A new study by geobiologists traces the cause of the first known mass extinction of animals to decreased global oxygen availability, leading to the loss of a majority of animals present near the end of the Ediacaran Period some 550 million years ago.A new study by geobiologists traces the cause of the first known mass extinction of animals to decreased global oxygen availability, leading to the loss of a majority of animals present near the end of the Ediacaran Period some 550 million years ago.