Right before the Jurassic started there was an extinction event that wiped out most of the crocodilians and amphibians. With the only ones surviving eventually evolved into modern day crocodilians and amphibians. But for some reason the dinosaurs were unaffected. This meant that now that the dinosaurs had no competition they could flourish. They then spread across the new earth and became more diverse. We now had the infamous huge dinosaurs we all know. This is when the great sauropods finally grew to their huge size and even the carnivores grew to huge sizes like allosaurus, ceratosaurus, megalosaurus, coelurosaurs etc. This was also the time when the armored and plated dinosaurs appeared, such as ankylosaurus and stegosaurus. This was also the time when the first species of ceratopsian, which was called yinlong downsi.
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