Protista -- includes eukaryotic one-celled organisms. You've probably heard of the amoeba, which does not have a skeleton, and feeds by engulfing organic material by extending its cell wall around a piece of food, and ingesting it after it is inside the cell. Fossils of amoeba-like organisms, the Radiolaria and the Foraminifera, are important groups, and there are many others. Radiolaria are tiny, and have siliceous tests (test = skeleton of a microorganism). Foraminifera have calcite tests, and are usually tiny, like these:
That's a microscope slide, so each little "foram" (or foraminifer, if you want to be formal) would fit on the head of a straight pin. They come in all shapes and sizes in an incredible diversity and are very important fossils, because it is so easy to find thousands of them, even in a small sample of an ancient sea bottom deposit.
A few lineages stretched the one-cell condition, seemingly to its limits, but not really -- there are even larger forms than these:
An important group of Permian foraminifera are the fusulinids, called that because of the fusiform, torpedo-like shape of the skeleton:
Don't let these large examples throw you off -- most foraminifera were tiny.
The tiny skeletons of radiolarians are even more varied! The following sketch shows some of the many forms:
Image Source: Wikipedia; See the page on Radiolaria .
Is it any wonder that microfossils are important as index fossils? Recall that one of the requirements of a good index fossil is that be easily recognizable -- has unique features, won't be confused with other fossils.