Ammonites - Ammonites

Japanese: アンモナイト - あんもないと（英語表記）ammonites

A general term for a group of extinct animals belonging to the class Cephalopoda of the phylum Mollusca, also known as ammonites, chrysanthemum shells, or chrysanthemum-faced stones. Academically, the correct name is Ammonoidea. The name comes from the ancient Egyptian sun god Amon (Greek), who had a ram's head, but it is likely because the spiral shell of ammonites resembled a ram's horns. Modern cephalopods are broadly classified into squids and octopuses (subclass Coleoidea), which have one pair of gills and are either internally shelled or shellless, and nautiloids (subclass Nautiloidea), which have two pairs of gills.

Ammonoids have a multi-chambered shell separated by numerous septa, similar to nautiloids (from the late Cambrian to the present day). However, their developmental and comparative anatomical characteristics and evolutionary records clearly distinguish them from nautiloids, and they are more closely related to coleopoda. They were once considered to be an independent group in the subclass Ammonoidea, but are now widely considered to be included in the order Ammonoidea, along with the order Coleopoda, in the subclass Neocephalopoda. Ammonoids are thought to have evolved from Bactrites, which have a cone-shaped shell, in the Early Devonian (about 420 million years ago). Since then, they have prospered in the oceans of the world, despite suffering three mass extinction events at the end of the Devonian, the end of the Permian, and the end of the Triassic, but they became completely extinct along with land dinosaurs at the end of the Cretaceous (about 65.5 million years ago). The most likely cause of their extinction is thought to be environmental changes, such as global cooling, caused by a huge asteroid colliding with the Earth.

[Tanabe Kazunari]

Individual structure

Although most of the soft body remains unknown, there is fossil evidence showing the presence of feeding organs such as jaw apparatus and radula, as well as the digestive tract. The radula, like that of modern coleomorphs, has seven teeth in a horizontal row, which is distinct from the nine teeth of the radula of nautilus. The shell consists of a chamber separated by numerous partitions, followed by a living chamber (body chamber) that is thought to have housed the soft body. The spiral tube that composes the shell begins as a hollow initial chamber that is spherical or ellipsoidal, and generally grows bilaterally symmetrically by coiling in the same plane. A chitinous tube (body tube) extends from the rear of the initial chamber through the partitions of the chamber and opens at the rear of the living chamber, and evidence from well-preserved fossils from America has revealed that it contained the soft body, consisting of arteries, veins, connective tissue, and epithelial tissue. The tips of the inter-chambered tubules swell like a balloon before the initial chamber and converge, from which a spatula-shaped or tubular prototube extends and attaches to the inner surface of the initial chamber. The spiral tube about one turn from the initial chamber has a constriction that is thought to have been formed during hatching, and the shell structure changes from this point onwards. The shell from the initial chamber to the constriction is called ammonitella, and corresponds to the embryonic shell formed inside the egg. Ammonites grew by moving the soft parts forward and forming the body tube, septa made of aragonite CaCO ₃ , and an exoskeleton. The surface of the spiral tube is often decorated with ribs, warts, thorns, constrictions, keels, etc. The septa of the chamber are folded from the center to the periphery in a much more complex way than those of nautiluses, and the suture line of the intersection with the exoskeleton draws a self-similar fractal curve. Sutures become more complex as an individual grows, but they remain consistent in shape depending on the species, making them an important classification trait.

All ammonoids were marine, carnivorous or carnivorous, and moved by ejecting seawater from the funnel, which was part of the mantle. Based on the similarity of the internal structure of the shell, it is thought that, like modern nautiluses, the air chamber contained low-density gas and a small amount of water, which maintained the buoyancy and stability of the body and led a free-swimming lifestyle. However, heteromorphs such as Nipponites and Hamites , which flourished in the Cretaceous Period of the Mesozoic Era, may have had a lifestyle close to that of the benthic.

[Tanabe Kazunari]

kinds

Ammonoids are classified into 14 suborders: Bactrites, Eigoniatites, Anarcestes, Geffroceras, Clymenia, Gonioclymenia, Tornoceras, Goniatites, Prolecanites (all from the Paleozoic Era), Ceratites (Permian Period to Triassic Period of the Mesozoic Era), Phylloceras (Triassic Period to Cretaceous Period of the Mesozoic Era), Lithoceras, Ammonites, and Ankyloceras (Jurassic Period to Cretaceous Period of the Mesozoic Era). When comparing at the same growth stage, the indentations of the sutures become more complex with age.

[Tanabe Kazunari]

fossil

Ammonite fossils are found in abundance in marine strata from the Paleozoic and Mesozoic eras around the world, and because their morphology changes dramatically over time, they are a prime example of a standard stone that is effective for determining the age of and comparing strata. They are also widely used in research into biological evolution. In Japan, they are found in strata from the Late Paleozoic to the Cretaceous period in the Kitakami region, Cretaceous strata in Hokkaido and Shikoku, and Jurassic strata in Yamaguchi and Fukui prefectures. Fossils in the Cretaceous strata in Hokkaido in particular are well preserved compared to the rest of the world, and there have been many research examples.

[Tanabe Kazunari]

"Japan Fossil Collection: Japanese Ammonites," 9 volumes compiled by the Japan Fossil Collection Editorial Committee (1984-1986, Tsukiji Shokan)" ▽ "Cretaceous Natural History, by Obata Ikuo (1993, University of Tokyo Press)" ▽ "Hokkaido Ammonite Museum, by Fukuoka Koichi (2000, Hokkaido Shimbun Press)" ▽ "Ammonite Studies: The Knowledge, Form, and Beauty of Extinct Creatures, compiled by the National Museum of Nature and Science and written by Shigeta Yasunari (2001, Tokai University Press)" ▽ "Ammonites: The Latest Illustrated Guide to Ammonite Fossils: Reviving Ancient Treasures, by Neil L. Larson, supervised translation by Tanabe Kazunari and translated by Sakai Masaru (2009, Ammolite Research Institute)"

Schematic diagram of ammonite's body structure
©Kanzo Otawa ">

Schematic diagram of ammonite's body structure

Gaudriceras denseplicatum (ammonite)
(Jimbo) Late Cretaceous period, Mesozoic era, diameter approx. 5.5cm, from Obira-cho, Rumoi-gun, Hokkaido, Japan . Photo by National Institute of Advanced Industrial Science and Technology, Geological Survey of Japan (GSJ F3242)

Gaudricellas denseplicatum (An…

Nipponites mirabilis (ammonite)
Yabe Late Cretaceous period of the Mesozoic era Specimen width: approx. 8cm Produced in Haboro, Tomamae-gun, Hokkaido Photo: National Institute of Advanced Industrial Science and Technology Geological Survey of Japan (GSJ F9094)

Nipponites mirabilis (Ammonite...

Bacrites Tanakae (Ammonite)
Matsumoto and Obata, Late Cretaceous period of the Mesozoic era, Length: approx. 13cm, Produced in Tomamae-cho, Tomamae-gun, Hokkaido, Photo: National Institute of Advanced Industrial Science and Technology, Geological Survey of Japan (GSJ F13924)

Bacrites Tanakae (Ammonite)

Ceratites semipartites (Ceratites)
von Montfort Mesozoic Triassic period Diameter: approx. 18 cm Produced in Württemberg, Germany Photo courtesy of the Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (GSJ F7616)

Ceratites semipartites (Ceratites semipartites)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

軟体動物門頭足綱に属する絶滅動物の一群の総称で、アンモン貝、菊石、菊面石ともよばれる。学術的にはアンモノイド類Ammonoideaが正しい名称である。その名は、古代エジプトの太陽神で、雄羊の頭をもつアモンΑμμος／Ammon（ギリシア語）に由来するが、アンモナイトの螺旋(らせん)状に巻いた殻が羊の角(つの)を連想させたからであろう。現生頭足類は1対のえらをもち、内殻ないし無殻性のイカ、タコの類（鞘形(しょうけい)亜綱Coleoidea）と、外殻性で2対のえらのオウムガイ亜綱Nautiloideaに大分類される。

　アンモノイド類はオウムガイ類（古生代カンブリア紀後期から現世）とよく似た多数の隔壁で仕切られた多室性の外殻を有する。しかし、発生学・比較解剖学的特徴や進化記録からはオウムガイ類と明らかに異なり、むしろ鞘形類と近縁である。かつてはアンモノイド亜綱Ammonoideaとして独立されていたが、現在ではアンモノイド目として鞘形目とともに新頭足亜綱Neocephalopodaに含める考えが有力である。アンモノイド類は、前期デボン紀（約4.2億年前）に直錐(ちょくすい)状の殻をもつバクトリテス類から分化したと考えられている。それ以降、デボン紀末期、ペルム紀末期、三畳紀末期の3回の大量絶滅事変を被りながら、世界中の海洋に大繁栄を遂げたが、白亜紀末期（約6550万年前）に陸上の恐竜類などと一緒に完全に絶滅した。その絶滅の要因については、巨大な小惑星の地球への衝突によって生じた寒冷化などの環境変動が有力視されている。

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個体の構造

軟体部の大部分は不明であるが、顎器(がっき)や歯舌などの摂食器官、消化管の存在を示す化石の証拠はある。歯舌は現生鞘形類と同様、横1列が7本の小歯からなり、9本の小歯からなるオウムガイ類の歯舌と区別される。殻体は、多数の隔壁に仕切られた部屋からなる気房部と、それに続く軟体部を収容したと考えられる住房（体房）部からなる。殻を構成する螺管(らかん)は、球状ないし楕円(だえん)球状をした中空の初期室に始まり、一般には左右相称で同一平面内に巻いて成長する。初期室の後方からキチン質の管（体管）が気房部の隔壁を貫いて延びて住房部後方に開口するが、その中には動脈・静脈・結合組織・上皮(じょうひ)組織からなる軟体部が入っていたことが、アメリカ産の保存のよい化石の証拠からわかっている。連室細管の先端は初期室の手前で風船状に膨らみながら収束し、そこからへら状または管状の原体管が延びて初期室の内面に付着する。初期室から約1巻目の螺管には孵化(ふか)の際にできたと考えられるくびれがあり、そこを境に殻の構造が変化する。初期室からくびれまでの殻をアンモニテラammonitellaとよび、卵の中で形成された胚殻に相当する。アンモナイトは軟部を前方に移動させ、体管やあられ石CaCO₃からなる隔壁や外殻をつくって成長した。螺管表面にはしばしば肋(ろく)、いぼ、とげ、くびれ、竜骨などの装飾を伴う。気房部の隔壁は中心から周辺に向かってオウムガイ類のものよりはるかに複雑に褶曲(しゅうきょく)し、外殻との交線の縫合(ほうごう)線suture lineは自己相似的なフラクタル曲線を描く。縫合線は個体の成長とともに複雑化するが、種類によって一定な形を示すので、重要な分類形質となる。

　アンモノイド類はすべて海生、肉食性ないし腐肉食性で、その運動は、外套膜(がいとうまく)の一部である漏斗(ろうと)funnelからの海水の噴射によった。殻の内部構造の類似から、現生オウムガイ類と同様、気房部には低密度のガスと少量の水が入っていて、それにより生体の浮力や安定性を維持しながら遊泳生活をしていたと考えられる。しかし、中生代白亜紀に繁栄を遂げたニッポニテスNipponitesやハミテスHamitesなどの異常巻の類では底生に近い生活様式をもっていた可能性がある。

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種類

アンモノイド類は、バクトリテス、エイゴニアタイト、アナルセステス、ゲフロセラス、クリメニア、ゴニオクリメニア、トルノセラス、ゴニアタイト、プロレカニテス（以上古生代）、セラタイト（古生代ペルム紀～中生代トリアス紀）、フィロセラス（中生代トリアス紀～白亜紀）、リトセラス、アンモナイト、アンキロセラス（以上中生代ジュラ紀～白亜紀）の14亜目に分類される。同一成長段階で比べると、縫合線の刻みは時代とともに複雑になる。

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化石

アンモナイトの化石は世界各地の古生代、中生代の海成層に多産し、形態の時代的変化が著しいので、地層の時代決定や対比に有効な標準化石の代表例である。また、生物進化の研究にも広く利用されている。日本では北上(きたかみ)地方の後期古生代から白亜紀までの地層、北海道や四国の白亜紀層、山口県や福井県のジュラ紀層などから産する。とくに北海道の白亜紀層中の化石は世界的にみて保存がよく、多くの研究例がある。

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『日本化石集編集委員会編『日本化石集　日本のアンモナイト』全9集（1984～1986・築地書館）』▽『小畠郁生著『白亜紀の自然史』（1993・東京大学出版会）』▽『福岡幸一著『北海道アンモナイト博物館』（2000・北海道新聞社）』▽『国立科学博物館編、重田康成著『アンモナイト学――絶滅生物の知・形・美』（2001・東海大学出版会）』▽『ニール・Ｌ・ラースン著、棚部一成監訳、坂井勝訳『アンモナイト――アンモナイト化石最新図鑑――蘇る太古からの秘宝』（2009・アンモライト研究所）』