Using the Fossil Record 

There only two groups of organisms that can be pinpointed with almost absolute certainty in their timing and transitory species for their move to land.  These are the vascular plants and the arachnid scorpions.

The reason that they can be pinpointed is the existence of clear transitory organisms that show their evolution in the fossil record.

For the terrestrialization of vertebrates, many species exist that could be considered half and half, so to speak.  We have the Eusthenopteron, and the Icthyostegalia among others, (both of which will be discussed later).

What we are really lacking in this record is a fossil of something like a Rhipidistian Crossopterygian that can clearly show some sort of structural or otherwise evidence to point to a clear connection between them and the primitive amphibians.  Without this, the abundance of slightly different species that might be considered ancestors will just spurn more debate over lineage.

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