1. Departments of Palaeobotany and Palaeozoology, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden
2. Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois 60637, USA
3. Department of Earth Sciences, University of Aarhus, DK-8000 Århus C, Denmark
4. Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK
5. Institute of Geosciences, Eberhard-Karls-University, D-72076 Tübingen, Germany
6. Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen, Switzerland

Correspondence to: Else Marie Friis¹Marco Stampanoni⁶ Correspondence and requests for materials should be addressed to E.M.F. (Email: else.marie.friis@nrm.se) or M.S. (Email: marco.stampanoni@psi.ch).

Over the past 25 years the discovery and study of Cretaceous plant mesofossils has yielded diverse and exquisitely preserved fossil flowers that have revolutionized our knowledge of early angiosperms¹, but remains of other seed plants in the same mesofossil assemblages^{2, 3} have so far received little attention. These fossils, typically only a few millimetres long, have often been charred in natural fires and preserve both three-dimensional morphology and cellular detail. Here we use phase-contrast-enhanced synchrotron-radiation X-ray tomographic microscopy to clarify the structure of small charcoalified gymnosperm seeds from the Early Cretaceous of Portugal and North America. The new information links these seeds to Gnetales (including Erdtmanithecales, a putatively closely related fossil group²), and to Bennettitales—important extinct Mesozoic seed plants with cycad-like leaves and flower-like reproductive structures. The results suggest that the distinctive seed architecture of Gnetales, Erdtmanithecales and Bennettitales defines a clade containing these taxa. This has significant consequences for hypotheses of seed plant phylogeny by providing support for key elements of the controversial anthophyte hypothesis, which links angiosperms, Bennettitales and Gnetales.

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