Source: Wikipedia. Pages: 52. Chapters: Marrella, Pikaia, Hippotherium, Scenella, Prodryas, Tullimonstrum, Paramylodon, Eosurcula, Polyptychoceras, Merychippus, Protoceratidae, Gomphotherium, Seymouria, Baltoceratidae, Graeophonus, Bactroceras, Basslerocerida, Harrington's Mountain Goat, Hynerpeton, Synthetoceras, Dawsonoceratidae, Actinoceras, Rayonnoceras, Encrinuridae, Geisonoceratidae, Pseudoprotoceras, Diadectes, Syndyoceras, Parahippus leonensis, Paratomarctus, Arctonasua, Plionarctos, Pachyarmatherium, Ursavus, Geisonocerina, Poabromylus, Paratoceras, Hypertragulidae, Proteoceratidae, Prosynthetoceras, Shuvosaurus, Leptotragulus, Leptoreodon, Cynarctoides gawnae, Canis cedazoensis, Rioceras, Nanotragulus, Heteromeryx, Adelphailurus, Hypertragulus, Pliometanastes, Rhabdiferoceras, Trigenicus, Kyptoceras, Michelinoceras, Hyracodontidae, Leptocyon, Pseudorthoceratidae, Leptomerycinae, Caedocyon, Pediomeryx, Cephalogale, Aphelops, Canis edwardii, Aceratheriinae, Parvitragulus, Toromeryx, Acleistoceras, Procranioceras, Peraceras, Claraia, Megalocephalus, Cyptendoceras, Odaraia, Diceratherium, Dinobastis, Synthetoceratinae, Lambdoceras, Protoceratinae, Encrinurus, Eosomichelinoceras, Pseudorthocerataceae, Kyptoceratini, Cyrtobaltoceras, Narthecoceratidae, Yumaceras, Hypisodus, Garryoceras, Pentacrinites, Donacoceras, Microbaltoceras, Discoceras, Metabaltoceras, Endorioceras, Hercynella, Aletomeryx, Clavilithes, Clinoceratidae, Sactoceras, Synthetoceratini, Richmondoceras, Cyrtoceras, Leurocycloceras, Neithea, Atrypa, Miocnus, Ficopsis, Discosorus, Adamsoceras, Sphaerocoryphe, Nephrolenellus, Acaenasuchus, Terebratula, Ogygopsis, Limnoscelis, Aviculopecten, Gulfoceras, Volutospina, Phymosoma, Leonaspis, Paleorhinus, Meristina, Spiriferina, Whiteavesia, Cheirurus, Discinisca, Oxytoma, Eucalyptocrinites, Columnaria, Cyphaspis, Belemnitella, Pleurocystites, Pentamerus, Solenomorpha, Hippurites, Platystrophia, Peronopsis, Wilkingia, Micraster, Gervillaria, Linthia, Tseajaia, Onnia, Tinodontidae, Volviceramus, Suchoprion, Asaphiscus, Teredina, Dicoelosia, Echinosphaerites, Cupressocrinites, Archaeocidaris, Burgessia, Diraphora, Temnocidaris, Cyathocrinites, Cardiola, Burgessochaeta, Sphaerexochus, Actinoconchus, Conulus, Tylocidaris, Metalopex, Glyptocrinus, Saccocoma, Kingena, Echinocorys, Reteocrinus, Tessarolax, Dimerocrinites, Cylindroteuthis, Sagenocrinites, Actinocrinites, Cyclothyris, Coelopleurus, Habelia, Pentremites, Hardouinia, Melonechinus, Turritella apicalis, Acroteuthis, Marsupiocrinus, Kinoceras, Fasciculus. Excerpt: Pikaia gracilens is an extinct animal known from the Middle Cambrian Burgess Shale of British Columbia. 16 specimens of Pikaia are known from the Greater Phyllopod bed, where they comprise 0.03% of the community. It was discovered by Charles Walcott and was first described by him in 1911. Based on the obvious and regular segmentation of the body, Walcott classified it as a polychaete worm. It resembles a living chordate commonly known as the lancelet and perhaps swam much like an eel. Scale diagram of various Burgess Shale invertebrates, P. gracilens in yellowDuring his re-examination of the Burgess Shale fauna in 1979, paleontologist Simon Conway Morris placed P. gracilens in the chordates, making it perhaps the oldest known ancestor of modern vertebrates, because it seemed to have a very primitive, proto-notochord. However, the status of Pikaia as a chordate is not universally accepted; its pre...
