On the geologic time scale are marked by major, striking, and worldwide changes in the type of fossils present.
a.
eras
c.
epochs
b.
periods
d.
all of the above

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.

The area of shoreline that is ultimately exposed and submerged is the

Number and variety of living organisms; includes genetic, species, and ecological types.