The Permian-Triassic extinction was the granddaddy of all post Cambrian mass extinctions. While the cause has been contentious, evidence has piled up in favor of Death by Greenhouse - a tremendous increase in global temperatures caused by very high levels of carbon dioxide, coupled with low levels of oxygen. Plants and animals do not do well at high temperature, and high ocean temperatures can limit marine oxygen.
Seen from our vantage point so long after, the Permian extinction was a repeat of what happened at the end of the Devonian, itself the first of what we now call greenhouse extinctions. Many more were destined to come at the end of the Triassic, multiple times in the Jurassic and Cretaceous, and ending with the last-known greenhouse extinction at the end of the Paleocene epoch, some 60 million years ago. But none were ever to be so great as the Permian event, or to unleash a more diverse assemblage of animals in the aftermath of extinction.
Ward, Peter. A New History of Life (p. 228). Bloomsbury Publishing. Kindle Edition.
High levels of CO2 are often paired with low oxygen, for reasons not entirely clear (to me). The low levels of oxygen throughout the succeeding Triassic likely contributed to the development of new body plans during that period including the first proto-mammals and dinosaurs.