Tutin Samuel L, Butler Richard J
School of Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom.
Acta Palaeontol Pol. 2017;62(3):563-573. doi: 10.4202/app.00355.2017.
Plesiosaurs were a highly successful group of marine reptiles occurring worldwide in the Jurassic and Cretaceous, but to date few studies have focused on their preservation through time. Here, we conduct the first detailed assessment of the quality of the plesiosaur fossil record. Data was compiled for 178 specimens representing 114 valid species. For each species we calculated the character completeness metric (CCM: percentage of phylogenetic characters from a cladistic dataset that can be scored for that species) and the skeletal completeness metric (SCM: percentage of the overall skeleton that is preserved for that species). Average CCM and SCM values were calculated for individual geological stages. A strong significant positive correlation was recovered between CCM and SCM, suggesting that the two metrics are recording the same signal, at least for this clade. Although a significant correlation between changes in sea level and changes in plesiosaur completeness was not recovered, an underlying negative relationship may be present but obscured by poorly sampled time bins. Plesiosaur completeness though time is not significantly correlated with that for contemporary terrestrial groups (sauropods, pterosaurs, birds), but is significantly correlated with that for ichthyosaurs, suggesting common controls on skeletal preservation in the marine realm. Significantly higher median completeness values in plesiosaurs and ichthyosaurs than in contemporary terrestrial groups support the hypothesis that the marine tetrapod fossil record is more complete than that of terrestrial tetrapods. A collector's curve for plesiosaurs shows a generally slow constant rate of discovery from the latter part of the 19 century until the 1990s, at which point the rate of discovery increased substantially and shows no sign of slowing. A significant but very weak negative correlation between SCM and the year in which a taxon was named suggests a weak tendency for more recently named species to have less complete skeletons.
蛇颈龙是一类非常成功的海洋爬行动物，在侏罗纪和白垩纪时期遍布全球，但迄今为止，很少有研究关注它们随时间推移的保存情况。在这里，我们首次对蛇颈龙化石记录的质量进行了详细评估。我们收集了代表114个有效物种的178个标本的数据。对于每个物种，我们计算了特征完整性指标（CCM：来自分支数据集的系统发育特征中可用于该物种评分的百分比）和骨骼完整性指标（SCM：该物种保存下来的整体骨骼的百分比）。计算了各个地质阶段的平均CCM和SCM值。CCM和SCM之间存在强烈的显著正相关，这表明这两个指标至少对于这个进化枝记录的是相同的信号。尽管没有发现海平面变化与蛇颈龙完整性变化之间存在显著相关性，但可能存在潜在的负相关关系，只是由于时间间隔采样不足而被掩盖了。蛇颈龙随时间的完整性与当代陆地类群（蜥脚类恐龙、翼龙、鸟类）的完整性没有显著相关性，但与鱼龙的完整性显著相关，这表明海洋领域对骨骼保存存在共同的控制因素。蛇颈龙和鱼龙的中位数完整性值明显高于当代陆地类群，这支持了海洋四足动物化石记录比陆地四足动物化石记录更完整的假设。蛇颈龙的收集者曲线显示，从19世纪后期到20世纪90年代，发现速度总体上较为缓慢且恒定，此后发现速度大幅增加且没有放缓的迹象。SCM与一个分类单元被命名的年份之间存在显著但非常微弱的负相关，这表明最近命名的物种骨骼不太完整的趋势较弱。
