Distance Matrix from Species richness and disparity of parareptiles across the end-Permian mass extinction.

The amniote clade Parareptilia is notable in that members of the clade exhibited a wide array of morphologies, were successful in a variety of ecological niches and survived the end-Permian mass extinction. In order to better understand how mass extinction events can affect clades that survive them, we investigate both the species richness and morphological diversity (disparity) of parareptiles over the course of their history. Furthermore, we examine our observations in the context of other metazoan clades, in order to identify post-extinction survivorship patterns that are present in the clade. The results of our study indicate that there was an early increase in parareptilian disparity, which then fluctuated over the course of the Permian, before it eventually declined sharply towards the end of the Permian and into the Triassic, corresponding with the end-Permian mass extinction event. Interestingly, this is a different trend to what is observed regarding parareptile richness, that shows an almost continuous increase until its overall peak at the end of the Late Permian. Moreover, richness did not experience the same sharp drop at the end of the Permian, reaching a plateau until the Anisian, before dropping sharply and remaining low, with the clade going extinct at the end of the Triassic. This observed pattern is likely to be due to the fact that, despite the extinction of several morphologically distinct parareptile clades, the procolophonoids, one of the largest parareptilian clades, were diversifying across the Permian–Triassic boundary. With the clade's low levels of disparity and eventually declining species richness, this pattern most resembles a ‘dead clade walking’ pattern.

羊膜动物支系中的副爬行动物（Parareptilia）类群颇具研究价值：该支系的成员展现出丰富多样的形态特征，成功占据了各类生态位，且成功渡过了二叠纪末大灭绝（end-Permian mass extinction）事件。为进一步阐明大灭绝事件对得以幸存的支系的影响机制，我们对副爬行动物在其整个演化历史中的物种丰富度与形态多样性（disparity）展开了研究。此外，我们将本次观测结果与其他后生动物支系的相关数据进行对比分析，以识别该支系所呈现的灭绝后存活模式。

本研究结果显示，副爬行动物的形态差异度在演化早期便出现显著上升，随后在二叠纪期间持续波动，最终在二叠纪末至三叠纪初期出现急剧下降，这一变化与二叠纪末大灭绝事件高度对应。值得注意的是，这一趋势与副爬行动物物种丰富度的变化趋势截然不同：后者几乎呈持续增长态势，直至晚二叠世末达到整体峰值。此外，物种丰富度并未在二叠纪末出现同等幅度的骤降，而是维持平稳直至安尼期（Anisian），随后才急剧下跌并长期维持在较低水平，该支系最终于三叠纪末彻底灭绝。

上述观测到的演化模式大概率源于以下事实：尽管多个形态特征独特的副爬行动物支系相继灭绝，但前棱蜥类（procolophonoids）——最大的副爬行动物支系之一——在二叠纪-三叠纪界线期间发生了辐射演化。结合该支系较低的形态差异度与最终下降的物种丰富度，这一演化模式最符合“行走的灭绝支系（dead clade walking）”的典型特征。