Seed structure in Cycadeoidea morierei and its significance for the relationship of Bennettitales and Gnetales

Cycadeoidea morierei is the species used most frequently to illustrate seed anatomy in Bennettitales, one of the most important and distinctive groups of Mesozoic seed plants. Re-examination of the single specimen of Cycadeoidea morierei using synchrotron radiation X-ray tomographic microscopy (SRXTM) provides new information on the cell layers surrounding the nucellus and demonstrates conclusively the presence of a thin integument enclosed by two seed envelopes. Comparison with other permineralized bennettitalean seeds shows that one or two envelopes are consistently present, although variably developed. In bennettitalean seeds preserved as compressions there is evidence of at least a single envelope, which is also seen in dispersed seeds of Erdtmanithecales and seeds of uncertain relationship from Early Cretaceous mesofossil floras. The characteristic chlamydospermous seed structure of Bennettitales is very similar to that of extant and fossil Gnetales, and especially seeds of Gnetum, in which the dispersal unit comprises the seed with its thin, single integument, surrounded by one or two more substantial envelopes. In Gnetales developmental and structural evidence indicates that the envelopes are modified bracts. That this is also the origin of the envelopes in Bennettitales is suggested by the quadrangular to pentangular form of Cycadeoidea morierei and similar seeds. The distinctively similar anatomy of bennettitalean and gnetalean seeds provides strong evidence that the two groups are closely related, an idea that has long been proposed but that has been controversial. Anatomical features of Cycadeoidea morierei including the recognition of chlamydospermous seed structure in Bennettitales and Gnetales favours the interpretation that bennettitalean seeds (and their sterile homologues, interseminal scales) are borne terminally on axes rather than on leaf homologues (sporophylls). Hence the bennettitalean seed head is multiaxial rather than uniaxial, which has important implications for understanding how bennettitalean reproductive structures should be compared with those of other seed plants.