Fossils of ancient organisms are found in deep layers of rock, while the fossils of more recent organisms are found in shallower layers. which two groups' common ancestor would be found in relatively shallow fossils

animals and plants

Explanation:

Fossils are remains or traces of animals, plants or other living things preserved in rocks, sediment, ice or amber. They are preserved as molds of the body or parts thereof, as well as tracks and footprints. Fossils and their presence in rock formations and sedimentary layers are known as the fossil record.

The fossil is covered by layers of sediment, which slowly compress until they become, after centuries or millennia, into rocks. The time required to build all this is millions of years and the oldest rocks are below. Often lower fossils are older than higher fossils. For example, if trilobites are found in the deepest layers, since they are more primitive beings; whereas plants and animals are found in the shallower layers.

the answer is:

carbon dioxide

the answer to your question is tooth decay.