Why is the fossil record of marine life more complete than that of organisms living in terrestrial ecosystems?

It is because sedimentation will more often occur in marine ecosystems

Explanation:

Formation of fossils (through the process of fossilization) is a rare event because animal bodies are often eaten and decayed (destroyed by scavengers and decomposed by worms). But, fossilization can be achieved after the quick burial of an animal.

So, for example, dead marine animals might be buried by sediment that is carried by the water. Also, sediment can precipitate on the ocean floor, covering the remaining of dead animals.