Shale may now prefix oil and gas but long before this, it gave us the strongest evidence of the Precambrian explosion. This was the Burgess Shale in Canada with its evidence of soft-celled invertebrates giving us a fortunate of life in those early oceans.
This gift is still giving: with a rather extraodianary hemichordate: a cousin of ours; a worm of some distinction. If Blackadder had been a palaeontologist, he would have been taken aback by the worm that looks like a thingy nestling in those warm waters. To view a modern equivalent, click on the link.
When I look down, is it a case of convergent evolution?