noun ダウンロード 🔗共有 示準化石 Any ancient remains of a plant or animal that lived during a specific geological period and that can be used to date the containing rocks. Examples : "Finding the index fossil Trilobite in a rock layer tells scientists that the rock is about 500 million years old. " 岩石層で示準化石の三葉虫を発見すると、科学者はその岩石が約5億年前のものであることがわかります。 geology archaeology biology science history nature environment organism animal plant period time AIとチャット語彙ゲーム読解練習