Threatened arthropods United States, and IUCN Red List for international purposes. Only a tiny fraction of the planet's endangered arthropods are formally recognized as such, as no one has ever evaluated the conservation status of the vast majority of arthropod species. It is difficult to estimate the total number of endangered arthropod species, since many of the taxa themselves have not been recorded. For example, in North America the estimated number of insect species exceeds 163,000, of which only about two thirds are taxonomically known. An even greater discovery awaiting, over 72 percent of North American arachnids are yet to be named
Small shelly fauna mostly of unknown origin. However it is known that some, including "Microdictyon", were produced by lobopods, animals that looked like worms with legs and are thought to be close to the ancestors of arthropods. Univalved and bivalved shells are fairly common. Some cap-shaped shells are thought to be the only sclerite covering their creators, while others are known to be parts of a more complex armor system like "Halkieria"’s. The Helcionellids are thought to be early molluscs with somewhat snail-like shells. Some have horizontal "exhaust pipes" on the concave edges of their shells, and there is debate about whether these