Were all Devonian seeds cupulate? A reinvestigation of Pseudosporognites hallei, Xenotheca bertrandii and Aglosperma spp.

• Premise of the research. Although the most comprehensively known Devonian seeds were borne in a ‘telomic’ cupule, some species have been postulated as being borne terminally on naked axes lacking a cupule. Uncertainty remains, however, if such seeds were shed from a cupule prior to preservation. We reinvestigate the Upper Devonian fossils Pseudosporogonites hallei and Xenotheca bertrandii from Belgium and the similar ovules Aglosperma quadrapartita and A. avonensis from Britain and A. sp. from North America to consider their structure and organisation, and to determine if they were cupulate.
• Pivotal results. Pseudosporogonites hallei, Xenotheca bertrandii and Aglosperma quadrapartita comprise individual ovules borne within a uni-ovulate cupule, with most specimens shed from the cupule. Cupules are small, radially symmetrical and surround the ovule pedicel and chalaza. Integument are entire at the chalaza but form flattened lobes distally, with the size and shape of lobes varying between species. While a cupule is unknown in Aglosperma avonensis its comparable integument morphology suggests it was shed from a uni-ovulate cupule. All species possess a domed pollen chamber with a tubular salpinx characteristic of hydrasperman-type reproduction. Although distinct from each other, similarity between the species shows they are closely related and belong to a single genus for which Pseudosporogonites has priority. We emend P. hallei in light of our findings and erect the combinations P. bertrandii (Stockmans) Prestianni et al., P. quadrapartitus (J. Hilton et D. Edwards) J. Hilton et al. and P. avonensis (J. Hilton) J. Hilton et al.
• Conclusions. The uni-ovulate cupule in Pseudosporogonites is distinct from multi-ovulate ‘telomic’ cupules of other Devonian seeds and expands the phenotypic diversity seen during the earliest phase of seed plant radiation that was geological instantaneous. Hydrasperman pollination in all proven Devonian seeds demonstrates evolution from a common ancestor, but finding morphological intermediates between seed and free-sporing plants remains a significant challenge to evolutionary plant biology.