Heterogeneous rates of molecular evolution and diversification could explain the Triassic age estimate for angiosperms

Dating analyses based on molecular data imply that crown angiosperms existed in the Triassic, long before their undisputed appearance in the fossil record in the Early Cretaceous. Following a re-analysis of the age of angiosperms using updated sequences and fossil calibrations, we use a series of simulations to explore the possibility that the older age estimates are a consequence of (i) major shifts in the rate of sequence evolution near the base of the angiosperms and/or (ii) the representative taxon sampling strategy employed in such studies. We show that both of these factors do tend to yield substantially older age estimates. These analyses do not prove that younger age estimates based on the fossil record are correct, but they do suggest caution in accepting the older age estimates obtained using current relaxed-clock methods. Although we have focused here on the angiosperms, we suspect that these results will shed light on dating discrepancies in other major clades.

基于分子数据的定年分析表明，冠群被子植物（crown angiosperms）曾存在于三叠纪，远早于其在早白垩世化石记录中被确凿无误证实的出现时间。在使用更新后的序列数据与化石校准对被子植物起源时间开展重新分析后，我们通过一系列模拟实验探讨了较早定年估算结果的潜在成因：一是被子植物基部附近序列进化速率发生显著改变，二是此类研究所采用的代表性类群采样策略，或二者共同作用。研究表明，这两类因素均会导致定年估算结果显著偏老。本分析并未证明基于化石记录的较年轻定年估算结果正确，但确实提示我们在采信当前松弛时钟定年方法（relaxed-clock methods）得到的较老定年结果时需保持审慎。尽管本研究聚焦于被子植物，但我们推测该结果可为其他主要演化支的定年分歧问题提供借鉴思路。