Evolution is a branching process - by which we mean that species often split in two.
Therefore: "Because evolution is a branching process that produces a complex bush pattern of related species rather than a linear process that produces a ladder-like progression, and because of the incompleteness of the fossil record, it is unlikely that any particular form represented in the fossil record is a direct ancestor of any other." — Rusty Cashman / Wikipedia In short, transitional fossils are best thought of as being close relatives of the species which actually link two groups.
Almost all of the transitional forms in this list do not actually represent ancestors of any living group or other transitional forms.
This website is dedicated to some of these so-called transitional fossils.
For those that subscribe to the theory of the living earth, it is the prokaryotes that maintain the homeostasis of the earth, rendering the biosphere habitable for all other life.
They maintain and recycle the atomic ingredients of which proteins, the essence of life, are made, including oxygen, nitrogen and carbon.
This definition is, in fact, such a gross oversimplification as be scientifically useless.
Although the Precambrian contains some seven-eighths of Earth's history, its fossil record is poor, with the majority of fossils being the stromatolites that are often heavily metamorphosed or deeply buried.