South Polar dinosaurs
The South Polar dinosaurs proliferated during the Early Cretaceous (145–100 Ma) 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.
Gondwana’s climate
During the Cretaceous, Earth on average was warmer than it is now, making the polar regions more habitable.
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 and 8 °C (32 and 46 °F). 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 (28 and 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 around 10 °C (50 °F) (higher than found by 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 very lopsided distribution of land and ocean around the South Pole would have forced the ocean currents and seasonal winds (monsoons) to flow across the polar area, stopping a cold pool from forming around the pole.
These studies show that during the Cretaceous there were no polar ice caps, and 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 inside 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, 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 possible placentals, 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.
Dinosaur fossils are rare in Australia, but dinosaurs found in the Victorian deposits include relics of the Jurassic period, such as a relative of Allosaurus, ornithomimosaurs, ankylosaurs, and hypsilophodont- like dinosaurs, the commonest and most diverse group found thus far. The hypsilophodont-like dinosaurs provide a clue to the habits of the dinosaurs that lived in these polar environments: one skull is interpreted as having 'large eyes', and casts of the brain of one individual has been interpreted as showing enlarged optic lobes. This interpretation has been used as a supporting argument for the theory that these dinosaurs may have had acute night vision; if this were the case it may suggest that the hypsilophodont-like dinosaurs may have lived in the polar areas for most if not all of the year, including the weeks or months-long polar night.
However these features may be a feature of ontogeny of the single taxon represented by a single skull fragment, and thus these characteristics may not indicate visual acuity in the adults of this taxon, for which no skull has been described. These polar characteristics would have been exposed to 6 months of near-endless daylight during the summer also, where enlarged eyes and optic lobes would not have proven necessarily advantageous.
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[2] 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."[2]
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.
See also
- Dinosaur Cove
- Dinosaur Dreaming
- Geology of Antarctica, including paleontology
- List of Australian and Antarctic dinosaurs
- Polar forests of the Cretaceous
Notes
- ↑ "Dinosaur Cove, Australia". This Dynamic Earth. USGS. Retrieved 14 March 2015.
- ↑ 2.0 2.1 Vickers-Rich, Patricia; Rich, Tom Hewitt (July 1993). "Australia's Polar Dinosaurs". Scientific American.