There is a sudden increase in the nimber of herbivorous fish due to the migration of some carnivorous fish from the river what effect will this have on primary producers and npp

Which of the following must be true for a fossil organism to be useful as an index fossil? a. the fossil organism must be widespread geographically. b. the fossil organism must be abundant. c. the fossil organism must be limited to a short span of geologic time. d. all of the above.