Scientists use index fossils to identify the times at which unidentified
fossils and certain sediments were deposited. They hypothesize that if a
particular fossil is found near an index fossil, in the same layer of rock,
then the two fossils were likely from the same time period. The shorter the
lifespan of the index fossil species, the more precisely scientists are able
to correlate that species with a particular era. Therefore, it is not
surprising that ideal index fossils are from shortlived, common, and easy-to-
identify species.
Which of the following is an example of the effective use of index fossils?
A. Scientists fix the time that Mesolenellus hyperborea existed in the late
Nevadella era, because they find Mesolenellus hyperborea near and in the same
level of rock as Limniphacos perspiculum, a recognized index fossil that
existed in that period.
B. Scientists determine that Mesolenellus hyperborea were probably mud bottom-
dwellers, because hyolitha, which existed during the same time period as
Mesolenellus hyperborea, were also bottom dwellers.
C. Scientists hypothesize that Mesolenellus hyperborea were wiped out by a
sudden climate change that occurred at the end of the Devonian period, when
many other trilobites were wiped out.
D. Scientists determine that Mesolenellus hyperborea did not exist in Iceland,
because Petrianna fulmenta, a similar trilobite, did not exist in Iceland.
