箭石类
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| 箭石下纲 Belemnoidea 化石时期： | ||||||||||
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 Well preserved Phragmoteuthis conocauda, showing arm hooks and outline of mantle
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化石
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箭石类动物（学名：Belemnoidea），旧作箭石下纲，是一种已经灭绝头足纲生物的分支，生活在泥盆纪至白垩纪之间。箭石在许多方面都与现代的鱿鱼相当接近[1]，而且与现代的乌贼关系密切。箭石也拥有墨囊[2]，不过它们有10条大约相同长度的触腕，其中并没有特别长的触腕[3]。
箭石的学名"belemnoid"是来自于希腊文βέλεμνον（belemnon，意指“飞镖或箭头”），以及另一希腊字είδος（eidos，意指“形式”），即“箭头形状”的动物。[4]
箭石类动物包括了箭石目、闭箭石目、沟箭石目和Diplobelida。
演化历史
箭石类在侏罗纪与白垩纪时数量相当多，所以它们的化石在中生代的海洋岩层中相当丰富，经常伴随著菊石类出现。不过箭石也跟菊石一起在白垩纪末期灭绝了。箭石是由杆石于泥盆纪时期演化而来，从密西西比纪（或早石炭世）至白垩纪末之间的地层中可以发现良好的箭石化石，其他头足类如杆菊石、鹦鹉螺类及棱菊石类等也可发现。
解剖学
Belemnoids possessed a central phragmocone made of aragonite and with negative buoyancy.[5] To the rear of the creature was a heavy calcite guard whose main role appears to have been to counterbalance the front (towards the head) of the organism; it positions the centre of mass below the centre of buoyancy, increasing the stability of the swimming organism.[5] The guard would account for between a third and a fifth of the length of the complete organism, arms included.[5]
Like some modern squid, belemnoid arms carried a series of small hooks for grabbing prey. Belemnoids were efficient carnivores that caught small fish and other marine animals with their arms and ate them with their beak-like jaws. In turn, belemnites appear to have formed part of the diet of marine reptiles such as Ichthyosaurs, whose fossilized stomachs frequently contain phosphatic hooks from the arms of cephalopods.
生态学
Belemnoids were effectively neutrally buoyant, and swam in near-shore to mid-shelf oceans.[5] Their fins could be used to their advantage in all water speeds; in a gentle current they could be flapped for propulsion; in a stronger current they could be held erect to generate lift; and when swimming rapidly by jet propulsion they could be tucked in to the body for streamlining.[5]
化石保存
箭石形成化石的部份大多是其结实的“护甲”，通常呈拉长的子弹形(though in some subgroups the rostrum may only exist as a thin layer coating the phragmocone). The hollow region at the front of the guard is termed the alveolus, and this houses a chambered conical-shaped part of the shell (called the phragmocone). The phragmocone is usually only found with the better preserved specimens. Projecting forwards from one side of the phragmocone is the thin pro-ostracum.
While belemnoid phragmocones are homologous with the shells of other cephalopods and are similarly composed of aragonite, belemnoid guards are evolutionarily novel and are composed of calcite or aragonite, thus tending to preserve well. Broken guards show a structure of radiating calcite fibers and may also display concentric growth rings.
The guard, phragmocone and pro-ostracum were all internal to the living creature, forming a skeleton which was enclosed entirely by soft muscular tissue. The original living creature would have been larger than the fossilized shell, with a long streamlined body and prominent eyes. The guard would have been in place toward the rear of the creature, with the phragmocone behind the head and the pointed end of the guard facing backward.
The guard of the belemnoid Megateuthis gigantea, which is found in Europe and Asia, can measure up to 46厘米（18英寸） in length, giving the living animal an estimated length of 3米（9.8英尺）.
Very exceptional belemnoid specimens have been found showing the preserved soft parts of the animal. Elsewhere in the fossil record, bullet-shaped belemnite guards are locally found in such profusion that such deposits are referred to semi-formally as "belemnite battlefields" (cf. "orthocone orgies"). It remains unclear whether these deposits represent post-mating mass death events, as are common among modern cephalopods and other semelparous creatures.
雷石
在数百年前的欧洲，尚未有古生物学概念的时代，当时的英国人认为箭石的化石是雷打到地面而形成的，所以称其为“雷石”，成为箭石化石的别称。[6]
用途
The stable isotope composition of a belemnoid rostrum from the Peedee Formation (Cretaceous, southeast USA) has long been used as a global standard (Peedee Belemnite, "PDB") against which other isotope geochemistry samples are measured, for both carbon isotopes and oxygen isotopes.
Some belemnoids (such as Belemnites of Belemnitida) serve as index fossils, particularly in the Cretaceous Chalk Formation of Europe, enabling geologists to date the age the rocks in which they are found.
分类
- 箭石类动物 Cohort Belemnoidea
- 基群及地位未定
- 沟箭石目 Aulacocerida
- 沟箭石科 Aulacoceratidae
- 科 Dictyoconitidae
- 科 Hematitidae
- 科 Palaeobelemnopseidae
- 科 Xiphoteuthidae
- 箭石目 Belemnitida
- 箭石亚目 Belemnitina
- 似箭石亚目 Belemnopseina
- 拟箭石科 Belemnitellidae
- 似箭石科 Belemnopseidae
- 科 Dicoelitidae
- 科 Dimitobelidae
- 科 Duvaliidae
- 箭乌贼亚目 Belemnotheutina
- 箭乌贼科 Belemnotheutidae
- 科 Chitinobelidae
- 科 Sueviteuthidae
- 目 Diplobelida
- 闭箭石目 Phragmoteuthida
- 闭箭石科 Phragmoteuthidae
- 科 Rhiphaeoteuthidae
参见
参考文献
- ^ Thomas E. Yancey, Christopher L. Garvie, Mary Wicksten. The Middle Eocene Belosaepia ungula (Cephalopoda: Coleoida) from Texas: structure, ontogeny and function. Journal of Paleontology. 2010/03, 84 (2): 267–287 [2018-04-02]. ISSN 0022-3360. doi:10.1666/09-018r.1 （英语）.
- ^ Lehmann, U. 1981. The Ammonites: Their life and their world. London: Cambridge University Press.
- ^ Peter Doyle, Emma V. Shakides. The Jurassic Belemnite Suborder Belemnotheutina. Palaeontology. 2004-07-01, 47 (4): 983–998 [2018-04-02]. ISSN 1475-4983. doi:10.1111/j.0031-0239.2004.00395.x （英语）.
- ^ Webster's New Universal Unabridged Dictionary. 2nd ed. 1979.
- ^ 5.0 5.1 5.2 5.3 5.4 Neale Monks, J. David Hardwick, Andrew S. Gale. The Function of the Belemnite Guard. Paläontologische Zeitschrift. 1996-11-01, 70 (3-4): 425 [2018-04-02]. ISSN 0031-0220. doi:10.1007/bf02988082 （英语）.
- ^ Vendetti, Jan. The Cephalopoda: Squids, octopuses, nautilus, and ammonites. UC Berkeley. 2006 [2013-06-07].
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