Sorry this post was so long in coming, I’ve been really busy with my PhD!
The Great Dying may sound like an exaggeration of a name, but if anything it is an understatement. The extinction that occurred at the end of the Permian, ushering in the Triassic, was the most catastrophic since animals appeared on the planet—96% of all marine species, and 70% of all terrestrial vertebrate species, were wiped out1. It was the only mass extinction of insects, 83% of all genera were destroyed: to appreciate this number, a genus is a group of species, and a genus is only considered extinct if every single species within it is wiped out.
This period of mass volcanism, runaway-greenhouse effect, increasing aridity and anoxia ended the synapsid and amphibian dominance over land: with all the large land-dwelling amphibians driven to extinction, their predators—such as Dimetrodon—also died out. The only large land amphibians that remained were aquatic, filling the niches occupied by modern-day crocodiles2,3.
Life on Earth would recover, but it would take 30 million years4, and synapsids would not reclaim their former dominance. Instead, diapsids would claim the land, diversifying into two major groups: lepidosaurs, the ancestors of modern lizards and snakes; and the formerly-mentioned archosauriformes. Meanwhile, ocean-dwelling diapsids saw the rise of Ichthyosaurs5, fish-like lizards that ruled the seas.
The ‘crocodile-lizard’ archosauriformes would diversify throughout the Triassic, with many forms, from small lizard-like animals to metres-long crocodile-like monsters. By the mid-Triassic, one group of archosauriformes would start to stand out: the archosaurs6.
Archosaurs are best recognised by their descendants, as they would proceed to dominate the world for the next two hundred million years. Earliest to break off were the ancestors of modern crocodylomorphs7—crocodiles and their nearest relatives.
At the end of the Triassic, two hundred million years ago, these groups had become well-established, and it was just in time: the Triassic closed with another mass extinction, the imaginatively-named Triassic-Jurassic Extinction Event10. This was the final nail in the coffin for any remaining giant amphibians, while marine life was devestated, with the extinction of over a third of all genera—including an entire class11. The cause of this extinction is unknown, but climate change marked with mass volcanism are two of the leading candidates. Whatever the cause, the Jurassic had begun.