This one is remarkable – a school of fish, admittedly very small fish, are preserved in a block of limestone. They are said to go back 50 million years ago but the fact a whole school of fish was fossilised in one event, necessarily very quickly, is astonishing as they were all still swimming in formation (at the point of extermination). The story is at www.dailymail.co.uk/sciencetech/article-7083817/ … and the post has lots of added information (of varying degrees of usefulness). They come from the geological strata, known as the Green River Formation, that runs across parts of Colorado, Wyoming, and Utah. What we have is nearly 300 fish swimming in formation (as in a school of fish). They are allotted to the Eocene and the researchers say they do not know how they died or why. They add that they died 'instantaneously' and we may note the sedimentary fabric in which they are preserved includes fine grained mud. Was the river bottom left suddenly without water – but the fish are in the act of swimming. Were they on the crest of a wave that came to an end – suddenly.