最近的努力导致开发了极其复杂的方法,用于在估计系统发育树的时间模式时合并整个树的数据并适应不确定性。但是对节点年龄的先验约束的分配仍然是最重要的因素。这在很大程度上取决于理解专家之间的实质性分歧(古生物学家,地质学家,和比较解剖学家),对于依赖这些数据作为下游用户的系统发育学家和分子生物学家来说,这通常是不透明的。这通常会导致误解与节点年龄最小值相关的不确定性是如何产生的,导致系统学家对这种不确定性的不适当处理。为了促进关于这一主题的对话,我们在这里回顾因素(系统发育,保存性的大型会议,四足动物化石记录中的空间和时间模式)使四足动物树中深度发散的先验节点年龄约束的分配复杂化,专注于冠群羊膜的起源,冠组两栖动物,和皇冠组四足动物。我们发现两栖动物和四足动物的节点先验表现出很高的系统发育不稳定性和不同的系统发育处理,将不同的分类单元识别为这些冠群的最早代表。这部分对应于众所周知的两栖动物起源问题,但越来越反映出早期四足动物系统发育的更深层次的不稳定性。相反,系统发育治疗的差异不会影响我们识别最早的冠群羊膜动物的能力,但会影响我们对最早的羊膜动物群的理解。早期四足动物化石记录的保存性和时空异质性在可靠地估计四足动物节点的年龄方面提出了无法识别的挑战;整个相关间隔的四足动物记录在空间上受到限制,并被与所有三个冠组的出现相吻合的几个主要最小采样间隔所破坏。展望未来,研究人员试图校准这些节点的年龄,以及后生化石记录中的其他类似深层节点,应该有意识地考虑主要的系统发育不确定性,保存性的大型会议,和时空异质性,最好检查来自多个研究小组的工作假设的影响。我们强调需要在经典的欧洲和北美部分之外进行主要的四足动物收集工作,特别是来自南半球的,并表明这种采样可能会极大地改变我们四足动物进化的时间表。
Recent efforts have led to the development of extremely sophisticated methods for incorporating tree-wide data and accommodating uncertainty when estimating the temporal patterns of phylogenetic trees, but assignment of prior constraints on node age remains the most important factor. This depends largely on understanding substantive disagreements between specialists (paleontologists, geologists, and comparative anatomists), which are often opaque to phylogeneticists and molecular biologists who rely on these data as downstream users. This often leads to misunderstandings of how the uncertainty associated with node age minima arises, leading to inappropriate treatments of that uncertainty by phylogeneticists. In order to promote dialogue on this subject, we here review factors (phylogeny, preservational megabiases, spatial and temporal patterns in the
tetrapod fossil record) that complicate assignment of prior node age constraints for deep divergences in the
tetrapod tree, focusing on the origin of crown-group Amniota, crown-group Amphibia, and crown-group Tetrapoda. We find that node priors for amphibians and tetrapods show high phylogenetic lability and different phylogenetic treatments identifying disparate taxa as the earliest representatives of these crown groups. This corresponds partially to the well-known problem of lissamphibian origins but increasingly reflects deeper instabilities in early tetrapod phylogeny. Conversely, differences in phylogenetic treatment do not affect our ability to recognize the earliest crown-group amniotes but do affect how diverse we understand the earliest amniote faunas to be. Preservational megabiases and spatiotemporal heterogeneity of the early
tetrapod fossil record present unrecognized challenges in reliably estimating the ages of
tetrapod nodes; the
tetrapod record throughout the relevant interval is spatially restricted and disrupted by several major intervals of minimal sampling coincident with the emergence of all three crown groups. Going forward, researchers attempting to calibrate the ages for these nodes, and other similar deep nodes in the metazoan fossil record, should consciously consider major phylogenetic uncertainty, preservational megabias, and spatiotemporal heterogeneity, preferably examining the impact of working hypotheses from multiple research groups. We emphasize a need for major
tetrapod collection effort outside of classic European and North American sections, particularly from the southern hemisphere, and suggest that such sampling may dramatically change our timelines of tetrapod evolution.