大部分在古生代岩层中发现的化石都是没有脊骨的无脊椎动物，例如珊瑚、软体动物和三叶虫。
Most of the fossils seen in Paleozoic rocks are invertebrate animals lacking backbones, such as corals, mollusks and trilobites.
200多万年前的马蹄蟹化石，与今天的马蹄蟹几乎完全相同：它们是三叶虫最近的亲戚。
Horseshoe crab fossils over 200 million years old are almost identical to contemporary animals: They are the closest living relatives of the trilobites.
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