Question

Why might ancient lake and sea beds be rich sources of fossils?

Answer 1

1- creatures have hard parts that can be preserved 
2- oxygen poor environment, slows decomposition 
3- fast burial

Answer 2

Answer:

    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.

    Water offers better conditions for fossilization because it carries sediment and dissolved minerals. They are the ingredients of sedimentary rocks, which can be built on the earth from the dune sands, for example. But the most common are formed under the sea, or in lakes and rivers. Not surprisingly, therefore, most fossils are formed at the bottom of the sea. To find marine fossils on dry land, something must have happened for rocks to rise, tides to fall, or both.