Macroscale rangeomorph fossils, with characteristic branching fronds, appear (571 Myr ago) after the Gaskiers glaciation (580 Myr ago). However, biological mechanisms of size growth and potential connections to ocean geochemistry were untested. Using micro-computerized tomography and photographic measurements, alongside mathematical and computer models, we demonstrate that growth of rangeomorph branch internodes declined as their relative surface area decreased. This suggests that frond size and shape were directly responsive to nutrient uptake.
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Specimen number ROM 63005 was loaned by the Royal Ontario Museum with the permission of the Rooms Museum, Newfoundland. CT-scanning was conducted by R. Asher at the Cambridge Biotomography Centre. Access to the specimen of Fig. 2d was provided at the South Australian Museum by J. Gehling and M.-A. Binnie, who found this fossil. This research was funded by an ELSI Origins Network (EON) Research Fellowship to J.F.H.C., supported by a grant from the John Templeton Foundation, and Palaeontological Association Research Grant number PA-RG201501 (J.F.H.C.). We thank N. Butterfield, A. Caulton and E. Smith for discussion of the manuscript.