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.
14-585: The Ascocerida are comparatively small, bizarre Orthoceratoids known only from Ordovician and Silurian sediments in Europe and North America, uniquely characterized by a deciduous conch consisting of a longer juvenile portion and an inflated short adult portion that separate sometime in maturity. The juvenile portion of an ascocerid consists typically of a narrow, circular cyrtocone that underwent periodic truncation. Cumulative length, including broken off segments, may have reached about 20 cm (8 in) in
28-552: Is a noteworthy paraphyletic order which 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,
42-436: Is an extinct order of Ordovician nautiloid cephalopods . They were the earliest-diverging members of the major group Orthoceratoidea , with which they share dorsomyarian muscle scars (a few large muscle scars concentrated at the top of the body chamber ). However, they also possess several plesiomorphic ("primitive") traits which are absent in other orthoceratoids: Cameral and endosiphuncular deposits are absent, and
56-496: Is established for the derived Upper Silurian Choanoceras , characterized by septa that form deep symmetrical cones and a subcentral expanded siphuncle with cyrtochoanitic and recumbent septal necks at maturity. The middle and possible upper Ordovician ancestral forms are placed in the Hebetoceratidae , which includes the orthoconic Hebetoceras and cyrtoconic Montyoceras . The fully mature breviconic ascocerid, with
70-453: Is formed by the septum of truncation which is about three times as thick as normal internal septa and about as thick as the external shell itself. The septa become confined to the dorsal side of the shell resulting in a series of dorsal chambers, or camerae, that provided stabilizing buoyancy. The ascocerid shell, or conch, is the thinnest and most fragile of any orthocone or cyrtocone of comparable size, which accounts for their rarity. Often it
84-413: Is only the mature, ascoceroid portion that is found. The Ascocerida are derived from the michelinocerid family Clinocertidae in the early Middle Ordovician, possibly from Clinoceras through such slender forms as Montyoceras and Hebetoceras . The connection between these slender deciduous forms and the typically ascoceroid Probillingsites from the early Upper Ordovician can be made through
98-799: 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, the Polymeridae and Rangeroceratidae, both established by Evans (2005). Riocerida Riocerida , originally named as Rioceratida ,
112-469: 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 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
126-667: 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 the Orthocerida, Ascocerida, Pseudorthocerda as in Wade (1988) plus
140-404: The juvenile longiconic portion discarded was no doubt a facile swimmer. The dorsal chambers would have provided a stable center of buoyancy directly above the center of gravity with the shell in a horizontal orientation. The hyponomic sinus, observed in some, indicates active, directionally controlled hydro-jet propulsion. Just how maneuverable these creatures were is another matter as is how high in
154-496: The largest individuals. The siphuncle is located halfway between the shell axis and the venter, is thin walled and tubular with short, orthochoanitic septal necks and segments that are only slightly inflated. The juvenile portion is known as the deciduous conch, as it is shed in a similar manner to deciduous trees losing their leaves. The adult, or mature ascoceroid conch, as it is referred to, typically consists of an expanded exogastric brevicone with unique features. The apical end
SECTION 10
#1732801619343168-576: The slightly inflated Redpathoceras . A phylogenetic sequence can be seen in the Ascoceratidae , from Probillingsites through Schucertoceras , then Billingsites , in the Upper Ordovician, and finally to the middle and upper Silurian Ascoceras . Changes involve the arrangement and complexity of septa in the mature conch and in its shape, ending with the more elongate and compressed mature Ascoceras . The Choanoceratidae
182-410: 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 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
196-507: The water column they spent their time. How juvenile forms spent their time is less obvious. Juvenile shells are found in the same location as adult, indicating they lived in the same area. Juvenile ascocerids were probably more benthic than their adult counterparts, perhaps spending their time at or on the sea floor. Orthoceratoidea Currently, Orthoceratoidea comprises the orders Riocerida , Dissidocerida , Actinocerida , Pseudorthocerida , Lituitida and Orthocerida . Orthocerida
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