From evolutionary standpoint , the older a fossil-bearing rocks is, the more likely it is to contain Options

The older a fossil-bearing rock is, the more likely it is to contain simpler and primitive organisms, and less likely to contain complex and advanced organisms. This is because simple and primitive organisms existed before complex and advanced organisms evolved. Therefore, the correct option in this case is "invertebrates as opposed to vertebrates". Invertebrates are simpler and more primitive than vertebrates and thus were the dominant form of life for a long time before vertebrates evolved.