Synchrotron radiation X-ray tomographic microscopy datasets for seed structure in Cycadeoidea morierei

The extinct plant group Bennettitales (Cycadeoidales) was a prominent part of Mesozoic vegetation. The group has been studied extensively and for more than 100 years it has been central in discussions of the relationships among the major groups of seed plants in which extinct plants were also considered. Despite much effort the phylogenetic position of the Bennettitales is still unresolved with a major obstacle being uncertainty about how to interpret the organisation and structure of the seed. The most widely accepted interpretation has been that the seed, like the seeds of most conifers, has a single integument around the nucellus and lacks any additional coverings. An alternative interpretation is that the seed integument is surrounded by an envelope comparable to the situation in extant Gnetales and extinct Erdtmanithecales. The petrified seed cone of Cycadeoidea (Bennettites) morierei described in the late nineteenth century from the Early Cretaceous of Normandy is one of the most informative bennettitalean reproductive structures with its superb preservation offering great potential for resolving the unsettled discussion regarding seed structure in Bennettitales.
C. morierei was carefully documented by Octave Lignier in his monumental study “Végétaux fossiles de Normandie. Structure et affinités du Bennettites morierei Sap. & Mar. (sp.)” from 1894, but despite his meticulous analysis of the fossil, Lignier left certain questions open in his interpretation and in his widely cited reconstruction of the structure of the seed. In the paper associated with the datasets provided here we have re-examined fragments of Cycadeoidea morierei using synchrotron X-ray tomographic microscopy (SRXTM) to attempt to answer some of these open questions.
Cycadeoidea morierei is based on a single petrified seed cone. The specimen was fractured by Lignier, who prepared several thin sections, in longitudinal as well as transverse directions in preparing his account. The original material was stored at the University of Caen, France, but most was lost during the second World War. However, several larger and smaller fragments of the seed cone were later recovered at the university and made available for the present study. Among the fragments studied using SRXTM were small isolated, abraded seeds and interseminal scales (e.g., fragments 6 and 8) and a few small groups of fractured seeds and interseminal scales still tightly packed together (e.g., fragment 17). Typically, these small fragments are broken at the base and only rarely are remains of the seed stalk preserved. One larger fragment (fragment 24) consists of several seeds with seed stalks intact. For comparison with Cycadeoidea morierei we also examined a fragment of another classic petrified seed cone, Cycadeoidea gibsoniana from the Early Cretaceous of isle of Wight, Southern England, using SRXTM. The results, combined with a comparative survey of other fossil material, indicate that the seeds of Bennettitales had at least one, perhaps two, envelopes covering the integument, comparable to what is seen among extant Gnetales, extinct Erdtmanithecales, and diverse fossil seeds recovered from Early Cretaceous mesofossil assemblages. Most of the SRXTM analyses of the Cycadeoidea material were carried during 2009-2023 on the TOMCAT beamline, SLS, Paul Scherrer Institute, Villigen, Switzerland. Fragment 24 was analysed at the new I-TOMCAT beamline at SLS in November 2025.