|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.
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:
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
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.
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).
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.
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.
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:
Ferns: Much like modern ferns. These ancient plants have been around for over 300 million years. They reproduce mostly via wind-blown spores. Various types, such as tree ferns and ground ferns, have been identified from Victoria.
Ginkgoes: Fossils go back for 260 million years for this type of plant. Only one species survives today - Ginkgo biloba, discovered in the gardens of some Chinese monasteries at the turn of the century. Ginkgoes are members of a distinct plant order only distantly related to other living groups. Females produce an orange coloured fruit that smells awful to humans (a lot like dog droppings!). It is also known as the maidenhair tree due to its fan-shaped leaves.
“Seed-ferns”: This is a loosely defined (and entirely extinct) group that includes several major orders of plants (Corystospermales, Pentoxylales, and Bennettitales) that are only distantly related to each other. Some types, such as the Bennettitales, were superficially like cycads or palms in their growth habits, but their reproductive structures show that they are not closely related to either of these groups.
Cycads: These palm-like plants reached their peak during the Jurassic period (146 to 208 million years ago), but were a rare floral component by tghe Cretaceous. Although palms are flowering plants (called Angiosperms), cycads are cone-bearing and are not closely related. They are found today along coastal areas and in isolated patches in central and south west Australia. Seeds of some species were eaten by the Australian Aborigines after being carefully processed to leach out the toxins.
Araucarians: Primitive conifers that are found scattered throughout the forests of north eastern Australia. They are common in botanical parks and gardens around Victoria (the Royal Botanic Gardens in Melbourne have some impressive specimens). Araucarians reached their peak during the Mesozoic. They are named after the Araucania, a group of South American Indians. Modern araucariacean trees include the hoop pine, bunya bunya pine, norfolk island pine, the kauri and the South American monkey puzzle tree.
Podocarps: Generally less robust than the araucarians, podocarp conifers were at least as diverse and perhaps more abundant during the Cretaceous. Podocarps, or plum-pines, are still important elements of temperate forests in the wetter parts of Australasia and even into southeast Asia.
Angiosperms: One of the oldest flowering plants is known from Victoria. It was identified in 1989 by Dr. Leo Hickey and Dr. David Taylor of Yale University, from a fossil discovered in the Koonwarra deposits in southeastern Victoria. The fossil includes two leaves and a flower, and superficially resembles the modern black pepper plant. It dates to between 115 and 118 million years old.
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.
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