Mesozoic era

The second geological era, counting from the present. It corresponds to about 186.17 million years between the Paleozoic and Cenozoic eras, from about 252.17 million to about 66 million years ago. It was named by British geologist J. Phillips in 1841. The strata and rocks formed during this period are called the Mesozoic. The Mesozoic is further divided into three periods, the Triassic, Jurassic, and Cretaceous, from the oldest to the oldest. The beginning and end of the Mesozoic are defined by major changes in the animal kingdom, which also coincide with the period when widespread terrestrialization occurred. At the end of the Paleozoic, many animal groups, such as trilobites, tetracorals, and fusulinids, became extinct, and brachiopods and crinoids also declined significantly. During the Mesozoic era, ammonites, belemnites (arrow stones), and giant reptiles (dinosaurs, etc.) made great strides, but they also became extinct at the end of the Mesozoic era, and were replaced by modern Cenozoic animals. The cause of the mass extinction at the boundary between the Cretaceous and the following Paleogene period (KT boundary. The first letter of the English Cretaceous period, C, overlaps with the first letter of the Cambrian and Carboniferous periods, so the name is a combination of the first letter K of Kreide, which means Cretaceous in German, and the first letter T of Tertiary, which means Tertiary in English. It is also called the KP boundary, combining the first letter P of the English Paleogene) is now believed to be the drastic change in the global Earth's surface environment caused by the impact of a large meteorite on the Yucatan Peninsula in Mexico. The great change in the plant kingdom occurred before that in the animal kingdom. The Mesozoic era was a time when gymnosperms flourished overall, but the latter half of the Cretaceous period was already the age of angiosperms. The Mesozoic era was a time when the supercontinent Pangaea, which had been formed at the end of the Paleozoic era, began to split up and move one after another. Europe and the areas on both sides of the Atlantic Ocean were relatively quiet, but in the Pacific Rim region, land masses carried by plates collided with and were added to the surrounding continental crust, resulting in violent crustal movement, igneous activity, and metamorphism. Many useful mineral deposits were formed during this period.

[Hayami Itaru and Ozawa Tomoo, August 19, 2015]

"Revised New Edition of Geological History, Volume 2" by Koichiro Ichikawa et al. (1967, Asakura Publishing) " "Geology of Japan" by Toshio Kimura, Itaru Hayami, and Shizuo Yoshida (1993, University of Tokyo Press) " "Atlas of the Evolution of Life and the Earth II, Devonian to Cretaceous, by Dougal Dixon, supervised translation by Ikuo Obata (2003, Asakura Publishing)"

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

地質時代区分で、現在から数えて二番目の代。古生代と新生代の間の約2億5217万年から約6600万年までの約1億8617万年に相当する。イギリスの地質学者J・フィリップスが1841年に命名した。この期間に形成された地層・岩石を中生界という。中生代（界）はさらに3分され、古いほうから三畳紀（系）、ジュラ紀（系）、白亜紀（系）となる。中生代の始まりと終わりは、動物界におこった大きな変革で定義されるが、これらは広範な陸化がおこった時期とも一致している。古生代末には三葉虫、四放サンゴ、紡錘虫など多くの動物群が絶滅し、腕足類、ウミユリなども大きく衰退した。中生代にはアンモナイト、ベレムナイト（矢石、箭石(やいし)）、巨大な爬虫類(はちゅうるい)（恐竜など）が大発展を遂げるが、これらも中生代末には絶滅して、現在に近い新生代の動物と交代する。この白亜紀と次の古第三紀の境界（K-T境界。英語の白亜紀の頭文字Cがカンブリア紀、石炭紀の頭文字Cと重なるため、ドイツ語で白亜紀を意味するKreideの頭文字Kと、英語で第三紀を意味するTertiaryの頭文字Tと組み合わせた呼称。古第三紀の英語Paleogeneの頭文字Pと組み合わせてK-P境界ともいう）の大量絶滅の原因については、現在ではメキシコのユカタン半島への大隕石(いんせき)の衝突による全球的地球表層環境の激変に帰せられている。植物界の大変革は動物界に先駆けておこっている。中生代は全体として裸子植物が繁栄した時代であったが、白亜紀後半はすでに被子植物の時代に入る。中生代は、古生代末に成立した超大陸パンゲアが次々に分裂、移動を始めた時代であった。ヨーロッパや大西洋両岸の地域は比較的静穏であったが、環太平洋地域ではプレートにのって運ばれた陸塊が周辺の大陸地殻に衝突、付加し、これに関連して激しい地殻変動や火成活動、変成作用が生じた。有用な鉱床の形成もこの時代に多い。

［速水　格・小澤智生　2015年8月19日］

『市川浩一郎他著『改訂新版地史学　下巻』（1967・朝倉書店）』▽『木村敏雄・速水格・吉田鎮男著『日本の地質』（1993・東京大学出版会）』▽『ドゥーガル・ディクソン著、小畠郁生監訳『生命と地球の進化アトラスⅡ　デボン紀から白亜紀』（2003・朝倉書店）』

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例