The accumulation rate for 400m of sediment over 35 million years is just over 1cm per 1000 years. So we should assume that ammonites could only be buried in an avalanche of sediment, on a sloping sea floor. This would be most likely to occur near the shore, where there would be other sediments mixed in with the lime, so we would not expect to see pure chalk fossils. Anyway, I don't like to mention it. I just do mention it. Still, as long as nobody minds, I suppose it's all right.