The trilobite seas of Utah

Trilobites were arthropods with a three-lobed shell that lived on the sea floor for over 250 million years, and they are among the commonest Cambrian fossils. The House Range and the Wheeler Formation yield some of the world's richest Cambrian trilobite faunas, and they have been central to calibrating the Cambrian timescale, because species succeed one another rapidly and recognisably. North America then sat on the equator and was largely drowned by a shallow inland sea — the continent on today's map is the dry part of something that was under water for most of its history.