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.

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).

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.