Issue link: https://wardsworld.wardsci.com/i/1491083
4 + ward ' s science a pseudocoel. These pseudocoelomates are soft-bodied, small to minute, and commonly parasitic; some of them have never been found as fossils. Some features of the cells of primitive animals resemble those of the single-celled Protozoa, especially the flagellates, which have long been believed to be animal ancestors. Molecular phylogenies have supported this idea and also suggest that the phylum Cnidaria arose separately from all other phyla that have been studied by this technique. Thus, animals may have evolved at least twice from organisms that are not themselves animals, thereby representing a grade of evolution and not a single branch (clade) of the tree of life. Sponges also have been suspected of an independent origin, and it is possible that some of the extinct fossil phyla arose independently or branched from sponges or cnidarians. The earliest undoubted animal fossils (the Ediacaran fauna) are soft-bodied, and they first appear in marine sediments nearly 650 million years ago. This fauna lasted about 50 million years and consisted chiefly of cnidarians or cnidarian-grade forms, although it contains a few enigmatic fossils that may represent groups that gave rise to more advanced phyla. Then, nearly 570 million years ago, just before and during earliest Cambrian time, a diversification of body architecture began that pro- duced most of the living phyla, as well as many extinct groups. The body plans of some of these groups involved mineralized skeletons that created, for the first time, an extensive fos- sil record because these skeletons are more easily preserved than soft tissues. The earliest mineralized skeletons are chiefly minute, including the small shelly fauna of earliest Cambrian age; moreover, although some of these fossils are primitive members of living phyla, many (perhaps 10 to 20 kinds of Fig. 4: Vertebrate evolution in relation to geologic time. Classes are shown in uppercase letters; subclasses are shown in lowercase letters, except that "hagfishes" and "lampreys" are common names for orders. (Credit: M. Hildebrand) Animal Evolution (continued)