However, several workers have disputed the monophyly of the Orthocerida as traditionally defined, and some have criticized it as a "nightmare for taxonomists" lacking clearly defined characters. Orthocerida as usually understood may thus be a polyphyletic group, having arisen as several lineages from early Ordovician cephalopods. Some workers have split off the Pseudorthocerida and Dissidocerida as separate orders, the latter on the grounds that it arose from a different baltoceratid ancestor. The pseudorthocerids are thought to be distinct because their protoconch and septal necks are so different. Orthocerids flourished in the Paleozoic Era, giving rise to such intriguing forms as the ascocerids. Moreover, the spherical protoconch, or first chamber, of some orthocerids suggests they were ancestors to the Bactritida, small orthoconic forms that gave rise to both the ammonoids and coleoids. Fossilized radulas from orthocerids also suggest a closer affinity with modern coleoids than with Nautilus. There is some dispute when the orthocerids finally became extinct. Although they are often said to have survived into the Triassic Period, the two genera that date from that period may actually be pseudorthocerids. In that case, the last orthocerids may date only to the Permian. However, the discovery in the Caucasus of a possible orthocerid from the Early Cretaceous Period suggests that they may have endured much longer as a ghost lineage. References Kröger, Bjorn, & Mapes, R. (2004) Embryonic orthoceratid nautiloids of the Imo Formation (Lower Carboniferous-Upper Chesterian) of Arkansas (USA). Journal of Paleontology 78: 560-573, Iowa City. pdf; GSA presentation Sweet, Walter C., (1964), Nautiloidea—Orthocerida, in Treatise on Invertebrate Paleontology. Part K. Mollusca 3. (Geological Society of America, and University of Kansas Press, New York, New York and Lawrence, Kansas) External links www.palaeos.com Orthocerida (Fossil Nautiloidea Page) Prehistoric cephalopod orders Mesozoic cephalopods Paleozoic cephalopods Early Ordovician first appearances Triassic extinctions Paraphyletic groups