At the end of the Cretaceous period clade Dinosauria, having ruled the world for 135 million years, was so diverse and so vigorous that it seemed nearly impossible to imagine a set of events that could wipe it out completely. It would have to take the perfect storm of catastrophic events to effect such a drastic change. At the very least an enormous basalt flow lasting millennia coupled with massive sea level change followed by a devastating asteroid impact scorching the Earth, filling the atmostphere with sulfur and carbon dioxide, throwing the global climate into chaos. That could, perhaps, do the job. You might be left with some remnant of Dinosauria, probably a portion of the flying maniraptorans, requiring less food and able to cover more ground more easily than all the others. But the giants that had ruled the Mesozoic era would be gone.
But what if the storm was less than perfect? What if the angle of the asteroid was different, or it veered off and hit the Pacific Ocean ra