Answer:
answ2. Current ideas support the concept of a large bolide landing in the Yucatan Peninsula in Mexico, a place called Chixculub, about 65 million years ago. This bolide was about 10km in diameter, and created a crater of about 180 km diameter. The bolide contained an unusual concentration of iridium, and Ir dust from this impact is found world wide.
This produced huge destruction and the increased clouds and particles would have chilled the atmosphere and killed off most life forms. The ones that survived were able to burrow (Tuatara), and of course the sharks.
Other possible causes would include great flood basalt events, and these would also poison the atmosphere.
There have been several other major catastrophes in Earth history.