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Goniatids, informally Goniatites, are ammonoid cephalopods that form the Order Goniatiida, derived from the more primitive Anarcestida during the Middle Devonian some 390 million years ago. Goniatites (goniatitida) survived the Late Devonian extinction to flourish during the Carboniferous and Permian only to become extinct at the end of the Permian some 139 million years later. == Morphology == All goniatites possessed an external shell, which is divided internally into chambers filled with gas giving it Buoyancy during the life of the animal. An open chamber at the front of the shell provided living space for the goniatitid animal, with access to open water through an aperture. The general morphology and habit of goniatites was probably similar to that of their later relatives the ammonites, being free swimming and possessing a head with two well developed eyes and arms (or tentacles). Goniatite shells are small to medium in size, almost always less than in diameter and often smaller than in diameter. The shell is always planispirally coiled, unlike those of Mesozoic ammonites in which some are trochoidal and even aberrant (called heteromorphs). Goniatitid shells vary in form from thinly discoidal to broadly globular and may be smooth or distinctly ornamented. Their shape suggests many were poor swimmers The thin walls between the internal chambers of the shell are called the ''septa'', and as the goniatite grew it would move its body forward in the shell secreting septa behind it, thereby adding new chambers to the shell. The ''sutures'' (or suture lines) are visible as a series of narrow, wavy lines on the surface of the shell. The sutures appear where each septa contacts the wall of the outer shell. The typical goniatitid has a suture with smooth saddles and lobes, which gives the name "goniatitic" to this particular suture pattern. In some the sutures has a distinctive "zigzag" pattern Not all goniatitid ammonoides have goniatitic sutures. In some the sutures are ceratitic, in others, even ammonitic. Nor are goniatitic sutures limited to the Goniatidia. The sutures of nautiloids are by comparison somewhat simpler, being either straight or slightly curved, whereas later ammonoids showed suture patterns of increasing complexity. One explanation for this increasing extravagancy in suture pattern is that it leads to a higher strength of the shell. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「goniatite」の詳細全文を読む スポンサード リンク
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