martin Kundrát of the Department of Ecology at Comenius University and the Center for Interdisciplinary Biosciences at the University of Pavol Jozef Šafárik, and Travis Tischler, Trish Sloan, George Sinapius, Judy Elliott and David Elliott, all of the Australian Age of Dinosaurs Museum of Natural History, describe two new Sauropod Dinosaur specimens from the early Late Cretaceous Winton Formation of Queensland.
The Winton Formation is an iron rich sandstone laid down in a shallow inland sea (the Etomanga Sea) and associated river systems that covered parts of Queensland and central Australia during the early Late Cretaceous (98-95 million years ago). which extends from Hungerford on the New South Wakes border northwest to the area around Kynuna, a distance of over 1000 kilometres. This formation is famous for its Dinosaurs, but also produces Crocodylians, Turtles, Fish and a wide range of Invertebrates. It is also noted for the production of opals, which are typically found in cracks in ironstone concretions (themselves formed by precipitation from water that has accumulated iron as it peculated through the feruginous sandstone), and is commonly called 'boulder opal'.
Map of Queensland, northeast Australia, showing the distribution of Cretaceous outcrop. Porapat et al. (2016).
The first specimen described is assigned to a new species and genus and named Savannasaurus elliottorum, where 'Savannasaurus' refers to the Savanah Grasslands where the specimen was found and 'elliottorum' honours the Elliott family for their contributions to Australian palaeontology. The specimen comprises a series of vertebrae and ribs plus a fragmentary scapula, a left coracoid, the left and right sternal plates, incomplete left and right humeri, a shattered ulna, the left radius a number of metacarpals and phalanges, fragments of the left and right ilias, the left and right pubes and ischia, fused together, the left astragalus the right third metatarsal and some other fragmentary remains.
Savannasaurus elliottorum. (a–e) Dorsal vertebrae (left lateral view). (f) Sacrum (ventral view). (g,h) Caudal vertebrae (left lateral view). (i) Left coracoid (lateral view). (j) Right sternal plate (ventral view). (k) Left radius (posterior view). (l) Right metacarpal III (anterior view). (m) Left astragalus (anterior view). (n) Co-ossified right and left pubes (anterior view). A number of ribs were preserved but have been omitted for clarity. Scale bar is 500 mm. Porapat et al. (2016).
The second of specimen is referred to the species Diamantinasaurus matildae, which has previously been described from the Winton Formation. This specimen comprises a left squamosal, a nearly complete braincase, a right surangular, several skull fragments, the atlas-axis, five post-axial cervical vertebrae, three dorsal vertebrae, a partial sacrum, some dorsal ribs, a right scapula, both iliac preacetabular processes, a paired pubes and ischia and some other fragmentary material. The preservation of skull material in this specimen is particularly noteworthy, as this is the first such material known not only for this species but for any Australian Sauropod.
Diamantinasaurus matildae, new specimen. (a,b) Braincase (left lateral and caudal views). (c,d) endocranium (left lateral oblique and ventral views). (e) Axis (left lateral view). (f) Cervical vertebra III (left lateral view). Abbreviations: bt, basal tuber; cca, internal carotid artery; coch, cochlea; crb, cerebral hemisphere; crbl, cerebellum; dds, dorsal dural sinus; fm, foramen magnum; hfp, hypophyseal fossa placement; ioa, internal ophthalmic artery; jug, jugular vein; lbr, endosseous labyrinth; mf, metotic foramen; midb, midbrain; mo, medulla oblongata; nc, nuchal crest; occ, occipital condyle; ofb, olfactory bulb; oft, olfactory tract; pp, paroccipital process; II, optic tract; III, oculomotor nerve; IV, trochlear nerve; V, trigeminal nerve; V1, ophthalmic branch of the trigeminal nerve; V2+3, maxillo-mandibular branch of the trigeminal nerve; VI, abducens nerve; VII, facial nerve; IX, glossopharyngeal nerve; X, vagus nerve; XI, accessory nerve; XII, hypoglossal nerve? structure of unknown or disputable identity/placement. Scale bar is 100 mm. Porapat et al. (2016).
Both of these specimens are adjudged to be Titanosaurs, a group of (often extremely large) Sauropod Dinosaurs that originated in South America and spread across much of the globe during the Cretaceous, which has interesting biogeographical implications for the Australian fauna of the Cretaceous. The extremely large size of Titanosaurs means that they are highly unlikely to have been dispersed across oceans by rafting or any similar mechanism, suggesting that they must have walked from South America to Australia overland. This was certainly possible during the Cretaceous, as the continents of Gondwana were still largely attached at the beginning of this period, and even at the end Australia was still attached to South America via a land-bridge across Antarctica. Titanosaurs, and Sauropods in general, appear to have favoured warmer climates, being absent from latitudes higher than 66° in either hemisphere, and much less diverse in higher latitudes than they were within the tropics. Antarctica was attached to both South America and Australia throughout the Cretaceous, but is known to have had a much cooler climate, with a distinct flora and fauna of its own that were apparently adapted to much cooler conditions during the Middle Cretaceous. This would appear to make a dispersal from South America across Africa, Madagascar and India during the Early Cretaceous (reaching Australia before 119 million years ago, when the connection between Indo-Madagascar and Australia was finally broken) the most likely method for these animals to reach Australia.
Palaeogeographic map of the mid-Cretaceous world. Showing the possible high latitude dispersal routes that might have been utilised by titanosaurs and other sauropods during the late Albian–Turonian. Porapat et al. (2016).
However, Australia has a reasonably good fossil record for part of the Middle Cretaceous (in this case roughly the period from 115-105 million years ago, which has produced a wide variety of Dinosaur specimens, but not yet to date any Sauropods. Neither have Titanosaurs thought to be closely related to the Australian species been recovered from the Middle Cretaceous deposits of Africa or the Late Cretaceous deposits of India or Madagascar, something which might be expected if the ancestors of these Sauropods had walked across Africa, India and Madagascar into Australia.
As an alternative route Porapat et al. suggest that the arrival of Titanosaurs in Australia may have occurred via Antarctica in the much warmer climate of the Late Cretaceous, after about 105 million years ago, when rapidly rising global temperatures brought a much warmer climate to Antarctica, possibly allowing rapid dispersal of warmth-loving Titanosaurs across the Antarctic land bridge.