Vast floods caused by sea level rise may have helped dinosaurs take over the planet.
The first dinosaurs evolved early in the Triassic period, about 245 million years ago, but these were rare. It took about another 20 million years before they came to dominate land ecosystems. Palaeontologists are unsure why the dinosaurs came to be so numerous and diverse, while other reptile groups like crocodiles didn鈥檛.
In 2019, researchers led by Tore Klausen, then at the University of Bergen in Norway, reported that in the late Triassic there was a vast river delta in what is now the Barents Sea between Norway and Russia. The floodplain was 10 times the size of the Amazon delta. It lay on the north coast of Pangaea: the vast landmass that existed at the time and included all the modern-day continents.
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Klausen and his colleagues have now found that most of this area flooded and became a shallow sea around 227 million years ago. Sediments characteristic of land were replaced by traces of wetlands and then seabed. Fossil pollen revealed that the plants also changed: those adapted to dry land were first replaced by more swamp-like plants, says Klausen, and then marine plankton.
The floods were caused by rising seas. Today the seas are rising because the water is warming and expanding, and because ice caps are melting, but in the Triassic the climate was consistently warm and there were no ice caps to melt.
Instead, Klausen says 鈥済lobal tectonic events鈥� that changed the shape of the seabed, such as rising underwater mountains, were responsible. Because the floodplain was so flat, even a small rise in sea level would inundate most of it, he says.
The huge floods must have had a big impact on land animals, says Klausen, which could explain how dinosaurs came to dominate. Klausen suggests that other reptiles were specialised in floodplain environments and became marginalised when this habitat was drowned.
In contrast, dinosaurs may have been better able to cope with hills and deserts, so were able to thrive. 鈥淭hey were occupying niches gradually,鈥� he says.
Terra Nova
