The is often represented as a chart in which a specific sequence of bases in mrna (a codon) is used to represent each amino acid building block found in the world of proteins.

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.