Over long spans of geological time, various groups of organisms may wax and wane, experiencing times of apparent success and contraction. These rises and falls are often said to reflect either opportunities created by climate change or the relative success of innovative characteristics. Phylum Brachiopoda was one of the most successful marine clades before the Permian/Triassic mass extinction (PTME), but after this event, they became marginal components of marine communities through to the present day. How brachiopod morphological innovations reacted to swiftly declining diversity has long remained poorly understood. Here we analyse morphological evolution over the 300 Myr (Permian–Quaternary) history of the four major Mesozoic–Cenozoic brachiopod orders (Terebratulida, Rhynchonellida, Spiriferinida, Athyridida). Unexpectedly, their disparities reached or exceeded pre-PTME levels, but were decoupled from generic richness, which was generally low. Distribution of taxa in morphospace and shifts in centroid indicate that all four orders exploited new morphospaces when adapting to post-Permian environments. A comparison of morphospace occupation and diversity evolution suggests that the high extinction rate of brachiopods and the limited diversification of new forms may have accounted for the depauperate nature of modern-day brachiopods.

在漫长的地质时期中，各种生物体可能会盛衰，经历明显的成功和收缩时期。人们通常认为这些涨跌要么反映了气候变化创造的机遇，要么反映了创新特征的相对成功。腕足动物门是二叠纪/三叠纪大规模灭绝（PTME）之前最成功的海洋进化枝之一，但在这次事件之后，它们一直成为海洋群落的边缘组成部分，直到今天。长期以来，人们对腕足动物形态创新如何应对多样性迅速下降的反应知之甚少。在这里，我们分析了四个主要中生代-新生代腕足目（Terebratulida、Rhynchonellida、Spiriferinida、Athyridida）在 300 迈里（二叠纪-第四纪）历史中的形态演化。出乎意料的是，它们的差异达到或超过了 PTME 之前的水平，但与普遍较低的通用丰富度脱钩。类群在形态空间中的分布和质心的变化表明，所有四个目在适应后二叠纪环境时都利用了新的形态空间。形态空间占据和多样性进化的比较表明，腕足动物的高灭绝率和新形式的有限多样化可能是现代腕足动物退化的原因。