Certain kinds of shells are always found in rock layers lower than layers with plant fossils. Dinosaur fossils show up only in layers above or with plant fossils what does this tell you about When these organisms lived

This shows that once animals were on the surface there were plants for them to eat.

The answer is number 3, rust deposits in the ancient ocean.

Explanation:

The question above is highly related to the "Great Oxidation Event" that occurred billions of years ago (or the ancient times).

This event led to an increase in the oxygen, both in the atmosphere, terrestrial and in the oceans. This was primarily because of the existence of "cyanobacteria," the only oxygen photosynthetic bacteria. Due to this, the oxygen oxidized with the iron in the ocean (which was said to be abundant in those days). Thus, this led to the formation of iron oxides (rust deposits) on the floor of the ocean.

This explains the evidence that the oxygen levels in the sea increased due to photosynthetic bacteria.