The Cretaceous Period ends with a mass extinction. An asteroid six miles in diameter is believed to have hit the Yucatan Peninsula. Shock waves reverberate around Earth. Debris flying high above the atmosphere rains down with incinerating heat. Later, dust and aerosols block sunlight, and temperatures plunge. Photosynthesis stalls. All animals over 55 pounds disappear, including the beloved dinos. Many plant species disappear, and the diversity of plankton and sponges falls sharply. Approximately 85 percent of ocean-dwelling protoctists and marine animal species are lost. It requires 20 million years for new life forms with high levels of diversity to reappear.