Untitled spec evo project?

What if animals with mesothermic metabolisms were the dominant fauna on the planet?

General
Timeline starts Permian-ish? Not sure how to work in mass extinctions

Paleocene-Eocene Thermal Maximum had much more lasting effects: world definitely warmer than in equivalent Holocene, more like Mesozoic temps.

Flora
Flowering plants do emerge but in somewhat different forms

Fauna
Primarily archosaurs and synapsids. Due to the constraints of mesothermic metabolism, some physical attributes and lifestyles are absent from these groups (excepting the select endothermic examples):
 * Need pterosaur analogues in both archosaurs and synapsids - maybe synapsid lineage ultimately persists?
 * Both lineages do eventually evolve fully endothermic species, but these are prevalent only in certain areas
 * No "true" feathers but similar structures
 * No species under a meter in length or so
 * No aquatic species