Fossil Worm Burrows Reveal Very Early Terrestrial Animal Activity and Shed Light on Trophic Resources after the End-Cretaceous Mass Extinction
After the Cretaceous mass extinction, worm burrows were some of the first fossil traces to appear.
Survivors of the end Cretaceous extinction event were able to
withstand both short- and longer-term repercussions of the bolide
impact and ongoing Deccan vulcanism. Global thermal radiation caused
by re-entry of asteroid impact ejecta apparently presented an
immediate and potentially deadly, short-term consequence of the impact
. Robertson et al.  suggest that this intense pulse of infrared
radiation would have killed exposed organisms, and infer that
survivors were protected within burrows, natural cavities, or bodies
of water. Earthworms not only live in the soil, but are also capable
of employing several mechanisms that allow them to withstand
unfavorable environmental conditions: their protective egg-bearing
cocoons can remain unhatched until conditions improve, and the worms
can enter a state of quiescence or diapause, or can burrow deeply to
avoid adverse conditions near the sediment surface ... Sheehan et
al.  suggested that detritivores would have thrived on existing
stores of dead and decomposing carbon resources despite decreases in
Worms are protected from radiation, and a postapocalyptic world is full of worm food.
Organisms sharing earthworm habitat could also survive - land crustaceans (isopods), other worm types like nematodes, and the ancient springtails. Microscopic soil animal also inhabit this soil sanctuary - mites, tardigrades and rotifers.
From comments: /what about plants?/
I did not realize that the extinction event counted plants among species going extinct. I think that for extinction events in our timeline plants did a lot better than animals - example:
No mass extinction for land plants at the Permian–Triassic transition
Killing all the plants is harder. A gamma ray burst that let anything live would spare some seeds and plants would bounce back to a world without herbivores and they would thrive. Let us assume the plan extinction is from higher incident UV. Plants that need full sun would die with the full UV. Survivors would be those that are ok in low light because shady niches would be shaded from UV as well. Given that some of the most ancient plant lineages are low light specialists today in our timeline (mosses, ferns) I would guess that the low light adapted ancestors in these lineages would be the survivors in a bright UV world. So those ancients get their world back. Exposed areas will be claimed by lichens which I suspect (but do not know) must have some sort of super UV durability given their unique ability to tolerate exposed areas at high altitudes.
The high UV circumstances would need to outlast any durable cones or seeds in the soil or once things ease up the gymnosperms will come back swinging.
There could be plant refugia like the Wollemia refuge in our timeline - a larger niche which by virtue of topology was protected from UV and where UV sensitive plant species that had been wiped out elsewhere might persist thru the UV apocalypse.