65 million years ago, a great big rock slammed into the Yucatan Peninsula in southern Mexico with enough force that debris from the impact rained down as far north as Kentucky. There is also a layer of carbon found everywhere on earth at a depth that indicates this layer of carbon also dates to 65 million years ago. The thickness of this carbon layer varies, but the fact that it is everywhere indicates that the entire surface of the earth burned.
Around the same time as the impact 65 million years ago, there was a huge increase in volcano activity in an area called the Deccan Traps. While not exactly on the opposite side of the world from where the impact hit, it’s kinda close, which means that this were likely an area where volcanoes were already prone to happen, but were also spurred on and made a lot worse by the shock wave from the impact, which would travel around the world and would concentrate again on the opposite side of the globe from the impact.
So not only did the impact burn everything to a crisp and throw up a huge amount of hot stuff into the atmosphere (you can basically imagine it raining molten lava for a bit), but then it was followed by huge volcanic eruptions that threw even more stuff into the atmosphere and kept throwing stuff into the atmosphere for decades.
The earth wasn’t a very happy place for a very long time after that.
The impact itself killed a lot of the life on earth. You’ll note that most of the survivors were small creatures that could burrow underground or could dive under the water to get away from all of the burning stuff. The creatures that did manage to survive the impact and the rain of fire then faced a new problem. There wasn’t any food. A lot of the plants that were burned didn’t die off completely, and would have sprouted new leaves and grown new fruit, but the lush forests were gone. Only a tiny fraction of the plant life remained. Creatures that lived on those plants starved to death. Creatures that ate the plant eaters then also starved to death.
Crocodilians did fairly well because they can go a long time in between meals and not starve to death. This is why they survived while other similar sized animals perished. IIRC, the crocodilians were the largest land animals to survive, partly because they could go underwater to survive the burning, and partly because they could go a long time without eating.
In the ocean, things weren’t quite as bad because the ocean didn’t burn (obviously). The lack of sunlight really wreaked a lot of havoc there though. Ocean plant life died off in large numbers, causing the plant eating animals to starve, which then caused the ocean predators to starve.
75 percent of the species on earth were completely wiped out. The 25 percent that remained had their numbers drastically reduced, but managed to squeak by and survive.
There’s nothing inconsistent or strange in the species that survived vs. those that were completely wiped out.