The fossil record of the origins of major groups is of great interests to many biologists, especially when the fossil record apparently conflicts with timings based on molecular clock estimates. Here we model the diversity of “stem” (basal) and “crown” (modern) members of groups as seen in the fossil record, using a “birth-death model”. Under background conditions, the stem group members must diversify rapidly until the modern crown group emerges, at which point their diversity rapidly collapses, followed shortly by their extinction. Mass extinctions can disturb this pattern to create very diverse stem groups such as the dinosaurs and trilobites. Understanding these null-hypothesis patterns is essential for framing ecological and evolutionary explanations for how major groups originate and subsequently evolve.

中文翻译：

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茎和冠群的动力学

许多生物学家都非常感兴趣主要群体的化石记录，特别是当化石记录明显与基于分子钟估计的计时冲突时。在这里，我们使用“出生死亡模型”对化石记录中所见的“干”（基）成员和“冠”（现代）成员的多样性进行建模。在背景条件下，茎群成员必须迅速多样化，直到现代的冠群出现为止，此时它们的多样性迅速瓦解，随后很快灭绝。大规模灭绝会干扰这种模式，从而形成非常多样化的茎类，例如恐龙和三叶虫。理解这些零假设模式对于为主要群体如何产生和随后进化建立生态和进化解释至关重要。