At one time, the animals present in these fossil beds were assigned to various modern animal groups, but most paleontologists now agree that all Tommotian fossils represent unique body forms that arose in the early Cambrian period and disappeared before the end of the period, leaving no descendants in modern animal groups. (52, TPO5)

paleontologist /ˌpeɪlɪɔnˈtɔləd ʒɪst/ n. 古生物学家

descendant /dɪ'send(ə)nt/ n. 子孙，后代

我是分界线，大家先自己分析哦。

At one time, the animals (present in these fossil beds) were assigned to various modern animal groups, but most paleontologists now agree that all Tommotian fossils represent unique body forms (that arose in the early Cambrian period and disappeared before the end of the period), (leaving no descendants in modern animal groups.)

分析：

修饰一：(present in these fossil beds)，形容词短语，修饰animals，注意是放在后面哦

中文：存在于这些化石床中

修饰二：(that arose in the early Cambrian period and disappeared before the end of the period)，从句，修饰body forms新托福口语模板

中文：出现在Cambrian时期早期，并在这个时期末期前消失

修饰三：(leaving no descendants in modern animal groups.)，非谓语动词修饰前面的主干

中文：在现代动物群组中没有留下后代

主干：most paleontologists now agree that，that后面整个宾语从句，从all到groups

参考翻译：

曾经，存在于这些化石床中的动物被分配到各种现代动物群组中，但现在大多数古生物学家都同意：所有Tommotian的化石代表了出现在Cambrian时期早期并在这个时期末期前消失独特的化石形式，在现代动物群组中没有留下后代。

这个句子的主要修饰成分就是从句、非谓语动词，大家务必想清楚，每一个修饰成分修饰的是什么，才能看清楚这个句子的结构。