Electromagnetic Flyers
Feathered mammals = good. Feathered mammals with electromagnetic powers = awesome
In the Late Cretaceous a lineage of deltatheroidean metatherians developed specialised, tightly packed hairs with branching filaments, superficially similar to plumes. This permitted an unique development in vertebrate history: by extending these hairs and creating friction, they developed the ability to hover on electromagnetic fields, much as with spiders and their silk.
Some species developed particularly large and fluffy tails, allowing them to be suspended in the air. This allowed them to glide away to cover more territory, but permitted relatively little control over their flight. These mammals spread over most of the world, but became extinction alongside non-avian dinosaurs in the KT event.
At least two lineages however extended their specialised fur into the limbs. In one the tail lost its "magic feathers" but the fur on the limbs became elongated instead, allowing the hovering to be more easily controlled. Enough to, say, control the direction of flight and hover with relatively little motion, allowing these mammals to sneak up on their prey like owls.
This seems to have made the difference, as they have made it into the Cenozoic. By the late Eocene one lineage seems to have extended their forelimb hairs into wings superficially similar to those of birds. This disrupted the appliance of electromagnetic hovering but instead allowed the formation of true airfoil by increasing the Van der Waals forces.
Previous mammalian flyers were constrained by size, as weights above 40 g became unsuitable for hovering. But these new airfoils permitted a substantial increase in size, and nowadays wingspans of four meters or more are common.