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Orthocerida
Orthocerida is an order of extinct nautiloid cephalopods also known as the Michelinocerda that lived from the Early Ordovician () possibly to the Late Triassic (). A fossil found in the Caucasus suggests they may even have survived until the Early Cretaceous (). They were most common however from the Ordovician to the Devonian.
== Shell form ==
The shell is usually long, and may be straight ("orthoconic") or gently curved. In life, these animals may have been similar to the modern squid, except for the long shell. The internal structure of the shell consists of concavo-convex chambers linked by a centrally-placed tube called a siphuncle. There is a tendency for the chambers to develop cameral deposits, which were used as ballast to balance the long gas-filled shell. Depending on the family, the siphuncle has orthochoanitic or cyrtochoanitic septal necks, which protrude from the septa. The shell surface may be (depending on the species or genus) smooth, transversely ribbed, or ornamented by a network of fine lirae. Fossils are common and have been found on many continents, including the Americas, Africa, Europe, and Asia.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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