Blastoids (sometimes called sea buds for their shape) have a long stalk with a head at the top. They went extinct about 260 million years ago (during the Permian), but they used to be abundant on Earth. Their heyday was during the Mississipian (about 350 million years ago). They lived anchored to rocks by their stalks, feeding by filtering small food from seawater. At the top of their head is a mouth, surrounded by grooves with tiny hairs for trapping food. The structure of their stalks and heads is what left a record of their existence. The stalk is made of stacked, calcified disks. The head is made of rows of tightly interlocking, calcified plates that often remain together in the fossil record. A set of holes in the top of head are evidence of the blastoid's unusual respiratory system. Folded structures (hydrospires) were used to extract oxygen from seawater that passed through the holes.

Echinoderms appear in the fossil record more than 500 million years ago, during the early Cambrian. What are usually left behind are hard mouthparts or parts of their skeletons, made of calcite plates. Rarely, an entire skeleton fossilizes, for example in a situation where it was quickly buried in sand. Even a piece of skeleton can provide information, because echinoderms have specific patterns in their skeletons. The echinoderms you see today have five-point (pentaradial) symmetry, often noticeable in five arms. While some of the earliest echinoderms were pentaradial, others had unusual body shapes. The “helioplacoids” had long, oval-shaped bodies with no arms, and a spiral pattern on the surface from tube feet wrapped around a central core. Helioplacoids went extinct even before the end of the Cambrian, as did a variety of other echinoderms, including the star-shaped Somasteroidea. Some echinoderms survived and diversified, becoming dominant in the oceans of the Paleozoic era.