Fossils in

layers of rock are generally estimated to be

than fossils found in the upper layers.

older, younger

Onewer, younger

older, older

newer, older

Fossils in older layers of rock are generally estimated to be older than fossils found in the upper layers.

Explanation:

The fossils that forms the layers are the fossil layers in the sedimentary rocks. These will be forming layer by layer. The sediments will be getting deposited layer by layer. These sediments are those materials that are the broken pieces if rocks, sand, pebbles, silt, animals, dead things, etc. They will get deposited as they are carried like sediments.

These gets deposited above each other forming different layers. The layers that are formed on the top side will have larger weight thereby it presses the layers in the bottom side. This will be resulting in the rock formation. Thus the Fossils in the older layers of a rock are older fossils than the fossils that are found in the upper layers.