25 June 2012

The long hunt for Australian dinosaurs

By and Roger Benson
For the last three decades, palaeontologists have been scouring promising sites in Queensland and Victoria looking for the remains of Australia's dinosaurs. Thirty-seven tantalising bone discoveries make up everything known about the continent's carnivorous dinosaurs, theropods.
Jaw of a small mammal that lived alongside the dinosaurs in Victoria about 120 million years ago.

Jaw of a small mammal that lived alongside the dinosaurs in Victoria about 120 million years ago. Credit: Steve Morton/Monash University

Dinosaur remains have been found on every continent on Earth and we know these creatures dominated the planet’s ecosystems for around 140 million years. But despite their abundance elsewhere, few discoveries have been made in Australia.

Among these limited discoveries are skeletons of Australian dinosaurs found in Queensland over the past century, including those of herbivores such as the largeornithopod Muttaburasaurus, the small ankylosaur Minmi and three long-necked sauropods.

But an article we recently published in the open-access journal PLoS ONE should help extend understanding of Australia’s dinosaur record.

For the past three decades, one of us (Tom Rich) has been leading a team of staff and volunteers from Museum Victoria and Monash University, in Melbourne, in a hunt for dinosaurs and mammals from Australia’s south coast.

In those 30 years, one of us (Tom) and colleagues uncovered 37 bones of carnivorous dinosaurs known as theropods, which the other (Roger) and colleagues were able to interpret.

So what did we learn from those 37 bones? More on that in a moment. First we need to understand a bit about what Australia looked like when these dinosaurs were roaming the continent.

Sedimentary rocks exposed in the Otway and Strzelecki Ranges of south Victoria were laid down 110 million years ago during the Cretaceous period. By this time, Earth’s northern and southern continents had divided into two supercontinents: Laurasia in the north and Gondwana in the south.

Australia was still joined to Antarctica, forming the eastern part of Gondwana. But continental fragmentation had begun, resulting in a deep rift valley in which broad rivers deposited muds, sands, and occasionally the bones of now-extinct animals.

Rifting continued (and continues today), ultimately forming the Southern Ocean, with Cretaceous sedimentary deposits still preserved on Australia’s south coast.

Articulated skeletons, comprising multiple bones connected as in life, are rare in these rift-valley rocks: only four partial ones have been found, all from small-bodied ornithopod dinosaurs such as Leaellynasaura, a bipedal, herbivorous dinosaur one to two metres long.

Nonetheless, our search in Victoria has resulted in around 1,500 dinosaur bones and teeth, as well as remains of mammals, amphibians, crocodiles, birds, lungfish, and plesiosaurs. (Of those 1,500 fossils, 37 were recognised by Roger as being from theropods he was able to identify.)

These fossils paint a picture of the Australo-Antarctic rift valley fauna during the age of dinosaurs. This picture has been phenomenally important for understanding the global context of Cretaceous dinosaur and mammalian evolution.

The focus of our initial study was theropods – mostly predatory dinosaurs and the rock stars of the dinosaur world, including Tyrannosaurus rex and Velociraptor. Theropods also gave rise to birds and consequently have been the focus of intense scientific interest.

Palaeontologists have paid a great deal of attention to the anatomy of these animals, allowing some to be recognised from isolated bones.

Nonetheless, many of the Victorian theropod bones found by Tom and associates provide little information about what kind of theropod they represent. This is especially true of bones from the tail, hands and feet. But other bones provide tantalising glimpses of high species diversity.

Small-bodied theropods the size of cats or dogs seem to have been especially diverse in the Victorian fauna. Although their remains are the least common, they suggest the presence of three species of maniraptorans, the closest dinosaurian relatives of birds.

Medium-sized theropods, perhaps two to three metres in length, include a small-bodied relative of T. rex known from two bones. Also known are a single tail vertebra strikingly similar to those of ornithomimosaurs, the possibly herbivorous “ostrich-mimic” theropods, and the astragalocalcaneum (an ankle bone) of a ceratosaur, representing a group of dinosaurs common elsewhere in the southern hemisphere.

Finally, the remains of large-bodied predators, estimated as seven to nine metres long, are relatively common. Most represent a single group, Allosauroidea, including an ankle bone, part of a large claw, other parts of the arm, a vertebra, and a collection of nearly 100 shed teeth from a single site near Inverloch.

By contrast, a single vertebra represents a spinosaurid, from a group of theropods with superficially crocodile-like skulls.

The Victorian theropod fauna found in our study teaches us two things about Australian dinosaurs.

First, the faunal composition is most similar to that in the northern continents. This suggests that although continental fragmentation was important in determining some aspects of Cretaceous dinosaur faunas, climatic zonation may have also played a key role.

So similarities between northern and Victorian dinosaur faunas – such as the presence of tyrannosauroid theropods and abundance of basal ornithopods (small plant-eating dinosaurs) – may reflect the cool-warm, wet climate that we know was present in both regions at the time. This differed from the predominant climate in South America and Africa at the time, which was highly arid.

Second, many species of theropods were able to coexist in high latitude southern dinosaur faunas. Indeed, because of continental drift, these dinosaurs lived within the Antarctic Circle, and at least three months of winter darkness occurred annually.

We know that, as close relatives of birds, many of these dinosaurs were insulated by a feathery or down-like covering, and were likely warm-blooded. This may have been the secret to their success in cool environments.

But the knowledge we have of these dinosaurs is based on very few specimens and those are isolated bones and teeth rather than entire skeletons. So while these bones can be assigned to known higher groups (such as tyrannosauroids, allosaroids and ceratosaurs), there is much more to be learned about them.

Further excavation at the sites known to be most productive – such as Flat Rocks near Inverloch, Victoria – will continue for the indefinite future.

After all, it took three decades to get the 37 fossil bones that Roger was able to do something with. So despite all the effort put in thus far, we are still at the bottom of the learning curve in our understanding of the carnivorous dinosaurs of Victoria.

The other way forward is to excavate a site near the hamlet of Koonwarra in South Gippsland. In the nature of its sediments and the common fossils found there, this site is strikingly similar to the fossil sites in the area northeast of Beijing, China, where feathered dinosaurs have been found in profusion over the last two decades.

The likely answer as to why feathered dinosaurs have not been found yet at the Koonwarra site is simply that not enough excavation has been done there. A simple statistical calculation suggests that to have a 99% chance of finding a feathered dinosaur, if they occur at Koonwarra at the same frequency as at the Chinese sites, an area of 50 square metres would have to be excavated.

Carrying out such an excavation at the known site or a similar one in the same region is a high-priority project.

Tom Rich is the Senior Curator (Vertebrate Palaeontology & Palaeobiology) at Museum Victoria and Roger Benson is a Post-doctoral research in palaeobiology at the University of Cambridge.
This first appeared online, on theconversation.edu.au
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