Why is it impossible to construct phylogenetic series for all evolutionary branches?

The study of fossil forms provides an incomplete picture of the evolution of flora and fauna. Most of the remains consist only of solid parts of organisms: shells, bones, etc., fossils that have preserved traces of holes and passages of ancient animals. It is far from always possible to find intermediate forms: two thirds of all previously existing organisms have been lost to science. Those animals that did not have a solid skeleton, teeth and other formations died out forever, disappeared from the face of the Earth, and their trace cannot be found, and their appearance cannot be compiled.