Early Cretaceous

During the Mesozoic Australia was at the end of the world, attached to the end of a long peninsula of Gondwanaland.  This isolation seems to have allowed older forms like allosaurids and labyrinthidonts to exist when they had become extinct elsewhere. Fossil remains are often very scrappy and several species have been identified based on single bones or even footprints. During the Cretaceous, Australia and Antarctica began to separate, sea levels rose and a large inland sea developed.

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Closed forests habitats were common in the temperate climate, and this may account for the small size of the dinosaurs found there. Minmi was named after a location in Australia and was about 10 feet long.  This is small for an ankylesaurid and Minmi doesn't seem to belong to either the nodosaur or ankylosaur families. It and the iguanodontid Muttaburrasaurus are two of the best-known Australian dinosaurs coming from relatively complete skeletons. 

Muttaburrasaurus,reaching 23 feet long, was a plant eater (possibly an omnivore) and often found in marshy areas. It had an intriguing and unique large bump on its nose, and walked on all fours, although able to stand on its rear legs. It had teeth designed for shearing rather than grinding, and a snout featuring a distinctively expanded hollow chamber, with large external nostrils that may have served as cooling chambers.  

Leaellynasaura, a small turkey-sized vegetarian, was a hypsilophodont (a wide spread group). Their fossils tend to be rare in areas where other, larger dinosaurs are found.  Hypsilophodonts are usually found in harsh environments that are unattractive to larger animals, like the uplands of North America and various island environments. The brain case suggests that Leaellynasaura has well-developed eyes even night vision.  This characteristic would have been an advantage during the dark arctic winters