The brachiopods are a very important group for paleontologists as they have a rich 600 million year old fossil history. With more than 12,000 fossil species described they were possibly the most abundant animals of the Paleozoic era, and important ancient reef builders. Several historical extinction events, most notably the Permian-Triassic extinction, dramatically reduced their numbers and diversity and today only about 335 living species exist.
There are two highly divergent extant classes of brachiopods. The Inarticulata (with about 45 species) appeared first in the fossil record at the beginning of the Cambrian; these were followed by the Articulata (290+ species). The extant genus Lingula shows minimal change from Ordovician fossils and are considered "living fossils".
(Brusca and Brusca 2003; UCMP Berkeley).