For instance, you know when the Devonian Period ends and the Carboniferous Period begins, because that's when you start finding fossils that belong to a species of conodont known as Siphonodella sulcata.

例如， 您知道泥盆纪何时结束和石炭纪何时开始，因为那是您开始发现属于一种名为 Siphonodella sulcata 的牙形石的化石的时间。