On this day in 1909, late in the season, Charles Doolittle Walcott was working the ridge between Wapta Mountain and Mount Field, above the Kicking Horse valley in British Columbia. Horse trail country, somewhere around 2,300 metres: thin soil, loose plates of dark shale, the sort of altitude where you eat lunch standing because the ground is cold. A slab split open, and inside were animals roughly 505 million years old, printed whole. Not the usual shells and carapaces. Gills. Limb filaments. Traces of gut. Creatures with nothing hard about them, which by every ordinary rule should have left no record at all.

The figure below shows Marrella splendens, the commonest animal in the quarry and one of the first Walcott named from it: two long antennae, four spines swept back from a head shield, and rows of feathery limbs. In the micrographs the limb filaments show as rusty orange fans in grey rock, fine as the barbs off a chickadee's flight feather. Tens of thousands of specimens of that one small arthropod have come out of the beds since.

What I keep in mind is the labour after the finding. Walcott came back for field seasons with pack animals and family and hired men, and quarried; then decades of surveyors, quarry crews and museum technicians splitting rock with cold hands, one plate at a time, in weather that turns to snow in September. Ada would tell you the same about any collection worth having. Somebody counted it.