So they turned out to be some of the most important index fossils from the whole Paleozoic Era.

因此，它们成为整个古生代最重要的指示化石之一。

Ammonites, for example, are excellent index fossils as their evolution was rapid, and they had a wide distribution.

例如，菊石是极好的指标化石，因为它们的进化迅速，而且分布广泛。

The presence of certain fossils in a rock layer—known as index fossils—can help identify the age of that layer.

岩层中某些化石（称为索引化石）的存在可以帮助确定该层的年龄。

They ended up being so successful, diverse, and numerous that their shells are now used as index fossils to define Periods in the Mesozoic.

它们最终变得如此成功、多样化和数量众多，以至于它们的贝壳现在被用作定义中生代时期的指标化石。