During the Early Cretaceous the continent of Australia was still linked to Antarctica as a remnant of Gondwana that had rifted from Africa and drifted southward. Much of this southern continent lay beneath the Antarctic Circle, and the climatic conditions that occurred there were unlike any that exist today. This in turn 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.

Gondwana’s climate

During the Cretaceous, Earth's average temperature was warmer than it is now, making the polar regions more habitable. The exact nature of the climate of Gondwana has been determined by various methods.

Several techniques have been used to deduce the ancient climate of Gondwana in the Early Cretaceous. One technique involves looking at the levels of oxygen isotopes in the rocks from the time. These have suggested estimated mean annual temperatures of between 0° C (32° F) (analogous to Hudson Bay) and 8° C (46° F) (analogous to Toronto). The rocks with associated mammal and dinosaur fossils show evidence of permafrost, features such as ice wedging , patterning and hummocked ground. Permafrost today occurs in temperature ranges of between −2° and 3° C (27° to 37° F).

Another method used to deduce the climate of the time is to use the types of plants found in the fossil record. The fossil record shows a floral community dominated by conifers, ginkgoes, ferns, cycads, bryophytes, horsetails and a few flowering plants. The plants indicated, through structural adaptations, a seasonal cold period and a mean annual temperature of around 10° C (50° F) (a higher value than that of the oxygen isotope data) and the presence of ferns and bryophytes indicates rainy conditions. A large inland sea that extended into central Australia modified its continental climate.

The conclusion of these studies is that during the Cretaceous there were no polar ice caps, meaning that forests would have extended all the way to the South Pole, and life could have flourished there during the summer. However, the Earth's axial tilt means that the regions below the Antarctic circle would still have experienced a polar night: a period of sunless darkness and cold of up to six months, during which only the hardiest life forms could survive. This combination of a habitable terrain with a long polar night is an ecological circumstance that has no present day analogue.

Gondwana’s ancient fauna

Much as in Australia today, Gondwana played host to many endemic animals, which included many relicts of families that had gone extinct in the rest of the Cretaceous world, among them giant amphibian labyrinthodonts. It is thought they probably survived in Gondwana because they were able to survive the cold, whereas their competitors the crocodiles were unable to live outside warm climates.

Mammals, including monotremes and possible placentals have been found, as well as 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.

Though dinosaur fossils are rare in Australia, dinosaurs found in the Victorian deposits include relics of the Jurassic era, such as a relative of Allosaurus; ornithomimosaurs , ostrich-like carnivorous dinosaurs; ankylosaurs; and members of the family Hypsilophodontidae , the commonest and most diverse group found thus far. The hypsilophodonts give us a big clue as to the habits of the dinosaurs that lived in these polar environments: they possessed large eyes, and casts of their brains show that they possessed enlarged optic lobes , which indicates acute night vision, which in turn suggests that the hypsilophodonts may have lived in the polar areas for most if not all of the year, including the weeks or months-long polar night.

In 1991, paleontologists in Antarctica discovered a previously unknown dinosaur species, Cryolophosaurus ellioti, the first dinosaur remains ever to be discovered on the continent, which was joined to Australia during the Cretaceous.

Post K/T dinosaurs?

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 well have survived the Yucatan Impact Event that is thought to have exterminated the dinosaurs and so many other of the world’s species at the time. At the present time, this is merely speculation, but Australia may yet prove to be the best shot at finding fossils of post-Cretaceous, non-avian dinosaurs.

External links

• Dinosaur trackways in Western Australia
• Polar dinosaurs in Australia
• Some Australian dinosaurs