We won't find a cow in the pre-cambrian because we know they're not there, tends to be small lifeforms, note that pollen has been found in the pre-cambrian but evolutionists have labelled such evidence, "anomolous"
Actually, I would have lableled it 'spurious'. You refer to pollen grains in the Hakatai Shale, I believe. However, those grains were shown to be concentrated in cracks in the rock. They were also white in color, which is the nature of modern, not fossil pollen, and they are not flattened into the plane of the shale laminations as would be expected.
In fact they would actually just argue contamination, and I have read that they have done this before. They also cast doubt on potentially compelling evidence such as human footprints (Laetoli) or even the human and dino footprints found together. It is at least interesting evidence but in one book Dawkins just says it's fake even though they shown how it wasn't fake.
In the case of the Hakatai, the interpretation just happens to be correct according to all of the evidence. You can complain, but that will not change things.