There are multiple reasons why the fossils of the animals and plants of the more distant past are much more rare than the fossils of the animals and plants from the past few hundred million years. The earlier life organisms were much less complex, often lacking solid body structures. That means that the chances that they can be preserved are very low from the start. The fossils can not last forever, and in order for fossils to be preserved over such a long time there has to be very specials conditions in which the fossils have been preserved, and that is very rare. The movement of the tectonic plates has resulted in the disappearance of parts of some plates, or even all of them, while also creating new ones. The parts of the oldest crust have almost entirely been destroyed through subduction, thus getting below other plates and being melted in the astenosphere.
