Data from: 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, 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.

Methods
SEM images of each specimen were used to digitalize landmarks, with digitization performed using TpsDig2 v. 2.25 (Rohlf 2016a). Due to the simple morphology of the ventral valve of Eohadrotreta, we apply a combination of landmarks and semilandmarks or sliding landmarks, for analysing homologous points locations together with curves measured on a sample of organisms (Zelditch et al. 2012; Gunz & Mitteroecker 2013; Mitteroecker et al. 2013).

Usage Notes
Appendix S1: Shape variation among ventral valve of Eohadrotreta zhenbaensis from the Cambrian Series 2 Shuijingtuo Formation at Wanhjiaping section of South China, as well as E. zhenbaensis and P. zhujiahensis from the Shuijingtuo Formation at Aijiahe and Wangzishi sections of South China for comparison, with indication of characteristic thin plate splines. All thin plate splines correspond to the shapes of 190 specimens in morphospace. The valve length is indicated by different colours.

Appendix S2: Principal components, size information, and GPA data of all the 158 specimens of E. zhenbaensis. n/a demonstrates that the height measurement is not obtained.

Appendix S3: Relative contribution of each landmark (1-38) of E. zhenbaensis.

Appendix S4: Multivariate regression of overall shape with centroid size of E. zhenbaensis, involving permutation tests on the correlations with 10 000 replicates.

Appendix S5: Multivariate regression of overall shape with valve height of E. zhenbaensis, involving permutation tests on the correlations with 10 000 replicates.

阿克罗特里斯是一种已灭绝的微形腕足动物，在其早古生代的快速演化过程中表现出显著的形态学变异。与其它腕足动物群相比，阿克罗特里斯的平面锥形壳具有独特性。尽管在早古生代群落中其多样性丰富且数量众多，但其起源、早期演化、生活史和系统发育仍被理解不足。本研究中，我们采用高级几何形态计量学方法，对来自中国南方早寒武纪已知最古老阿克罗特里斯物种的腹瓣进行定量研究，以探究其个体发育变异和异速生长。我们的研究结果表明，早阿克罗特瑞特兹氏（Eohadrotreta zhenbaensis）具有显著的形态学变异，并在个体发育过程中呈现出抛物线状的形态学轨迹，显示出在后期个体发育阶段对幼体形态的显著逆转。阿克罗特里斯独特的（体型缩小且平面锥形）身体计划是在两个异速生长阶段逐渐形成的，最初是在寒武纪辐射的最终阶段，从一个原始的低等、等铰合舌形身体计划演化而来。与非锥形形态相比，锥形瓣的发育似乎导致了总体体型较小。在不同异速生长阶段产生这些个体发育变异的异时性过程可能促进了阿克罗特里斯腕足动物在早古生代的进化多样性。

方法：
每个标本的扫描电镜图像被用于数字化特征点，数字化过程使用 TpsDig2 v. 2.25（Rohlf 2016a）进行。由于 Eohadrotreta 腹瓣的简单形态，我们采用特征点和半特征点或滑动特征点的组合，以分析同源点的位置以及测量在样本生物上的曲线（Zelditch 等人，2012年；Gunz 和 Mitteroecker，2013年；Mitteroecker 等人，2013年）。

使用说明：
附录 S1：来自中国南方万嘉平剖面 Cambrian Series 2 Shuijingtuo 形成的 Eohadrotreta zhenbaensis 腹瓣的形态学变异，以及来自中国南方 Aijiahe 和 Wangzishi 剖面的 E. zhenbaensis 和 P. zhujiahensis 的 Shuijingtuo 形成的比较，并标示特征性的薄板样条。所有薄板样条均对应于形态空间中 190 个标本的形状。瓣长以不同颜色表示。
附录 S2：所有 158 个 E. zhenbaensis 标本的成分、大小信息和 GPA 数据。/na 表示高度测量未获得。
附录 S3：E. zhenbaensis 每个特征点（1-38）的相对贡献。
附录 S4：与 E. zhenbaensis 标本中心体型大小相关的整体形态的多变量回归，涉及与 10,000 个重复的排列测试。
附录 S5：与 E. zhenbaensis 瓣高相关的整体形态的多变量回归，涉及与 10,000 个重复的排列测试。