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''Belemnotheutis'' , as officially spelled, is an extinct coleoid cephalopod genus from the middle and upper Jurassic, related to but morphologically distinct from belemnites. ''Belemnotheutis'' fossils are some of the best preserved among coleoids. Remains of soft tissue are well-documented in some specimens, even down to microscopic muscle tissue. In 2008, a group of paleontologists even recovered viable ink from ink sacs found in several specimens. This genus was the subject of a dispute between several eminent 19th century British paleontologists, notably between Richard Owen and Gideon Mantell. Some authors spell the genus ''Belemnoteuthis'' following the usual spelling ''teuthis'' (τευθίς) for 'squid'. ==Description== The genus ''Belemnotheutis'' is characterized by an internal shell consisting of a conical phragmocone covered apically by a thin rostrum, or guard, homologous to the bullet-shaped rostrum of true belemnites, a short forward projecting proostracum, and ten hook bearing arms of equal length. ''Belemnotheutis'' fossils are sometimes found in remarkable states of preservation, some specimens retaining permineralized soft tissue. The mantle, fins, head, arms, and hooks are well-documented from remains preserved in ''Lagerstätten''. One specimen recovered from Christian Malford, Wiltshire and currently displayed in the Paleontology Department of the Natural History Museum in London is fossilized clasping a fish. ''Belemnotheutis'' is not a 'true' belemnite (suborder Belemnitina) but a closely related coleoid.〔 Both belemnotheutids and belemnites resembled modern squids except that they had chambered internal skeletons called phragmocones. The apical portion of the ''Belemnotheutis'' internal skeleton is called the rostrum (plural: rostra) or the guard. The rostrum of ''Belemnotheutis'' differs significantly from that of true belemnites. Unlike the bullet-shaped dense guards of belemnites, the rostrum of ''Belemnotheutis'' is only present as a very thin sheath. It was also composed of aragonite rather than the heavy calcite of belemnites.〔 In large specimens the rostrum can reach a maximum of only in thickness near the tip. The outer surface was covered by a thin organic layer in the live animal.〔 In true belemnites, the large dense rostra acted as a counterbalance, keeping the animal horizontally oriented when swimming.〔 It was long assumed that ''Belemnotheutis'' were confined to shallow waters, unable to venture into deeper waters due to the absence of the heavy rostra. The discovery of cameral desposits in the phragmocones of ''Belemnotheutis'' in 1952 made it clear that they were capable of controlling buoyancy.〔 The phragmocone of ''Belemnotheutis'' is short and blunt, measuring around to in length.〔 The outer wall of the phragmocone is called the conotheca, distinct from the rostrum. It begins approximately from the tip of the phragmocone and consists of a nacreous outer layer and an inner lamellar layer. The outer layer gradually thins from in thickness to only about thick at about further down the shell until it eventually disappears around the opening of the phragmocone (the peristome). Sometimes there is a hollow gap between the rostrum and the lamellar layer of the conotheca, indicating either organic content that have since disappeared or disintegration of the lamellar layer itself. The phragmocone of ''Belemnotheutis'' had about 50 chambers that were originally aragonitic, though they are usually replaced by calcium phosphate during the process of fossilization.〔 Donovan D.T & Crane M.D. 1992. The type material of the Jurassic cephalopod ''Belemnotheutis'', Palaeontology vol35, issue 2| pp273–296 ()〕 At the very tip of the phragmocone beneath the rostrum is an embryonic shell known as the protoconch. In ''Belemnotheutis'', like in other belemnotheutids, the protoconch is roughly cup-shaped and sealed.〔〔 This was thought to be another method of distinguishing it from other belemnites which usually have ball-shaped protoconchs.〔Reitner, J. & Engeser,T., 1982. Phylogenetic trends in phragmocone-bearing coleoids (Belemnomorpha); Konstruktions-Morphologie, pp157–158, E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart. ()〕 However, it is probably a taphonomic artefact, with the protoconch being spherical like other belemnites.〔 The long, weakly tapering structure in the dorsal anterior part of the internal skeleton is called the proostracum. It is striated longitudinally and often shows minute holes left by boring organisms usually less than 1 μm in diameter.〔 The length of the proostracum is one to two times the length of the phragmocone. The proostracum was a thin, delicate structure substantially narrower than the phragmocone.〔 Its original composition is unknown and it is always found separated from the phragmocone and the rostrum and often crushed.〔〔 Whether the proostracum was connected to or derived from the phragmocone is still a subject of debate among paleontologists.〔 Its general morphology, however, resembles that of true belemnites rather than those from other 'unusual' belemnoid coeloids with short rostra like ''Permoteuthis'' and ''Phragmoteuthis''.〔Jeletzky, J.A. 1966. Comparative Morphology, Phylogeny, and Classification of Fossil Coleoidea; Mollusca pp 1–162; The University of Kansas, Paleontological Contributions ()〕 The head is not well preserved in known specimens. It comprised approximately 20% of the body length (excluding the arms). Brain cartilage is observed in some specimens, as well as a pair of aragonitic statoliths which helped the animal determine horizontal orientation when swimming.〔 ''Belemnotheutis'', like most of the other belemnoids, had ten arms of equal length lined with two rows of inward-pointing hooks each.〔 Each of the hooks were composed of several sections. The curved pointed tip is called the uncinus and was probably the only part of the hooks exposed. The rest of the hook (the shaft and the base) were embedded in a sheath of soft tissue below the orbicular scar, a small groove where the tissue attachment terminated. They are also believed to have been stalked and mobile, helping the animal manipulate its prey.〔 Traces of functional suckers have been found and these constitute a single row on each arm, alternating in between the pairs of hooks. The size of the suckers decreases distally along the arms, with the largest (around in diameter) being closest to the head.〔 The length of the arms varies with the size of the individual but may have reached in larger specimens.〔 ''Belemnotheutis'' had a cylindriconical muscular mantle covered by an outer and inner skin (tunic).〔〔 Traces of a criss-cross pattern composed of collagen fibers have been observed in the surface of some well-preserved fossils. The cross section of the exceptionally preserved body wall of a specimen from the Oxford Clay formations also reveals alternating bands of concentrically and radially oriented body fibers. They imply that ''Belemnotheutis'' were powerful swimmers and agile predators, similar to modern shallow-water squid species.〔 Philip R. Wilby et al 2008. Preserving the unpreservable: a lost world rediscovered at Christian Malford, UK. Geology Today Vol 24(3). Blackwell Publishing Ltd. ()〕 The animal reached in length, including its arms. The body diameter was around 12 to 14% of the mantle length.〔〔Jack Sepkoski 2002. A compendium of fossil marine animal genera (Cephalopoda entry). Bulletins of American Paleontology, 364 p.560 () 〕 At the center of the dorsal surface of the rostrum is a narrow V-shaped groove running about 3/5ths the length of the phragmocone from the apex, with two rounded ridges at its left and right sides. These grooves are one of the most distinctive features of the Belemnotheutidae and are theorized to have served as attachments to terminal oval or oar-shaped fins like in some modern squids.〔 The siphuncle is marginal and located ventrally.〔 Directly in front of the phragmocone was an ink sac that could reach long in large specimens. Intestinal casts (cololites) as well as the orientations and positions of fossilized remains reveal that the animal preyed on fish and other coleoids in life. Their great abundance in certain formations indicate that ''Belemnotheutis'' were highly gregarious animals, congregating in large monospecific or polyspecific shoals.〔 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「Belemnotheutis」の詳細全文を読む スポンサード リンク
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