To human eyes, the most noticeable parts of every ecosystem are the big, charismatic organisms; there’s a reason the blog is called “Valley of the Mastodon”. But in terms of numbers of individuals and, usually, total biomass, small organisms actually dominate ecosystems. That’s often reflected in the fossil record as well.
Pupilla muscorum is a tiny, air-breathing terrestrial snail (snails are one of a fairly small number of organisms to successfully invade the land). The black bars under the shell shown above are each 1 mm wide, so the entire shell is less than 4 mm long. Shells from this species are quite common in the Diamond Valley Lake deposits; below are the specimens recovered from a single DVL locality:
Today, P. muscorum is found across the northern hemisphere, but especially in Europe. It appears that many of the modern North American occurrences are actually descended from recent invasive European populations, and there is some debate as to whether or not native North American specimens such as the ones from DVL are actually the same species. The shells are extremely similar, so genetic testing of modern populations will probably be necessary to answer this question.