Springer Mark S, Emerling Christopher A, Meredith Robert W, Janečka Jan E, Eizirik Eduardo, Murphy William J
Department of Biology, University of California, Riverside, CA 92521, USA.
Museum of Vertebrate Zoology, University of California, Berkeley, CA 94720, USA.
Mol Phylogenet Evol. 2017 Jan;106:86-102. doi: 10.1016/j.ympev.2016.09.017. Epub 2016 Sep 19.
The explosive, long fuse, and short fuse models represent competing hypotheses for the timing of placental mammal diversification. Support for the explosive model, which posits both interordinal and intraordinal diversification after the KPg mass extinction, derives from morphological cladistic studies that place Cretaceous eutherians outside of crown Placentalia. By contrast, most molecular studies favor the long fuse model wherein interordinal cladogenesis occurred in the Cretaceous followed by intraordinal cladogenesis after the KPg boundary. Phillips (2016) proposed a soft explosive model that allows for the emergence of a few lineages (Xenarthra, Afrotheria, Euarchontoglires, Laurasiatheria) in the Cretaceous, but otherwise agrees with the explosive model in positing the majority of interordinal diversification after the KPg mass extinction. Phillips (2016) argues that rate transference errors associated with large body size and long lifespan have inflated previous estimates of interordinal divergence times, and further suggests that most interordinal divergences are positioned after the KPg boundary when rate transference errors are avoided through the elimination of calibrations in large-bodied and/or long lifespan clades. Here, we show that rate transference errors can also occur in the opposite direction and drag forward estimated divergence dates when calibrations in large-bodied/long lifespan clades are omitted. This dragging forward effect results in the occurrence of more than half a billion years of 'zombie lineages' on Phillips' preferred timetree. By contrast with ghost lineages, which are a logical byproduct of an incomplete fossil record, zombie lineages occur when estimated divergence dates are younger than the minimum age of the oldest crown fossils. We also present the results of new timetree analyses that address the rate transference problem highlighted by Phillips (2016) by deleting taxa that exceed thresholds for body size and lifespan. These analyses recover all interordinal divergence times in the Cretaceous and are consistent with the long fuse model of placental diversification. Finally, we outline potential problems with morphological cladistic analyses of higher-level relationships among placental mammals that may account for the perceived discrepancies between molecular and paleontological estimates of placental divergence times.
爆发式、长导火索式和短导火索式模型代表了关于胎盘哺乳动物多样化时间的相互竞争的假说。爆发式模型假定在白垩纪-古近纪（KPg）大灭绝之后发生了目间和目内的多样化，对该模型的支持源于形态学分支系统学研究，这些研究将白垩纪真兽类置于胎盘类冠群之外。相比之下，大多数分子研究支持长导火索式模型，即目间分支发生在白垩纪，随后在KPg边界之后发生目内分支。菲利普斯（2016年）提出了一种软爆发式模型，该模型允许在白垩纪出现少数几个谱系（异关节类、非洲兽类、灵长总目、劳亚兽总目），但在其他方面与爆发式模型一致，即假定在KPg大灭绝之后发生了大多数目间的多样化。菲利普斯（2016年）认为，与大体型和长寿命相关的速率转移误差夸大了先前对目间分歧时间的估计，并进一步表明，当通过排除大体型和/或长寿命类群中的校准来避免速率转移误差时，大多数目间分歧发生在KPg边界之后。在这里，我们表明速率转移误差也可能以相反的方向发生，并且当省略大体型/长寿命类群中的校准时，会将估计的分歧日期向前拖动。这种向前拖动的效应导致在菲利普斯所偏好的时间树上出现了超过5亿年的“僵尸谱系”。与幽灵谱系不同，幽灵谱系是不完整化石记录的逻辑副产品，当估计的分歧日期比最古老冠群化石的最小年龄更年轻时，就会出现僵尸谱系。我们还展示了新的时间树分析结果，这些分析通过删除超过体型和寿命阈值的分类单元来解决菲利普斯（2016年）所强调的速率转移问题。这些分析恢复了白垩纪所有的目间分歧时间，并且与胎盘类多样化的长导火索式模型一致。最后，我们概述了胎盘哺乳动物之间高级关系的形态学分支系统学分析中可能存在的潜在问题，这些问题可能解释了分子和古生物学对胎盘类分歧时间估计之间的明显差异。
