The was a period of rapid change during which the ancestors of most animal groups emerged. a layer of soot found between rock layers worldwide, known as might indicate that a large meteorite collided with earth.

poor water quality affects most estuarine organisms, including commercially important fish and shellfish. the pollutants that have the greatest impact on the health of estuaries include toxic substances like chemicals and heavy metals, nutrient pollution (or eutrophication), and pathogens such as bacteria or viruses.