The skeleton of a dinosaur in a museum reminds us of the great extinctions. The Earth has lived at least five mass extinctions throughout its history. The most tragic was the great extinction of the Permian, 225 million years ago. It killed 95% of living things, including trilobites.
Micrometeorites Dictionary of astronomy, letter M They are particles from extraterrestrial space, some thousandths of a millimeter in diameter, that rain incessantly on Earth settling on the bottom of the oceans or on polar caps, where their layers accumulated by time can be easily recognizable.