fish eat zooplankton so without fish the zooplankton lose their predators and the population will increase. seals have other things they can eat (penguins and sea gulls), and phytoplankton will decrease because there will be more zooplankton to eat them
Index fossils are used by scientists to help them figure out the dating of discovered fossils or the correlation. They are also very known to appear in a (very) certain/specific age range.
