Only the calcitic scleretomes ("armour plates") of these worms tend to be preserved in the fossil record. These are tiny, and usually found disarticulated: articulated specimens reach about a centimeter in length, and are incredibly rare – hence the limited degree of study since their description in 1857. Scleritomes which bear a strong resemblance to the machaeridians are found in the small shelly fauna of the early Cambrian, 530 million years ago, suggesting an early origin of the group. The machaeridians are characterized by having serialized rows of calcitic shell plates. The dorsal sclerites were convex and almost isometric; lateral sclerites were flatter and longer. The plates comprised two calcite layers: the outer layer is thin and formed by lamellar deposition, whereas new elements were added to the thicker inner layer as it grew. Scales are ridged with growth lines, implying that they grew episodically. A few taxa experimented with different approaches to scale formation; some were only very weakly calcified and may have mainly been organic in nature. They were never moulted, and each scale could be moved with an attached muscle. The front two segments of the machaeridians were commonly different from the rest, bearing fewer spiny projections. The plumulitids are flattened from above and looks much like the coat of mail armour of chitons. The two other families are laterally compressed and some lepidocoleids formed a dorsal hinge, which make these machaeridians look like a string of bivalves.
In the Virtual Museum there are total 59 samples