Orthoceratoidea is a major subclass of nautiloid cephalopods . Members of this subclass usually have orthoconic (straight) to slightly cyrtoconic (curved) shells, and central to subcentral siphuncles which may bear internal deposits. Orthoceratoids are also characterized by dorsomyarian muscle scars (a small number of large scars concentrated at the top of the body chamber), extensive cameral deposits , and calciosiphonate connecting rings with a porous and calcitic inner layer.
12-436: Pseudorthocerida is an order of generally straight longiconic orthoceratoids with a subcentral to marginal cyrtochoanitic siphuncle composed of variably expanded segments which may contain internal deposits that may develop into a continuous parietal lining. (Sweet 1964). Cameral deposits are common and concentrated ventrally. Apices typically have a slight to moderate exogastric curvature The Pseudorthocerida are included in
24-465: A Silurian orthoconic genus with a wide, nearly central, tubular siphuncle , suborthochoanitic septal necks, and endosiphuncular rod; in many aspects similar to rod-bearing orthocones of the Ordovician. It is thought by some that Ordovician-Silurian orthocones with continuous longitudinal endosiphuncular laminar deposits constitute a single phylogenetic stock. The Dissidocerida as understood includes
36-887: A broad in-group of generally orthoconic cephalopods known as the Orthoceratoidea (Kroger 2008) along with the Ascocerida , Dissidocerida , Lituitida , and Orthocerida . The Pseudorthocerida were among the last living orthoconic nautiloids. One family, the Trematoceratidae , survived into the Triassic Period. By current understanding the Pseudorthocerida contains the following families: The Pseudorthoceratidae through Spyroceratidae are presented as subfamilies in Sweet (1964) and are included in
48-532: A result, true pseudorthocerids were limited to the Pseudorthoceratidae while the earlier Ordovician look-a-likes were retained in the Orthocerida (Michelinocerida Flower) Orthoceratoidea Currently, Orthoceratoidea comprises the orders Riocerida , Dissidocerida , Actinocerida , Pseudorthocerida , Lituitida and Orthocerida . Orthocerida is a noteworthy paraphyletic order which
60-790: Is ancestral to the major cephalopod groups such as the extinct ammonoids and living coleoids (cephalopods without external shells, including squids, octopus, cuttlefish, etc.). As a superorder, Orthoceratiodea was one of six superorders within the Nautiloidea, the others being the Plectronoceratoidea (= Ellesmeroceratoidea of some) from which the others are derived, the Endoceratoidea , Actinoceratoidea , Discosoratoidea , and Nautilitoidea . Current classifications (e.g. Kröger 2008 and Teichert 1980) separate orthoceratoids, endocerids and actinocerids as co-equal taxa to
72-731: Is used as a subclass, referring to McCoy (1844), revising his (Kröger 2004) perception of the order Orthocerida Kuhn (1940) as part of the Nautiloidea, in the sense of Sweet 1964 in the Treatise on Invertebrate Paleontology . In the classification of Teichert (1988), the subclass Orthoceratoidea is expanded to include the orders found in the Plectronoceratoidea and Orthoceratoidea of Wade (1988), combining forms with thick-ringed ventral siphuncles with those with thin-ringed central siphuncles. The Orthoceratoidea in Kröger (2008) includes
84-416: The Nautiloidea, which becomes much reduced in scope. Wade (1988) instead proposed separating the Nautiloidea into phylogenetically related superorders while retaining the basic concept of the subclass: externally shelled cephalopods with simple concave septa and retrochoanitic siphuncles from which the convexly septate Ammonoidea with prochoanitic siphuncles are distinguished. In Kröger 2008, Orthoceratoidea
96-653: The Orthocerida, Ascocerida, Pseudorthocerda as in Wade (1988) plus the Dissidocerida and Lituitida . The Lituitida, or Lituitiae, has been shown to have evolved from within the Tarphycerida by a straightening of the adult shell, a common character of many tarphycerids. The Dissidocerida, separated from the Orthocerida by Zhuravleva (1964), includes the Troedssonellidae and two small monogeneric orders,
108-532: The Polymeridae and Rangeroceratidae, both established by Evans (2005). Dissidocerida See text Dissidocerida is an order of Early Ordovician to the Early Silurian orthoceratoid cephalopods in which the siphuncle has a continuous lining or a longitudinal rod-like structure within. The order Dissidocerida was proposed by Zhuravleva (1994) for Dissidoceras undosum Zhuravleva, 1964,
120-720: The Pseudorthoceratidae sensu Sweet (1964). The Carbactinoceratidae are removed from the Actinocerida in Kroger and Mapes (2007). Pseudorthocerids were previously known as the Pseudorthocerataceae, a superfamily within the Orthocerida (Sweet 1964) which included the Silurian and later Pseudorthoceratidae and Middle Ordovician to Middle Silurian Proteoceratidae. Both have siphuncles that are partly cyrochoanitic with expanded segments and internal deposits but differ in
132-422: The later Pseudorthoceratidae, begin orthochoanitic with subcylindrical segments and ontogenetically become cyrtochoanitic with expanded segments. As such they matured into "pseudorthoceratoids" from "orthoceratoid" juveniles. From this Flower (1976) concluded that the Ordovician and Silurian pseudorthocerids (sensu Sweet 1964) were derived from different orthocerids (Orthocerida) and were therefore polyphyletic . As
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#1732793347813144-445: The manner in which they develop. Flower (1976) distinguished Middle Ordovician pseudorthocerids from Silurian and later pseudorthocerids on the basis of how the siphuncle changes during the life of the animal. The earlier Proteoceratidae begin cyrtochoanitic with expanded segments and end up orthochoanitic with subcylindrical segments. As juveniles they are "pseudorthoceratoid", as mature individuals they are "orthoceratoid". In contrast
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