Mesozoic Dinosaurs of WA

Main Club November Meeting

Our speaker, Mikael Siversson, is the head of the Department of Earth and Planetary Sciences at the WA Museum. He manages the Palaeontology collection of 1.5 million specimens at the museum, which contains for example: stromatolites from the Pilbara, fish from Gogo in the Kimberley and megafauna from Margaret River. He has published research on various vertebrate groups from the Mesozoic Era and is regarded as a leading authority on Cretaceous Lamniform Sharks.

Mikael began his presentation at the event 66 million years ago when a meteor collided with the earth, making its impact off what is now Mexico. This created a “nuclear winter” effect blocking sunlight and which, along with firestorms, an infrared radiation pulse and persistent dust clouds, wiped out nearly all the dinosaurs and thousands of other species. This included the iconic eight-tonne Theropod Tyrannosaurus rex. In the palaeontological record, fungal spores were the first living things to make a resurgence on land, followed by ferns, then higher plants, and then birds that were dependent on insects and seeds, and other vertebrates.

We are actually in an ice age now—the third of three over the last 600 million years. (Ice ages are defined as having ice on land at sea-level.) But the environment was very different in the Mesozoic. With temperatures much higher than now, the sea level was higher, covering much of the continents and thus fragmenting them. Animals spread around because of the movement of continents that we know as plate tectonics. (We have marsupials here in Australia because this was the last continent they spread to.) Atmospheric CO2 was high because of geological emissions due to the fast movement of continents. Due to these high CO2 levels, plants probably grew quickly, perhaps explaining the very large size possible for dinosaurian herbivores.

Palaeontologists need to know where to look for dinosaur fossils so that their searches are not futile. In Australia, ancient flood plains like the Canning Basin are promising. The Great Sandy Desert has Mesozoic rocks with good exposure due to erosion. The best places are where the land has risen and allowed erosion to expose fossils. But in some places skeletal material has been bleached or oxidised. The Bringo railway cutting east of Geraldton has only revealed two dinosaur bones, and Molecap Hill Quarry near Gingin has revealed only a dinosaur toe bone.

In the 1940s on the Dampier Peninsular near Broome, it was discovered that low tides expose not skeletal material but footprints of three-toed dinosaurs imprinted in the sandstone. Since then thousands more have been discovered, of many types: Theropods, Sauropods, Stegosaurs and Ankylosaurs. Amazingly, the ground has buckled as much as 70cm under the weight of some of these dinosaurs’ feet. Instead of removing the footprints to record them, silicon casts are made in the short time between tides. Also, drones have been used to photograph the trackways.

Below left: Tracks of Megalosauropus broomensis (this was the dominant theropod species in the area)

Above right: Dinosaur thoroughfare (ground trampled by sauropods) – Credit both: Steve Salisbury, University of Queensland.

The Broome site had ideal conditions for the formation of the tracks—possibly a muddy, braided river system—and now has ideal conditions for them to be exposed, on an intertidal sandstone platform.

The Indigenous people of the area had always regarded the three-toed Theropod tracks as the footprints of the Emu Man. This claim is closer to the scientific consensus view than we might think, in the light of palaeontological knowledge, because birds are now known to be one of several Theropod dinosaur groups. The remains of many rapto-dinosaurs have been found in China. These were small carnivorous dinosaurs with feathers, long arms and probably even the unique avian two-way, lung-and-air-sac system which enhances stamina. Melanin has been found, showing that the feathers were coloured. However, unlike modern birds they had teeth and a bony tail. Only the bird-grade Theropods with beaks, living at the same time as these, survived the effects of the asteroid impact and became modern birds.

The obvious objection is that birds don’t look like typical dinosaurs. But Mikael pointed out that whales don’t look anything like bats, and yet they are both unquestionably mammals.

Mike Gregson

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