I want to wind up this discussion on gaps by pointing out a couple of things. First, when looking at evolutionary trees, it is always good to be cognizant of what is found as fossils and what is conjecture. The dark lines on the diagram illustrate the relative abundance of fossils of the different groups.
However, the interconnects between the major divisions are show with dashed lines. These are theoretical connections, not actually found in the fossil record.
Reference: M. Denton, Evolution: A Theory In Crisis, pg.173