Summations Only

But is this kind of error (which I take on faith at this point btw) frequent or rare? How much information is sufficient to make an educated estimate?

No, you first asserted that you had issues with dating methodology. This was the second time I suggested you should look at that thread and see what you can explain. Your previous response was to ask a question rather than to go to that thread.

Looking at some of my local (ie Australian) geology:
In the mid Proterozoic rocks here I can only find fossils of stromatolites. Your claim seems to be that nearly all other genera existed but were somehow not fossilised.
I move up the sequence a little to late Proterozoic, I'm still seeing stromatilites but I also see Ediacara. Nothing else.
Above this is the Cambrian where I start seeing arthropods as well as stromatolites. Ediacaran fossils have also been reported in the Cambrian. However, no fossils of amphibians, reptiles, birds, mammals etc.
By the time we get to Ordovician rocks in eastern Australia we start seeing verterbrates - jawless fishes as well as arthropods, etc, etc. No marine mammals, no marine reptiles, no marine amphibians and no marine birds for some reason.I could go on, but you know the pattern. In every succeeding geological epoch we see, not just new species, but often new genera, plus nearly always examples of the pre existing genera. Isn't it a much more elegant solution to this observation to admit that the over time new species and genera are appearing rather than to try and come up with some convoluted theory that genera start coming out of hiding for no apparent reason? It might be worth having a look at the cognitive dissonance thread.

Ichthyopterygia ("fish flippers") was a designation introduced by Sir Richard Owen in 1840 to designate the Jurassic ichthyosaurs that were known at the time, but the term is now used more often for both true Ichthyosauria and their more primitive early and middle Triassic ancestors.[1][2]

Ichthyosaurs aren't "very mammal like". They're plainly reptiles.

Traditionally, reptiles are members of the class Reptilia comprising the amniotes that are neither birds nor mammals.[1] (The amniotes are the vertebrates with eggs featuring an amnion, a double membrane that permits the embryo to breathe effectively on land.) Living reptiles can be distinguished from other tetrapods in that they are cold-blooded and bear scutes or scales.

Why did you post an unsourced quote that directly contradicted your claim ?

Well, I am glad we cleared that up.

But how do you deal with the difficulty of this supposed "sudden appearance"? Only it seems to be graver for your ideas than ours. We would expect the land-bound protoichthyosaurs to be confined to one place, since they were not yet fully marine (as turned out to be the case with whales when paleontologists finally found the fossils) and perhaps we haven't found the place yet. But according to you, all the ichthyosaurs, though perfectly capable of roaming the oceans, must instead have been concealed in an undiscovered location or locations until they came out to play in the Triassic --- which they did in order of how adapted they were to marine life, with the most poorly adapted coming out of hiding first. Are we meant to find your idea more plausible? Why?

Except that biologists, when they're classifying ichthyosaurs, biologically classify them as reptiles.

So your guess is completely uninformed and based on your biased opinions rather than on any review of empirical objective data. It's a WAG (wild-ass guess).

Curiously, science is based on evaluation of the best information we have available. Sometimes that information is incomplete due to gaps in the evidence available, and when any new information becomes available to fill those gaps -- as we see here in southern Mexico -- then there is either the possibility of confirming previous thought or of changing previous thought -- as we see here on the timing of the movement of tectonic plates in this area.It is fine to be skeptical, but you then need to investigate, to research the information, to see if your cursory impressions match fact..So again, perhaps it is time for you to actually investigate this, and you can start by reading Age Correlations and An Old Earth, Version 2 No 1

So there's no possibility that all those alleles arose in such geographically limited area in 4500 years. No fludde, nope, no how, no way.

For some reason those ichthyopterygians least adapted to a marine lifestyle managed to spread and flourish before the true ichthyosaurs, which were still hiding in this elusive large sea until the Jurassic. Some of them waited 'til the Cretaceous. Meanwhile, as the later forms turned up, the earlier forms were driven out ... apparently they could all co-exist in a single sea, but the whole of the oceans didn't have enough niches for them. (Maybe you should postulate more large seas that no-one's managed to find yet, in which they were originally segregated.)If this is what you would call a "logical progression", I wonder what you would find bizarre and counter-intuitive

Yup, pigs are unclean. That's why I chose them. Maximum four alleles, in "reality" fewer. Unless, of course, there were extra stowaway pigs scuttling around unnoticed and hiding in mouseholes with the mice.