Molecular evolutionary time scales are expected to predate the fossil evidence, but, particularly for major evolutionary radiations, they can imply extremely protracted stem lineages predating the origin of living clades, leading to claims of systematic overestimation of divergence times. We use macroevolutionary birth-death models to describe the range of total-group and crown-group ages expected under constant rates of speciation and extinction. We extend current predictions on origination times for crown- and total-groups, and extinction of stem-groups, demonstrating that there is broad variance in these predictions. Under constant rates of speciation and extinction, we show that the distribution of expected arthropod total-group ages is consistent with molecular clock estimates. The fossil record cannot be read literally, and our results preclude attempts to interpret the antiquity of clades based on the co-occurrence of stem- and crown-representatives.

分子进化的时间尺度预计早于化石证据，但是，特别是对于主要的进化辐射来说，它们可能意味着在生命进化枝的起源之前极其漫长的茎谱系，导致人们声称系统性地高估了分歧时间。我们使用宏观进化出生-死亡模型来描述在物种形成和灭绝速率恒定的情况下预期的总群体和冠群年龄的范围。我们扩展了当前对冠群和总群起源时间以及茎群灭绝时间的预测，表明这些预测存在很大差异。在物种形成和灭绝速率恒定的情况下，我们表明预期节肢动物总群体年龄的分布与分子钟估计一致。化石记录不能从字面上解读，我们的结果排除了根据茎和冠代表的共现来解释进化枝的古代性的尝试。