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
Opabinia family tree is that modern tardigrades may be "Opabinia"s closest living evolutionary relatives. Although Zhang and Briggs (2007) disagreed with Budd's diagnosis that "Opabinia"s "triangles" were legs, the resemblance they saw between "Opabinia"s lobe+gill arrangement and arthropods' biramous limbs led them to conclude that "Opabinia" was very closely related to arthropods. In fact they presented a family tree very similar to Budd's except that theirs did not mention tardigrades. "Opabinia" made it clear how little was known about soft-bodied animals, which do not usually leave fossils. When Whittington described it in the mid-1970s, there was already a vigorous debate about