>> new discoveries can never give us a later date for when something appeared, but they can give us an older date
But neither are useful. The fact that a structure appeared at time X is not an indication that it continued until time Y. Evolution is not linear at smaller scales. Structures evolve/appear only to disappear through the same processes that created them. The fact that this critter had a particular shape of eye is not an indication that it passed that structure to later lifeforms. It is certainly possible, but the concept that something happened "first" is irrelevant in a world where near-identical structures regularly evolve independently.
In practice, paleontologists look at the passage of traits down evolutionary lineages regularly. You are correct in stating that the same features can develop independently, but that does not mean they do every time. I don't think the assertion that 'evolution is not linear at smaller scales' holds up to much scrutiny. Life as we know it results from the passage of traits encoded in genes, otherwise there would be no evolution; we would have basic unicellular life spontaneously arising from the primordial ooze over and over again Groundhog Day style.
> The fact that this critter had a particular shape of eye is not an indication that it passed that structure to later lifeforms.
In the parlance of the geosciences, depending on the context, we'd call this a suggestion rather than an indication, as there may be weaker evidence or credible alternative hypotheses. However depending on the scales of the time and speciation differences between the species under consideration it may be very strong evidence.
For example, it's highly unlikely that the presence of limbs in both Homo erectus and Homo sapiens is coincidental and Homo sapiens have limbs that evolved independently of Homo sapiens. This is very different than asserting that wings in both bats and birds are indications that one evolved from the other.
Thats right, an evolutionary advantage from birth defect does not 100% result in a successful lineage. It can just die and not even pass it on, then later the same defect can appear again and again until if results in a lineage.
Its an advantage to see better, but it does not guarantee survival just a slightly higher percentage (which is enough to thrive)
But neither are useful. The fact that a structure appeared at time X is not an indication that it continued until time Y. Evolution is not linear at smaller scales. Structures evolve/appear only to disappear through the same processes that created them. The fact that this critter had a particular shape of eye is not an indication that it passed that structure to later lifeforms. It is certainly possible, but the concept that something happened "first" is irrelevant in a world where near-identical structures regularly evolve independently.