The sheet floralgae replaced its ancestor, the tripgae. The way tripgae grew, in only one dimension, was better than its ancestor, but increased surface area was always better. So, some tripgae that had grown outward were more successful, due to being able to catch more sunlight. Soon it became normal for all tripgae to do this, and so they were replaced, not by competition, but by the gradual change from one form into another. This does not mean they were not successful, however. If conditions are good, they can keep growing until they are ten by ten centimeters, but usually they break apart after this because, like their ancestor, they are only one cell thick.
Living Relatives (click to show/hide)
None found. Note that this does not necessarily mean it has no living relatives at all, but that, assuming all taxonomy is filled in, its entire phylum is extinct; any relatives it does have likely do not resemble it.