https://youtu.be/vuet3t9geXo?t=11m59s https://youtu.be/vuet3t9geXo?t=11m59s Start with this. The size of the asteroid they thought would be the size of Staten

https://youtu.be/vuet3t9geXo?t=11m59s https://youtu.be/vuet3t9geXo?t=11m59s Start with this. The size of the asteroid they thought would be the size of Staten Island, NY. The impact would generate a huge shock wave and high temapture, fire everywhere, so killing animals nearby (say 100 miles) within minutes. Tsunami came next and wiped out everything nearby hundreds of miles+. The smoke and dust covering the air (think of volcano) so sunlight cannot enter Earth so there was a long cold winter, killing even more animals.