Nocturnal bottleneck

[1] While some mammalian groups later adapted to diurnal (daytime) lifestyles to fill niches newly vacated by the extinction of non-avian dinosaurs, the approximately 160 million years spent as nocturnal animals has left a lasting legacy on basal mammalian anatomy and physiology, and most mammals are still nocturnal.

The emerging archosaurian sauropsids, including pseudosuchians, pterosaurs and dinosaurs and their ancestors, flourished after the Early Triassic Smithian–Spathian boundary event and competitively displaced the larger therapsids into extinction, leaving only the smaller burrowing cynodonts.

[3] The surviving cynodonts could only succeed in leftover niches with minimal competitions from the more dominant, diurnal dinosaurs, evolving into the nocturnal, small-bodied, insectivorous and granivorous dwellers of the forest undergrowths.

[4] While the early mammals continued to develop into several probably quite common groups of animals during the Mesozoic, they all remained relatively small and nocturnal.

Mammals experienced a significant radiation from the angiosperm revolution during the Middle/Late Cretaceous, but only with the massive end-Cretaceous extinction event did the dinosaurs' demise leave the stage open for the establishment of new mammalian faunae.