Dissidocerida

:<U>Systematic Paleontology</u> <br/>
: Class: [[Cephalopoda]]
: Subclass: [[Nautiloidea]]
: Superorder: [[Orthoceratoidea]]
: Order: '''Dissidocerida'''

'''Dissidocerida''' is an order of orthoceratoid cephalopods proposed by Zhuravleva (1994) for the monotypic Silurian genus ''Dissidoceras'' Zhuravleva, 1994, a longiconic orthocone with a wide, nearly central, tubular siphuncle, suborthochoanitic septal necks, and a rod-like endosiphuncular deposit, such as found in certain Late Ordovician genera. 

The Dissidocerida, as understood, contains the Polymeridae, Rangeroceratidae, Troedssonellidae, and Dissidocertidae.  Polymeridae and  Rangeroceratidae were proposed by Evans in 2005, respectively for ''Polymeres'' Murchison 1839 (type) and ''Rangeroceras'' Hook and Flower 1977.  Troedssonellidae was established by Kobayashi  in 1935 and the Dissidoceratidae by Zhuravleva for ''Dissidoceras undosum'' by Zhuravleva (1994)

The diagnostic description of the Troedsonnellidae differs from that indicated for the Dissidoceratidae. Troedssonnellids are characterized by a continuous endosiphuncular lining that forms long narrow endocones within the siphuncle, which may be derived from the ventral endosiphuncular rods of rod-bearing baltoceratids through a genus such as ''Tajarocers''. Dissidocerids are described as having intrasiphonal deposits in the shape of a longitudinal stalk or rod, more like that of true rod-bearing Baltoceratidae.  True rods have a convex upper surface and lie typically along the ventral interior of the siphuncle. In general form they resemble the ventral processes found in the endocerid ''Cacheoceras'', and ''Williamsoceras''. The siphuncle lining in ''Buttsoceras'', Troedsonnellidae, instead is concave circumferentially, although thicker ventrally.  


==References==

Bjorn Kroger, 2008. A new genus of middle Tremadocian orthoceratoids and the Early Ordovician origin of orthoceratoid  cephalopods. Brief Report Acta Palaeontologica Polonica 53 (4): 745–749

A. A. Shevyrev 2004, The Cephalopod Macrosystem: A Historical Review, the Present State of Knowledge, and Unsolved Problems: 2. Classification of Nautiloid Cephalopods

'''Credits'''

[[User:John M|John M]] Created with discussion 8 Oct 2014