Almost a hundred new animal species that survived a mass extinction event half a billion years ago have been discovered in a small quarry in China, scientists revealed Wednesday. However, it came to a sudden end when up to half of all animals died off 513 million years ago. This mass extinction, known as the Sinsk event, is thought to have been caused by declining oxygen levels. The Sinsk event is not considered among the best-known "Big Five" mass extinctions in our planet's history. Scientists have long debated whether dinosaurs were in decline before an asteroid smacked the Earth 66 million years ago, causing mass extinction.