Silurian period

Japanese: シルル紀 - しるるき（英語表記）Silurian period

The third oldest geological period of the Paleozoic Era, it corresponds to the 24.2 million years between the Ordovician and Devonian periods, from about 443.4 million years ago to about 419.2 million years ago. The strata formed during the Silurian Period are called the Silurian System. The name Silurian Period originated in 1835, when British geologist Murchison, in his research on the Welsh Mountains, named the strata that were rich in marine fossils below the Old Red Sandstone layer after the ancient people who lived in the area, the Silurians. The Silurian Period has also been used in a broad sense to include the present Ordovician Period, and has also been called the Gotlandian Period. It was the heyday of Paleozoic invertebrates, and in particular, uniaxial graptolites reached their peak, with many species differentiating, and detailed zonation of strata has been carried out in Wales and Bohemia using graptolites. The brachiopods, which are tentacle animals, and the corals, which are cnidarians, also flourished. Among the corals, the families of tabular corals, such as the family Polyporididae and Polyporididae, formed large colonies and, together with stromatoporoids, formed reefs. In addition, the arthropods trilobites and euptera (sea scorpions), and the echinoderms crinoids and sea apples are abundantly known. Of the vertebrates, only primitive fishes are known, including the jawless cephalopods, which are closely related to the modern cephalopods (including cyclostomes and lampreys). At the end of this period, the placoderms, which possessed primitive jaw organs, appeared. As for plants, the cephalopods, which are considered to be the oldest vascular plants, were discovered at the end of this period.

The strata of this period are distributed in the UK, Northern Europe, eastern North America, and are known to be distributed in Iwate, Gifu, Kochi, and Miyazaki prefectures in Japan, and produce chain corals and trilobites. The late Silurian period was the Caledonian orogeny, and there was active crustal movement.

[Tomo Ozawa and Kozo Watanabe, August 19, 2015]

"Atlas of the Evolution of Life and the Earth I: From the Origin of the Earth to the Silurian Period" by Richard T. J. Moody and Andrei Yu. Zhuravlev, supervised by Ikuo Obata (2003, Asakura Publishing)

Halysites gracilis (table coral)
Chain coral, Silurian period of the Paleozoic era, Sweden Photo/Fukui Prefectural Dinosaur Museum Image Library ">

Halysites gracilis (table coral)

Favosites (table coral)
Coral sp., Silurian period of the Paleozoic era, specimen width approx. 10cm, from Ono city, Fukui prefecture, photo provided by National Institute of Advanced Industrial Science and Technology, Geological Survey of Japan (GSJ F3511)

Favosites (table coral)

Pterygotus macrophthalmus (sea scorpion)
Silurian Period of the Paleozoic Era: North American Photographs / Fukui Prefectural Dinosaur Museum Image Library ">

Pterygotus macrophthalmus (sea urchin)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

古生代の古いほうから第3番目の地質時代で、オルドビス紀とデボン紀の間の約4億4340万年前から約4億1920万年前までの約2420万年間に相当する。シルル紀に形成された地層をシルル系という。シルル紀の名称の由来は、1835年にイギリスの地質学者マーチソンが、イギリスのウェールズ山地の研究で、旧赤色砂岩層の下位にある海産化石を多産する地層に対して、この地方に住んでいた古代民族シルルの名にちなんで命名したのに始まる。シルル紀は、現在のオルドビス紀を加えた広義に用いられたり、ゴトランド紀の名でよばれたこともあった。古生代型無脊椎(せきつい)動物の全盛期で、とくに、単軸型筆石類(ふでいしるい)は全盛を極め、多くの種が分化し、ウェールズやボヘミアなどで筆石類を用いて地層の詳しい分帯が行われている。触手動物の腕足類や刺胞動物のサンゴ類も繁栄を極める。サンゴ類では、床板サンゴ類に属するクサリサンゴ科、ハチノスサンゴ科などのものが大きな群体を形成し、層孔虫類などとともに礁(しょう)を形成した。このほか、節足動物の三葉虫類および広翼類（ウミサソリの仲間）、棘皮(きょくひ)動物のウミユリ類・ウミリンゴ類などが豊富に知られている。脊椎動物では原始的な魚類のみが知られ、現生の頭甲類（円口類、ヤツメウナギを含む）に近縁な無顎(むがく)類があげられる。この時代の終わりには、原始的なあごの器官を有する板皮(ばんぴ)類が出現した。植物では、最古の維管束植物とされるマツバラン類が当紀末に知られている。

　この時代の地層は、イギリスをはじめ、北ヨーロッパ、北アメリカ東部などに分布し、日本でも岩手県、岐阜県、高知県、宮崎県などに分布が知られており、クサリサンゴ類や三葉虫類を産する。シルル紀の後期はカレドニア造山運動期にあたり、活発な地殻変動があった。

［小澤智生・渡辺耕造　2015年8月19日］

『リチャード・T・J・ムーディ、アンドレイ・ユウ・ジュラヴリョフ著、小畠郁生監訳『生命と地球の進化アトラスⅠ　地球の起源からシルル紀』（2003・朝倉書店）』