Micro-vertebrates and sieving

Sediment from productive horizons — Cretaceous channel lags, Eocene lake margins, Miocene phosphate beds, Pleistocene cave earth — is bagged in the field with the horizon recorded, then wet-sieved through nested screens (typically 4 mm, 1 mm and 0.5 mm) to remove clay and concentrate the fraction that holds teeth, jaws, scales and bone. Screen-washing boxes built of window screen over a wooden frame are traditional; stacked laboratory sieves are quicker for small samples.

The dried concentrate is then picked under a stereo microscope at 10–20×, spread thinly on a gridded tray, with a fine wetted brush to lift each fossil into a labelled gelatine capsule or micro-vial. The picking is where the hours go: a kilogram of concentrate can hold a hundred identifiable teeth and take an evening.

Each vial gets a number and a label with the site and horizon; the fossils are too small to number individually and a vial of teeth from one sample is one lot. Keep the unpicked concentrate in labelled bags; it is often worth a second pass.

Micro-vertebrate faunas are where the mammals, lizards, amphibians and small sharks of a formation are found, which is why researchers ask about them and why a collector who has kept sample data has something a museum will take. On federal land, vertebrate microfossils are vertebrate fossils and need a permit; on private land, take the landowner's agreement on the bulk sediment as well as the finds.

Riker mounts and slides are the display; a micromount box with a foam bed holds the working collection.