A genus representing the Orthoceradinida order of mollusks, but generally refers to the Orthoceratida order in general. Also called the Orthoceradinida. It flourished in the Ordovician period and survived until the Triassic period. "Ortho" means "straight" and "ceras" means "horn." This group originated as early as the Early Ordovician from the Ellesmeroceratida order, which forms the basis of the Nautiloids, and the early Orthoceradinida gave rise to the Bactritida (considered to be the ancestors of the Ammonites and Coleoidea). Source: Heibonsha World Encyclopedia, 2nd Edition Information |
軟体動物,頭足類のオルトケラス目を代表する属だが,一般にはオルトケラス目Orthoceratida全般をさす。直角石類ともいう。オルドビス紀に全盛をきわめ,以後三畳紀まで生存した。orthoは“まっすぐ”,cerasは“角(つの)”の意。オウムガイ亜綱の根幹をなすエレスメロケラス目Ellesmeroceratidaから早くもオルドビス紀初期に由来したグループで,オルトケラス類の初期のものからバクトリテス類Bactritida(アンモナイト亜綱や鞘形亜綱の先祖とみなされる)を生じた。
出典 株式会社平凡社世界大百科事典 第2版について 情報 |
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