**Data accessibility.** Scans of *Milleropsis* specimen BP/1/720 are available on Morphosource (https://www.morphosource.org/concern/media/000125334?locale=en).

**Declaration of AI use.** We have not used AI-assisted technologies in creating this article.

**Authors' contributions.** X.A.J.: conceptualization, formal analysis, investigation, methodology, resources, visualization; R.B.J.B.: conceptualization, investigation, resources, writing—original draft, writing—review and editing; D.P.F.: supervision, writing—original draft, writing—review and editing; C.B.: resources; V.F.: resources; E.G.: visualization; J.C.: conceptualization, resources, supervision, writing—original draft, writing—review and editing; B.R.P.: conceptualization, resources, software, supervision, writing—original draft, writing—review and editing.

All authors gave final approval for publication and agreed to be held accountable for the work performed therein.

**Conflict of interest declaration.** We declare that we have no competing interests.

**Funding.** This study was partially funded by National Research Foundation of South Africa and its African Origins Platform Grant GUN to J.C. (no. 136516).

**Acknowledgements.** We are indebted to Sifelani Jirah and Bernhard Zipfel at ESI, Zaituna Skosan at the ISAM and Elize Butler, Jennifer Botha and Will Archer for assistance in accessing specimens in their care. We acknowledge the European Synchrotron Radiation Facility (ESRF) for provision of synchrotron radiation facilities under proposal no. ES823 and we would like to thank the beamline staff for assistance and support in using beamline ID19. We thank the National Research Foundation of South Africa and its African Origins Platform Grant. We deeply appreciate Gabriela Sobral and two anonymous reviewers for thorough reviews of this manuscript.

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