What can you conclude about a time in earth's past when only marine fossils formed?

The safest conclusion would be that the marine organisms dominated the living world at that time. It is really hard to say with certainty that there were no land organisms just from the lack of their fossils in that era. The process of forming the fossils is quite fragile and requires various conditions to be fulfilled ( perfect temperature, moisture, the composition of the ground etc.), so a lack of fossils can also be due the bad conditions for fossilization at that time, not the lack of organisms present.