Permian–Triassic phylogenetic and morphologic evolution of rhynchonellide brachiopods

The Rhynchonellida is a major group of brachiopods that survived the “big five” mass extinctions and flourished after the Permian-Triassic (P-Tr) crisis. However, phylogenetic and character evolution in the Rhynchonellida across the P-Tr transition is poorly understood. In view of the widespread homoplasy across this order, we employ a tip-dated Bayesian analysis to reconstruct phylogenetic relationships for Late Permian–Triassic rhynchonellides. The same data were also analyzed using three other methods: undated Bayesian, equal-weighting, and implied-weighting parsimony. Compared with trees generated by other methods, those constructed by tip-dating best account for the homoplasy in this group, and are closer to previous assumptions on the evolution of this group. Based on the analyses of multiple trees, the major increase in lineage richness occurred in the Early and early Middle Triassic, and the richness in the Anisian almost reached its highest level in the Triassic. According to fossil records, a pronounced reduction in shell size and in the development of ornamentation occurred after the P-Tr extinction, which is largely due to the loss of large and highly-sculptured genera, and the diversification of small-sized and weakly-ornamented genera. Ancestral state estimation of shell size and development of ornamentation, coupled with comparisons of other characters, indicate that the Early–Middle Triassic mature “small-sized taxa” may have characters displayed by juveniles of their ancestors. This suggests that for these genera, paedomorphosis was possibly a strategy to survive and diversify in the harsh environment after the P-Tr extinction.

穿孔贝目（Rhynchonellida）是一类成功历经“五大”生物大灭绝事件，并在二叠纪-三叠纪（P-Tr）生物大灭绝危机后再度繁盛的腕足动物（brachiopods）大类群。然而，学界对于穿孔贝目在P-Tr过渡时期的系统发育与性状演化机制仍知之甚少。鉴于该目类群广泛存在的同塑性（homoplasy）现象，我们采用末端定年贝叶斯分析法（tip-dated Bayesian analysis）对晚二叠世至三叠纪的穿孔贝目类群开展系统发育关系重建。本研究同时采用三种其他分析方法对同一数据集进行分析：无定年贝叶斯分析法（undated Bayesian）、等权简约法（equal-weighting parsimony）与隐含加权简约法（implied-weighting parsimony）。相较于其他方法构建的系统发育树，末端定年法得到的结果最能解释该类群的同塑性现象，且与此前学界对该类群演化的推测更为吻合。基于多棵系统发育树的分析结果显示，支系丰富度的显著增长发生在早三叠世至中三叠世早期，而安尼期（Anisian）的支系丰富度几乎达到三叠纪时期的峰值。根据化石记录，P-Tr生物大灭绝事件后，腕足动物的壳体大小与壳饰发育程度均出现显著退化，这一现象主要源于具大型壳体与强烈壳饰的属类群灭绝，而小型壳体、弱壳饰的属类群发生辐射演化。对壳体大小与壳饰发育程度的祖先性状重建（Ancestral state estimation），结合其他性状的对比分析表明，早-中三叠世成熟的“小型类群”可能保留了其祖先类群幼体阶段的性状特征。这暗示对于这些属类群而言，幼态延续（paedomorphosis）或许是它们在P-Tr大灭绝后严酷环境中得以存活并实现辐射演化的适应性策略。