Australia's Polar Dinosaurs

It was once thought that dinosaurs had avoided the colder regions of the world, much like modern cold-blooded reptiles. Then in 1960 dinosaur footprints were found at Spitsbergen, an island half way between Norway and the North Pole. In the late 1970s/early 1980s dinosaur remains were discovered along the southern coast of Victoria, in southeastern Australia, an area that would have been within the Antarctic circle at the time the animals lived. In 1987 dinosaur remains were also found within the Arctic circle in Alaska, and have since been recovered from Antarctica, New Zealand, Northern Canada and Siberia.

Values represent degrees of South Latitude
Rocks containing dinosaur fossils in this part of Australia date to between 105 and 115 million years ago, during the Early Cretaceous period. At this time Australia was still attached to Antarctica (see the figure to the left). As Australia tore slowly northward from Antarctica it formed a huge rift valley. Most of the fossil bearing remains from southern Victoria were formed in lakes or stream channels that would have run through the rift valley. The main dinosaur fossil locations in south eastern Australia (red dots) would have been located at approximately 75 degrees South latitude, which in modern terms would place them within the Antarctic circle. Average temperature estimates for this part of the world during the Early Cretaceous vary. Sedimentary structures associated with cold temperatures (called cryoturbation) suggest an annual temperature range of between about -6° and 3° celsius (21-37 fahrenheit). Oxygen isotope studies yield a mean annual temperature of -2° +/- 5° celsius (which means there is a 95% chance the annual temperature fell within -12° to 8° celsius). These values are the average temperatures for the entire year. Winter would have been much colder, and the summers milder. The modern city of Fairbanks, Alaska, has a mean annual temperature of -2.9° celsius (27° fahrenheit), at a latitude of 65 degrees North.

This part of the world would have experienced freezing winters with up to three months of near-darkness, with only the stars and the moon providing any light. Palaeontologists from Australia and South Africa think that a rock formation near the base of a sedimentary cliff at Flat Rocks near Inverloch, about 150 kilometres southeast of Melbourne, may be the result of seasonal freezing and thawing about 115 million years ago, associated with permafrost. A russian permafrost expert has since confirmed their suspicions. There is even evidence that icebergs may have existed at certain times of the year in the southern reaches of the shallow sea that covered most of the interior of the continent.

In order for dinosaurs to have lived in such a hostile climate, they would have to have either:

Migration

Many of the dinosaurs known from south eastern Australia may have been too small to have migrated over large distances. The most common types of dinosaur known from the fossil sites, small plant eaters called hypsilophodontids, rarely grew to be more than about 2 metres in length, with some only reaching half that. Global sea levels were extremly high at this time, and a shallow inland sea split Australia into several large islands. This may have prevented animals from migrating due *north or northeast for any great distance, although perhaps leaving the north west as a possible migration route, a long arduous journey following the rift valley. It is possible that many of the larger species of dinosaur may have migrated to avoid the winters, however this would have been extremely difficult for the smaller dinosaur species.

Hibernation

Many cold-blooded animals such as modern reptiles and amphibians live in cold climates, becoming active during the short summers and going into a state of suspended animation during the winters. Regular hibernation tends to leave recognisable signs within the bones, known as LAGs (Lines of Arrested Growth). Like the growth rings in trees, they form when bone grows faster at some times of the year (such as the warmer months), but slows or stops entirely at other times (during winter hibernation/fasting). Studies of the fossil bones found in south eastern Australia have found such LAGs in one type of dinosaur, a human-sized theropod known as Timimus. However such lines are not found in the bones of hypsilophodontids, suggesting that although some dinosaurs may have hibernated (or at least stopped eating at certain times of year), the smaller plant eaters seem to have remained active. There would have been several species of evergreen trees and plants for them to eat during the winter, with perhaps a few species of predators staying around to feed on the hypsilophodontids. This research prompted Australian artist Peter Trusler to create a painting depicting a hibernating Timimus and several active hypsilophodontids (called Qantassaurus), beneath an auroral polar sky (see the cover art for the book Dinosaurs of Darkness).

Special Adaptations

This leaves the last possibility: that certain dinosaurs had special adaptations to allow them to live in such a cold climate. The fossil material known for a small (1 metre long) hypsilophodontid known as Leaellynasaura includes something extremely rare among dinosaur fossils - a cast of the brain. After the creature died the brain rotted away, and was replaced by soft sediments. These sediments hardened inside the brain cavity, forming a natural cast of the brain known as an endocast. Studies of the brain cast of Leaellynasaura show that this small dinosaur had extremely well developed optic lobes, the parts of the brain associated with vision (the bump on the lower left of the picture, see below). The skull also indicates that these tiny dinosaurs had quite large eyes. This suggests that some hypsilophodontids may have remained active throughout the long dark winters, which would explain the absence of lag lines in the bones.


Click for a larger image of the brain cast

For any animal to be able to stay active in sub-zero temperatures it would almost certainly have to have been warm blooded, more like modern birds and mammals than cold-blooded reptiles. This has prompted several artists to depict Australia's polar dinosaurs with insulating hair-like coats. In recent years just such hair-like feathery coats have been found on several types of dinosaur, such as Sinosauropteryx (a small compsognathid), Beipiaosaurus (a therizinosaur), and Sinornithosaurus (a basal dromaeosaur).

In 2009 Dr Anthony Martin published a paper describing probable animal burrow structures in rocks from the Otway Group dating to the Early Cretaceous. These structures closely resemble the burrows of the North American hypsilophotontid Oryctodromeus from Montana. It seems likely that at least some of Australia's hypsilophotontids also lived in burrows, an adaptation that would have allowed them to retreat from particularly cold conditions, and provide a safe place to hide from predators.


The Environment

Pollen and spores found at the dinosaur sites, as well as excellent plant and invertebrate (insects etc) fossils known from other localities, provide a good indication of the type of environment that dinosaurs would have lived in during the Early Cretaceous. The plants that grew in the rift valley include:

The Koonwarra site in southeastern Victoria (Strzelecki group) has some exquisitely preserved plant, fish and invertebrate fossils dating to 115-118 million years ago. It also contains the impressions of several feathers (either from birds or theropod dinosaurs). A range of insects is known from the site, including wasps, ants, fleas, flies, mayflies, bugs, and water beetles. Spiders, earthworms, and a horseshoe crab are also known. The range of insects is similar to some modern aquatic insect ecosystems, especially that of alpine areas, indicating they were adapted to cold waters. It is thought that these fossil deposits formed when part of a lake froze over during the winter, killing fish and insects alike as they either froze, or the oxygen content of the water dropped too low.

Palynological (pollen) studies indicate that although there are plenty of pollen and spores available to study in the fine-grained mudstones and siltstones in southern Victoria, the diversity of plant species appears quite low (which is fairly typical for an Early Cretaceous spore/pollen assemblage). This may indicate that only a selected number of plant species were sufficiently adapted to surviving the dark, harsh winters. Ferns, lycophytes and sphenophytes (horsetails) seem to have dominated the understorey vegetation, with seed-ferns and smaller conifers constituting mid-storey elements, and tall ginkgoes and podocarp and araucarian conifers dominating the canopy. Some trees reached substantial heights judging from petrified tree trunks, but whether the vegetation had a closed or open canopy is difficult to determine from fossils. Tree rings from the fossilised wood indicate wide summer bands of growth, interspersed with very narrow winter growth bands when the trees all but shut down.

The picture we get from these plant and insect fossils is of a cool but well vegetated rift valley dominated by ferns, gingkoes and conifers, which had to endure freezing winters which were made up for by three months of daylight in the summer. This evidence supports the notion of a polar forest environment, although one unlike anything around today.

List of all dinosaurian fossils from Australia
List of non-dinosaurian fossils from southeastern Australia.

References

Chinsamy, A., T.H.Rich & P.Vickers-Rich 1998 Polar dinosaur bone histology. Journal of Vertebrate Paleotology18:385

Constantine, A., A.Chinsamy, T.H.Rich, & P.Vickers-Rich. 1998. Periglacial environments and polar dinosaurs. South African Journal of Science 94:137-141.

Douglas, J.G., & G.E.Williams. 1982. Southern polar forests: The Early Cretaceous floras of Victoria and their palaeoclimatic significance. Palaeogeography, Palaeoclimatology, Palaeoecology 39:171-185.

Martin, A.J. 2009 Dinosaur burrows in the Otway Group (Albian) of Victoria, Australia, and their relation to Cretaceous polar environments. Cretaceous Research 30(5):1223-1237 (PDF)

Parrish, J.T., R.A.Spicer, J.G.Douglas, T.H.Rich, & P.Vickers-Rich. 1991. Continental climate near the Albian South Pole and comparison with climate near the North Pole. Geological Society of America, Abstracts with Programs 23:A302.

Rich, T.H. 1996 Significance of polar dinosaurs in Gondwana. Memoirs of the Queensland Museum 39:711-717.

Rich, P.V. and T.H.Rich 1999 Wildlife of Gondwana. The 500-million year history of vertebrate animals from the ancient southern supercontinent. Second Edition. Reed books, Sydney.

Rich, P.V., L.S.Rich and T.H.Rich 1996 Australia's Lost World. A history of Australia's backboned animals. Kangaroo Press, Monash Science Centre, Monash University.

Rich, P.V., Rich, T.H., Wagstaff, B.E., McEwen-Mason, J., Douthitt, C.B., Gregory, R.T. and Felton, E.A. 1988. Evidence for low temperatures and biologic diversity in Cretaceous high latitudes of Australia. Science 242:1403-1406.

Rich, T.H., Rich, P.V., Wagstaff, B., McEwen-Mason, J., Douthitt, C.B. and Gregory, R.T. 1989. Early Cretaceous biota from the northern side of the Australo-Antarctic Rift Valley. In J.A.Crame (ed.) Origins and Evolution of the Antarctic Biota. Geological Society Special Publication 47:121-130.

Rich, T.H., R.A.Gangloff, & W.Hammer. 1997. Polar Dinosaurs. In P.J. Currie & K.Padian (eds.), Encyclopedia of Dinosaurs, pp. 562-573. Academic Press.

Rich, T.H. and P.V.Rich 1988. A juvenile dinosaur brain from Australia. National Geographic Research. 4(2):148.

Rich, T.H. and P.V.Rich 1989 Polar dinosaurs and biotas of the Early Cretaceous of southeastern Australia. National Geographic Research 5:15-53.

Rich,T.H., P.Vickers-Rich & R.A.Gangloff 2002 Polar Dinosaurs. Science 295:979-980

Vickers-Rich, P., T.H.Rich 2000 Dinosaurs of Darkness. Indiana University Press

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