“If everything around you is dying, go to ground. Feed on their corpses to survive. Mate. When trees regrow, take advantage of them. No matter how long it seems, the hard times will end. In the end, it is not the largest or the strongest that will dominate, but the survivors.”

— Purgatorius, great-grand-daddy of us, 66 million years back

In life, it would have resembled a squirrel or a tree shrew (most likely the latter, given that tree shrews are one of the closest living relatives of primates, and Purgatorius is considered to be the progenitor to primates)

The oldest remains of Purgatorius date back to ~65.921 mya, or between 105 thousand to 139 thousand years after the K-Pg boundary.[4]

https://en.wikipedia.org/wiki/Cretaceous–Paleogene_extinction_event

The Cretaceous–Paleogene (K–Pg) extinction event,[a] formerly known as the Cretaceous-Tertiary (K–T) extinction event,[b] was a major mass extinction of three-quarters of the plant and animal species on Earth[2][3] approximately 66 million years ago. The event caused the extinction of all non-avian dinosaurs. Most other tetrapods weighing more than 25 kg (55 lb) also became extinct, with the exception of some ectothermic species such as sea turtles and crocodilians.

A wide range of terrestrial species perished in the K–Pg mass extinction, the best-known being the non-avian dinosaurs, along with many mammals, birds,[22] lizards,[23] insects,[24][25] plants, and all of the pterosaurs.[26] In the Earth’s oceans, the K–Pg mass extinction killed off plesiosaurs and mosasaurs and devastated teleost fish,[27] sharks, mollusks (especially ammonites and rudists, which became extinct), and many species of plankton. It is estimated that 75% or more of all animal and marine species on Earth vanished.[28] However, the extinction also provided evolutionary opportunities: in its wake, many groups underwent remarkable adaptive radiation—sudden and prolific divergence into new forms and species within the disrupted and emptied ecological niches. Mammals in particular diversified in the following Paleogene Period,[29] evolving new forms such as horses, whales, bats, and primates.

K–Pg boundary mammalian species were generally small, comparable in size to rats; this small size would have helped them find shelter in protected environments. It is postulated that some early monotremes, marsupials, and placentals were semiaquatic or burrowing, as there are multiple mammalian lineages with such habits today. Any burrowing or semiaquatic mammal would have had additional protection from K–Pg boundary environmental stresses.[94]

Due to the wholesale destruction of plants at the K–Pg boundary, there was a proliferation of saprotrophic organisms, such as fungi, that do not require photosynthesis and use nutrients from decaying vegetation. The dominance of fungal species lasted only a few years while the atmosphere cleared and plenty of organic matter to feed on was present. Once the atmosphere cleared photosynthetic organisms returned – initially ferns and other ground-level plants.[172]