why are some fossils in some layers of the earth crust and then disappear
Fossils are typically found in sedimentary rock layers, which are formed by the accumulation of sediments over time. The fossilization process occurs when the remains of an organism are buried and preserved in these sedimentary layers. However, fossils may not always be found in every layer of sedimentary rock, as the conditions necessary for fossilization to occur may not have been present in all areas or during all time periods.
Additionally, the Earth's crust is constantly changing due to processes such as erosion, tectonic plate movement, and volcanic activity. As a result, older rock layers may be eroded away or uplifted, making fossils in those layers more difficult to access or completely disappearing. This is why some fossils are found in certain layers of the Earth's crust and then may disappear in subsequent layers.