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ellesmerocerida
The Ellesmerocerida is an order of primitive cephalopods belonging to the subclass Nautiloidea with a widespread distribution that lived during the Late Cambrian and Ordovician.〔(The Paleobiology Database Ellesmerocerida entry )〕
==Morphology==
The Ellesmerocerida are characterized by shells that are typically small, some even tiny, with close-spaced septa and relatively large ventral siphuncles. In some genera (e.g. ''Palaeoceras''), the septa are uniformly spaced. Shells of ellesmerocerids are typically smooth and compressed and vary in form. They may be breviconic (short) or longiconic (elongate), straight (orthoconic) or curved (cyrtoconic). Cyrtoconic forms are usually endogastric, with longitudinally convex ventral margins. The apeces of straight forms typically have an endogastric curvature. Some may have grown to as much as 15 cm.
Siphuncle segments are tubular or concave. Septal necks are short. connecting rings which may appear layered are thick and typically wedge shaped with their maximum width at or near where they join the previous septum. The siphuncle interior is commonly crossed by irregular partitions, known as diaphragms, but are otherwise free of internal deposits〔〔
As soft parts are not prone to fossilization, little can be surmised as to their soft part anatomy. Preserved muscle attachment scars indicate that they may have had segmented muscles reminiscent of primitive monoplacophoran molluscs. As for arms or tentacles, little can be said except that eight or ten, retained in modern coleoids, seems to be the primitive or ancestral number.
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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