South Polar region of the Cretaceous

Dinosaur Cove has moved north over the last hundred million years

The South Polar dinosaurs proliferated in the Polar forests of the Cretaceous, 145–66 mya, while the continent of Australia was still linked to Antarctica to form East Gondwana, a continent that had rifted from Africa and drifted southward. Much of this southern continent lay inside the Antarctic Circle, and the climate there was unlike any that exists today. This led to fauna and flora that were unique to the time. Much of what is known about the fauna of Polar Australia comes from fossil beds found in Dinosaur Cove and Flat Rocks on the Victorian coast of southeast Australia.

Landscape

Main article: Polar forests of the Cretaceous

East Gondwana may have been similar to present day South American temperate forests (above).

The landscape of the Middle Jurassic polar region has been reconstructed from the remains of forests exposed in New Zealand, which were between 70 and 80°S during the Jurassic. The vegetation was largely made up of conifers, cycads, and other gymnosperms, as well as ferns; on the forest floor grew lycopods, bryophytes, fungi, and algae. It had a temperate climate with heavy rainfall.

In the Early Cretaceous, East Gondwana (Australia, Antarctica, India, and Madagascar) had started to split away from South America, and India and Madagascar also began to separate at around the same time period. The tropics zone may have extended, during the Cretaceous, to 32°S, allowing year-round tree growth in the Antarctic in polar forests. However, it is also possible that the plant life may only be representative of the warm summer months. Much of what is known of the plantlife of East Gondwana during the Cretaceous consists of pollen remains and leaf compressions from the northern Antarctic Peninsula. Depending on the latitude, the polar winters may have lasted from six weeks to four and a half months.

The Cretaceous is characterized by warm global temperatures caused by the high amounts of carbon dioxide and possibly methane greenhouse gases in the atmosphere. This caused a lack of permanent ice coverage in the polar regions, though the carbon dioxide level dropped between 115 and 66 million years ago (mya), possibly allowing some permanent ice cover. It is possible that, throughout the Cretaceous, several small ice sheets developed. Tectonic activity prevented glaciation and increased global temperatures, and temperatures may have been up to 15 °C (59 °F) warmer than they are in the modern age.

The discovery of several mature evergreen and deciduous trees indicate a warm-to-cool temperature with moderate seasons lacking widespread freezing, at least between the latitudes 70 and 85°S. The canopy of the polar forests around present-day Alexander Island, which was around 75°S in the Cretaceous, were predominantly evergreen, and likely most South Pole polar forests were as well, and comprised mainly araucarian and podocarp conifers. It is thought that these trees remained dormant throughout the polar winters until summers under the midnight sun. Indicated by the size of the tree rings on fossil trees, the Antarctic polar forests featured a cooling trend throughout the Maastrichtian age 72 to 66 mya, from a mean annual temperature of 7 °C (45 °F) to a more seasonally extreme 4–8 °C (39–46 °F).

Evidence of flowering plants dating to around 80 mya suggests the existence of temperate forests–similar to those in present day Australia, New Zealand, and southern South America. Some flower remains were discovered near 60°S, and it is possible at such a low latitude that this area was subject to polar winters and seasonal weather, though the flowers suggest an annual temperature range of 8–15 °C (46–59 °F) and a rainy climate. Pollen remains from southeastern Australia are identical to living plant species of Australia: conifers, flowering plants that inhabit areas with high rainfall and a Mediterranean climate, and sclerophyllous scrublands, indicating a unique landscape of rainforest and open bushland.

However, rocks from the Early Cretaceous Wonthaggi Formation in southeastern Australia evidence seasonally frozen ground. Its geographical positioning in the Early Cretaceous at around 78°S indicates this area experienced one to three months of darkness in winters, though this area is representative of a glacial-fed floodplain. Evidence of Early Cretaceous glaciation and cold climates was discovered in sediments in Eromanga Basin in modern-day central Australia, or 60 to 80°S in the Early Cretaceous. Nonetheless, similar formations could have been created by simply debris flow, and so it is possible that glaciation did not ever occur there. The total polar ice coverage during the Mesozoic may have been a third of the size as it is in modern times, though cold snaps of subfreezing temperatures possibly occurred throughout the Early Cretaceous.

Gondwana's ancient fauna

See also: List of Australian and Antarctic dinosaurs and List of dinosaurs and other Mesozoic reptiles of New Zealand

Australovenator is the most complete theropod found in Australia.

Dinosaur fossils are rare from the South Polar region, and major fossil-bearing locations are the James Ross Island group; Beardmore glacier in Antarctica; Roma, Queensland; Mangahouanga stream in New Zealand; and Dinosaur Cove in Victoria, Australia. The dinosaur remains of this region, such as those found in Victoria, consist only fragmentary pieces, making identification controversial. Disputed identifications of an allosaurid, the ceratopsian Serendipaceratops which could be an ankylosaur, and the ornithomimosaur/oviraptorosaur/dromaeosaur Timimus have consequently been made.

The supercontinent Pangaea of the Jurassic allowed major dinosaur clades to achieve a global distribution, and several closely-related cognates existed between South Polar forms and forms found elsewhere despite separation by the Tethys Ocean. The Victorian theropod remains have been assigned to seven different clades: Ceratosauria, Spinosauria, Tyrannosauroidea, Maniraptora, Ornithomimosauria, and Allosauroidea. Tyrannosauroids and ornithomimosaurians are not known from other Gondwanan continents, and are more known from the northern Laurasia. The South Polar iguanodontian Muttaburrasaurus is closely related to European iguanodontians. The Cretaceous South Polar Kunbarrasaurus is identified as being the most basal (primitive) ankylosaur, which is significant as ankylosaurs are known from both Gondwana and Laurasia.

Three titanosaurs–Savannasaurus, Diamantinasaurus, and Wintonotitan–one macronarian–Austrosaurus–and Rhoetosaurus consist the sauropod assemblage of Cretaceous Australia, however it is these creatures probably avoided the polar regions as they are completely absent in Southeast Australia which was within the South Polar zone in the Cretaceous. However, these sauropods likely migrated to Australia from South America, which would have required them to pass through Antarctica. It is possible the Bonarelli Event in the Early Cretaceous, which caused a great increase in global temperatures, may have made Antarctica more hospitable to sauropods.

It is unlikely that South Polar dinosaurs migrated out of the polar forests during the winter, as they were either too massive–such as ankylosaurs–or too small–such as troodontids–to travel long distances, and a large sea between East Gondwana and other continents impeded any such migrations in the Late Cretaceous. It is possible, to cope with the winter conditions, some dinosaurs hibernated, such as the theropod Timimus.

Illustration of the hypsilophodont-like Diluvicursor

The most common and diverse group found so far are the hypsilophodont-like dinosaurs, making up half of the dinosaur taxa found in southeastern Australia, perhaps indicating some kind of advantage over other dinosaurs. The Leaellynasaura hypsilophodont-like dinosaurs had large eye sockets, larger than more equatorial hypsilophodont-like dinosaurs, and may have had acute night vision, suggesting that the hypsilophodont-like dinosaurs lived in the polar areas for year-round or most of the year, including polar winters. Bonegrowth was continuous throughout its life, indicating it did not hibernate, being possible by perhaps the dinosaur being endothermic or poikilothermic. However, it is possible that the large eyes are merely due to ontogenesis, that is, relatively large eye-sockets may have been a feature only seen in juveniles or perhaps was a birth-defect, since there is only one specimen known.

Much as in Australia today, East Gondwana played host to many endemic animals, which included many relict species of families that had gone extinct in the rest of the Cretaceous world, among them giant amphibian labyrinthodonts, such as Koolasuchus. It is thought that since they survived in Gondwana, they could survive the cold, in regions where it was too cold in winter for their competitors, the crocodiles.

Mammals, including monotremes and the multituberculate Corriebaatar, have been found, and fragmentary remains of flying pterosaurs. The teeth of plesiosaurs (long-necked fish-eating reptiles) have also been found, suggesting that they lived in the rivers of Gondwana. Lungfish and possible crocodile teeth have also been found; both taxa are associated with distinctly non-polar conditions today, which further confuses our understanding of the climatic conditions of these fossil localities.

Paleocene dinosaur

Main article: Paleocene dinosaur

Given that the dinosaurs and other fauna of Cretaceous were well adapted for living in long periods of dark and cold weather, it has been postulated that this community might have survived the Cretaceous–Paleogene extinction event (66 Ma) which exterminated the non-avian dinosaurs and many other of the world's species at the time. At the present time, this is merely speculation, but according to palaeontologists Patricia Vickers-Rich and Tom Rich, Australia may yet prove to be the best place to find fossils of post-Cretaceous, non-avian dinosaurs.

"Reports earlier this year that dwarf mammoths survived to early historical times, in islands off the coast of Siberia, give force to such speculation. If dinosaurs found a similar haven in which they outlived the rest of their kind, then we think polar Gondwana, including southeastern Australia, is a likely place to look for it."

However, small dinosaur fossils (teeth, bits of bone) found after the Cretaceous–Paleogene boundary are likely to be derived fossils washed out of eroded Mesozoic deposits, or remains of dinosaurs that died in the Cretaceous–Paleogene extinction event that were later washed into a lake or the sea. Likewise, several smaller animals, such as many mammal groups, did not survive the KT event.

See also

• Dinosaur Cove
• Dinosaur Dreaming
• Geology of Antarctica, including paleontology
• List of Australian and Antarctic dinosaurs
• Polar forests of the Cretaceous

Further reading

• Riffenburgh, B. (2007). Encyclopedia of the Antarctic. Vol. 1. Taylor and Francis. ISBN 978-0-415-97024-2.{{cite book}}: CS1 maint: ref duplicates default (link)
• Rich, T. H. V. (2007). Polar Dinosaurs of Australia. Museum Victoria Nature Series. Museum Victoria. ISBN 978-0-9758370-2-3.

External links

• Dinosaur trackways in Western Australia
• Polar dinosaurs in Australia
• Dinosaur Dreaming Web site
• Dinosaur Dreaming in Action
• Monash Science Centre
• Some Australian dinosaurs