Go large or go conical: allometric trajectory of an early Cambrian acrotretide brachiopod

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 a juvenile morphology during later ontogenetic stages. The evolutionary novel body plan (diminutive and plano-conical) of Acrotretida was established gradually during two phases of allometry, f...

阿特列贝类（Acrotretides）是一类已灭绝的微型腕足动物，在古生代早期的快速演化历程中展现出显著的形态变异。相较于其他腕足动物类群，阿特列贝类的平锥形壳体特征鲜明；尽管该类群在古生代早期群落中兼具多样性与丰度，但其起源、早期演化、生活史及系统发育关系仍研究不足。本研究采用先进的几何形态测量学（advanced geometric morphometrics）方法，对产自华南寒武纪早期的已知最古老阿特列贝物种的腹壳，开展个体发育变异与异速生长模式的定量探究。研究结果显示，镇巴始厚壳贝（Eohadrotreta zhenbaensis）存在显著的壳体形态变异，其个体发育过程呈现抛物线形的形态轨迹，证实了其在发育后期会出现向幼体形态的显著逆转。阿特列贝目（Acrotretida）演化出的小型平锥形特有躯体构型，通过两个异速生长阶段逐步形成，f...