When species become extinct it is for God. There are remnants of past species in fossilized rock past a certain age. For example, certain shelled species seem to fossilize well and so did the dinosaurs along with woolly mammoths. Woolly mammoths are reported to have become extinct 200,000 years ago, so, with this information and due to weather conditions we would assume the rock is about this old or at least this is when carbonization occurred with this being why they find whole skeletons fully formed. God informs us about what is needed when other smaller species become extinct, usually it is a measure of how the planet is performing. There is equilibrium in ecology when there is a balance in nature. Depending on the temperature of the ground at any one given moment in the history of the planet will result in the fossilization of species. Previous larger mammals would normally have needed an ice age, so, they are almost "snap" frozen for preservation and you could imagine nothing else rendering these species extinct other than lack of food. Some small species will be found if they have been covered but mostly God would have to show you where to find them, when found, these could be the vital clues for judging the age of larger mammals. We are just stumbling around in the dark without God's help. Certain people will be nominated by God to find these fossils and others will just walk right over them. I suppose it is like walking right on by a masterpiece in the Louvre Museum, only certain people are blessed with eyes to see.