Dissidocerida
Encyclopedia
The Dissidocerida comprise an order of Early Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

 to the Early Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

 orthoceratoid cephalopods in which the siphuncle
Siphuncle
The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and Spirula...

 has a continuous lining or a longitudinal rod-like structure within.

The order Dissidocerida was proposed by Zhuravleva (1994) for Dissidoceras undosum Zhuravleva, 1964, a Silurian orthoconic genus with a wide, nearly central, tubular siphuncle
Siphuncle
The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and Spirula...

, 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 laninar deposits constitute a single phylogenetic stock.

The Dissidocerida as understood includes the Polymeridae, Rangeroceratidae, Troedssonellidae, and Dissidocertidae. The Polymeridae and Rangeroceratidae were proposed by Evans in 2005 for Polymeres Murchison 1839 and Rangeroceras Hook and Flower 1977, respectively. Troedssonellidae
Troedssonellidae
Troedssonellidae is a family of orthoceroid cephalopods from the Ordovician, derived from rod-bearing Baltoceratidae, that have a continuous lining within the siphuncle that resembles very thin and slender endocones. Shells are generally slender and orthoconic. The siphuncle is central or...

was established by Kobayashi in 1935 and is based on the genus Troedssonella. The Dissidoceratidae was established by Zhuravleva for Dissidoceras undosum

References

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