Cretaceous: 145–65 million years ago

Reptiles rule as flowers evolve

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Illustration of an early cretaceous plant community.
Image: Karen Carr
Source: Karen Carr

120 million years ago.
Image: Ron Blakey. Altered by Cally Bennet and Fons VandenBerg
Source: Colorado Plateau Geosystems

Cretaceous

At the start of the Cretaceous, the world was warm and free of polar ice caps. The largest animals were reptiles – dinosaurs, flying pterosaurs, dolphin-like ichthyosaurs and giant sea turtles. Mammals remained small. Flowering plants evolved and spread across the world.

All this changed 65.5 million years ago at the end of the Cretaceous when a major catastrophe led to the extinction of dinosaurs, many marine reptiles, all flying reptiles (pterosaurs) and many marine invertebrates. There is strong evidence that the culprit was a huge asteroid that formed a large crater on what is now the Yucatan Peninsula of Mexico. Vast clouds of dust produced by the impact would have blocked out sunlight across much of the Earth, killing plants and affecting the food chain for almost all animals. Smaller animals and those that could feed on detritus would have been able to survive.

Despite a warmer world, Victoria was still close to the south pole and it was one of the coolest places on the planet at the time. Victoria and Antarctica were covered in temperate forests of conifers and ferns. Many dinosaurs, some only the size of a chicken, adapted to life in these cooler forests, as did other reptiles, mammals and birds.

Life inside the polar forests of Victoria presented challenges to the dinosaurs that lived there, such as long periods with little sunlight. Fossils of Victorian dinosaurs show adaptations to coping with this environment, such as large eyes and an enlarged optic lobe – the part of the brain that interprets visual signals from the eyes.

Australia continued to separate from Gondwana, creating a rift valley with streams, forest and volcanoes. In the streams lived many species of fishes, aquatic reptiles, and ferocious amphibious tetrapods such as Koolasuchus.

Photograph of a yellow plastic Tyranasaurus Rex