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.