奥陶系

例句：

1. 1. The Ordovician periodis characterized by the appearance of the firstvertebrate animals.
   奥陶纪以首次出现脊椎动物而闻名。
2. 2. Thediscovery of fossils from the Ordovician period has helped scientists understand theearly life on Earth.
   奥陶纪化石的发现帮助科学家理解了地球早期的生命。
3. 3. The Ordovician system isa geological time period that spannedfrom about 4.5 to3.5 billion years ago.
   奥陶系是一段地质时期，从大约45亿年前到35亿年前。