The Spence Shale of Utah represents a window into the time of the “Cambrian explosion” when evolution got into full swing, and many new phyla and families appeared in the fossil record. The Spence Shale represents a slightly earlier time than the Burgess Shale, famous for its preservation of soft parts. The Spence records many nice hard parts, and we see a proliferation of trilobites and other arthropods. Also recorded are the shells of primitive mollusks, brachiopods and echinoderms, groups which become much more diverse in the Ordovician. In fact, every phylum visible in the middle Cambrian became much more diverse by the Ordovician Period.