Acrotretides are extinct micromorphic brachiopods that exhibited considerable morphological variation during their rapid evolution in the early Palaeozoic. The plano-conical shells of acrotretides are distinct in comparison to other brachiopod groups and despite their diversity and abundance in early Palaeozoic communities, their origins, early evolution, life history and phylogeny are poorly understood. Here, we employ advanced geometric morphometrics to quantitatively investigate ontogenetic variation and allometry in the ventral valve of the oldest known acrotretide species from the early Cambrian of South China. Our results identify substantial shape variation for Eohadrotreta zhenbaensis, along with a parabolic morphological trajectory through ontogeny, demonstrating a remarkable reversal to PC1 values equivalent to those obtained for juveniles, during later ontogenetic stages. The evolutionary novel body plan (diminutive and plano-conical) of Acrotretida was established gradually during two phases of allometry, formed initially during the final stage of the Cambrian evolutionary radiation from an ancestral low, equivalved lingulide body plan. The development of a conical shaped valve seems to have resulted in an overall smaller body size, when compared with non-conical forms. The heterochronic processes responsible for generating these ontogenetic modifications at different allometric phases may have facilitated the evolutionary diversification of acrotretide brachiopods during the early Palaeozoic.

中文翻译：

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变大或变圆锥：早期寒武纪腕足动物的异速生长轨迹

Acrotretides 是已灭绝的微形腕足类动物，在古生代早期的快速进化过程中表现出相当大的形态变异。与其他腕足类群相比，acrotretides 的平锥形壳是不同的，尽管它们在早古生代群落中具有多样性和丰富性，但对它们的起源、早期进化、生活史和系统发育知之甚少。在这里，我们采用先进的几何形态计量学来定量研究华南早寒武世最古老的已知阿克曲肽物种腹侧瓣膜的个体发育变异和异速生长。我们的研究结果确定了真巴真角石斑鱼的实质性形状变化，以及通过个体发育的抛物线形态轨迹，表明在后期个体发育阶段，PC1 值的显着逆转与幼鱼获得的值相当。Acrotretida 的进化新体计划（小型和平锥形）是在异速生长的两个阶段逐渐建立的，最初形成于寒武纪进化辐射的最后阶段，来自祖先的低等舌体计划。与非锥形阀门相比，锥形阀门的发展似乎导致了整体较小的阀体尺寸。负责在不同异速生长阶段产生这些个体发育修饰的异时过程可能促进了古生代早期 acrotretide 腕足动物的进化多样化。