Describe how marine animals become fossilised?

Cartilaginous organisms are usually not fossilized due to the easy decomposition of cartilage. This is visible in human skeletons in which no nose and ears are present because they are made solely of cartilage and are decomposed easily.
However, the hard bones of fish are fossilized when they settle down to the sea floor and are enveloped by the sediment. If conditions are right, the bones may become well preserved. This is how many fish fossils have been found from places that were once covered in oceans.