MV palaeontologist Tom Rich, along with colleagues Roger Benson, Patricia Vickers-Rich, and Mike Hall, today published a review of all the theropod dinosaurs known from early Cretaceous period deposits in southern Australia. In doing so, they present the first complete snapshot of local theropod diversity around 120-105 million years ago.
Theropods are a group of mostly carnivorous dinosaurs that walked on two legs and had three-toed feet. Included among the theropods are the infamous T.rex, the small and agile Deinonychus, the feathered Archaeopteryx and modern birds. Tom and his colleagues have been pulling theropod fossils out of Victoria's coastline deposits since the 1970s and in this review, they considered 37 bones and over 90 individual teeth. They conclude that the local Cretaceous theropod fauna comprised nine major groups (or taxa), including allosauroids, tyrannosauroids, spinosauroids and the recently-discovered ceratosaur.
Some of the fossils reviewed in this examination of southern therapod diversity. These are large theropod manual phalanges, or bones from the 'hands' of these dinosaurs.
Source: Benson et al.
A summary cladogram (evolutionary tree) of the therapod dinosaurs, showing the relationships between the major groups within the suborder Therapoda.
Source: Benson et al.
Like the unique fauna of Australia living today, our prehistoric fauna was distinctive too, with some groups dominating the fossil record and others seemingly absent. In the past, palaeontologists have considered several explanations why the types of dinosaurs that lived in Australia were so different to the types found in other continents, even our nearby Gondwanan neighbours. Did certain groups evolve in other continents after Gondwana had split up, so those groups never dispersed to Australia? Or were there patterns of regional extinctions reflecting the differences in climate between the continents as they drifted apart?
As more fossils are uncovered and studied, the picture gets a little clearer. It now appears that many high-level dinosaur taxa, such as the tyrannosauroids and allosauroids, emereged earlier than previously estimated and were distributed all over the world during the Jurassic. This suggests they've been missing from Australian records simply because our dinosaur fauna is poorly known. The Australian fossil record is patchy – whether it's because the fossils have not been preserved or simply not discovered or properly interpreted yet – and often only one or two bones represent an entire group of animals.
However the isolation of Gondwana and Australia from the rest of the world, and the unique conditions here, did help shape a unique assemblage at the species level. During the early Cretaceous, Australia was still attached to Antarctica and was much closer to the South Pole than it is now. Earth's climate was much warmer, the poles were free of icecaps and Victoria and Antarctica were covered in lush, ferny temperate forests. Long periods of winter darkness and extended summer daylight influenced the evolution of endemic dinosaurs whereas in other parts of the world, their distant relatives were contending with quite different environments.
Approximate position of Australia 120 million years ago during the Cretaceous era.
Image: Ron Blakey. Altered by Cally Bennet and Fons VandenBerg
Source: Colorado Plateau Geosystems
The possibility remains that some dinosaurs, such as the long-necked quadrupedal sauropods, which were present in Queensland but have not been found in Victoria, could not survive in cool, dark Cretaceous southern Australia and and so they did not enter this area.
(2012) Theropod Fauna from Southern Australia Indicates High Polar Diversity and Climate-Driven Dinosaur Provinciality. PLoS ONE 7(5): e37122.doi: 10.1371/journal.pone.0037122
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