Data from: Quantifying historical trends in the completeness of the fossil record and the contributing factors: an example using Aves

Improvements in the perceived completeness of the fossil record may be driven both by new discoveries and by reinterpretation of known fossils, but disentangling the relative effects of these processes can be difficult. Here, we propose a new methodology for evaluating historical trends in the perceived completeness of the fossil record, demonstrate its implementation using the freely available software ASCC (version 4.0.0), and present an example using crown-group birds (Aves). Dates of discovery and recognition for the oldest fossil representatives of 75 major lineages of birds were collected for the historical period ranging from 1910 to 2010. Using a comprehensive phylogeny, we calculated minimum implied stratigraphic gaps (MIG range) across these 75 lineages. Our results show that a reduction in global MIG values of 1.38 Ga (billion years) occurred over the past century in avian paleontology. A pronounced increase in the average rate of global MIG reduction is noted in the post-1970s interval (290.5 Myr per decade) compared to the pre-1970s interval (31.9 Myr per decade). Although the majority of the improvement in the fossil record of birds has come from new discoveries, substantial improvement (~22.5%) has resulted from restudy and phylogenetic revision of previously described fossils over the last 40 years. With a minimum estimate indicating that at least 1.38 Gyr of gaps remain to be filled between the predicted and observed first appearances of major lineages of crown Aves, there is much progress to be made. However, a notable tapering off in the rate of global MIG reduction occurs between 1990 and 2010, suggesting we may be approaching an asymptote of oldest record discoveries for birds. Only future observations can determine whether this is a real pattern or a historical anomaly. Either way, barring the discovery of fossils that substantially push back the minimum age for the origin of crown-clade Aves, new discoveries cannot continue to reduce global MIG values at the average post-1970s rate over the long term.

化石记录（fossil record）的感知完整性的提升，既可能源于新化石的发现，也可能来自对已知化石的重新解读，但厘清这两类过程的相对影响颇具难度。本文提出了一种评估化石记录感知完整性历史变化趋势的全新方法，并借助可免费获取的ASCC软件（版本4.0.0）演示了该方法的实现流程；同时以冠群鸟类（Aves）为例展开应用分析。我们收集了1910年至2010年这百年间，鸟类75个主要支系的最古老化石代表的发现与识别时间数据。基于一套全面的系统发育树，我们计算了这75个支系的最小隐含地层间隙（minimum implied stratigraphic gaps，MIG）范围。研究结果显示，过去一个世纪以来，鸟类古生物学领域的全球MIG值累计减少了1.38吉年（Ga，即十亿年）。与1970年代前的年均缩减速率（每十年31.9百万年（Myr））相比，1970年代后的全球MIG平均缩减速率出现了显著提升（每十年290.5百万年（Myr））。尽管鸟类化石记录的改善大多源自新化石的发现，但在过去40年里，约22.5%的改善来自对已描述化石的重新研究与系统发育修订。据最低估算，冠群鸟类主要支系的预测首次出现时间与实际观测首次出现时间之间，仍存在至少1.38吉年的地层间隙有待填补，未来仍有大量研究空间。不过，1990年至2010年间，全球MIG缩减速率出现了明显放缓，这暗示我们或许正趋近于鸟类最古老化石记录发现的渐近线。唯有未来的观测研究才能确定，这一现象是真实存在的模式，还是历史记录的异常值。无论如何，除非发现能够大幅推高冠群鸟类起源最小年龄的化石，否则从长期来看，新化石发现将无法再以1970年代后的平均速率持续缩减全球MIG值。