Temporal niche expansion in mammals from a nocturnal ancestor after dinosaur extinction | Nature Ecology & Evolution
There is a curiosity of vertebrate color vision (Evolution of color vision, Evolution of colour vision in vertebrates - eye1998143.pdf). Many diurnal fish, reptiles, and birds have very good color vision: four kinds of cones, covering roughly red, green, blue, and near-ultraviolet. But most mammals, including most diurnal ones, have only two, for yellow and blue. Dogs are typical in that regard.
This has led to speculation that ancestral mammals were nocturnal. Since they needed all the photons that they could get, they sacrificed some color resolution. But it is difficult to tell from their color vision when diurnal ones' ancestors became active in the daytime.
From Ars Technica,
They collected descriptions of behavior and then coded species by when they are active. They found that mammals are ancestrally nocturnal, as one might expect, and that the diurnal ones' ancestors became diurnal in the Cenozoic, sometimes early in the Cenozoic.Zoologists and geneticists in Israel and England have thus collated the behavior of 2,415 different mammalian species, spanning more than 90 percent of mammalian families and all 29 mammalian orders, to try to reconstruct the ancestral activity patterns of mammals and see when diurnality first showed up.
Meaning that the disappearance of the (non-avian) dinosaurs allowed them to be active in the daytime.
The first strictly diurnal group to emerge was the simians, around 5233 million years ago. It was the Old World ones and some of the New World ones that acquired the best mammalian color vision: three kinds of cones.