Question

Oysters are an invertebrate the lives today in brackish waters along coastal zones. When a geologist working in stratified rocks observes a limestone bed with oyster fossils overlain by beds with clam and echinoderm fossils indicative of marine conditions, the fossils probably indicate

Answer 1

The possible answers are:

A) the oysters evolved into clams and echinoderms

B) the oysters went extinct while these rocks were being deposited

C) the environment changed as sea level rose, displacing the oysters environment with marine conditions represented by the clams and echinoderms

D) There is not sufficient information to reach any conclusion from these observations.

The correct answer is C

Brackish waters are salt waters with less salinity than seawater due to their proximity to the coast and the confluences rivers that are bringing fresh water and lowering the salinity of the salt water in these areas.

If a geologist studies layers of stratified rocks an notices a change of the fossil type from oysters that live in brackish waters to clams and echinoderms that are marine organisms he can conclude that there was a rise in sea level.

After the rise of sea level at the place where there used to be a coastal brackish zone with oysters there was deep salty ocean water.

Answer 2

When a geologist working in stratified rocks observes a limestone bed with oyster fossils overlain by beds with clam and echinoderm fossils indicative of marine conditions, the fossils probably indicate ----- that the environment had changed as the sea level rose and it displaced the oysters environment with marine conditions represented by the clams and echinoderms.