Liquid water is held together by hydrogen bonds. (Liquid water has fewer hydrogen bonds than ice.) Oils and fats not have any polar part and so for them to dissolve in water they would have to break some of water's hydrogen bonds. Water will not do this so the oil is forced to stay separate from the water.
Index fossil, any animal or plant preserved in the rock record of the Earth that is characteristic of a particular span of geologic time or environment.
