J. Plant Res. 107 : 479-492, 1994 JPR Symposium Journal of Plant Research (~) by The Botanical Society of Japan 1994 Morphology and the Evolution of Cycadeoidales Harufumi Nishida International Budo University, 841 Shinkan, Katsuura, Chiba, 299-52 Japan One Cycadeoidea stem, one cycadeoidalean gynoecium and a bisporangiate cone attached to a slen- der cycadeoidalean trunk Cycadeoidella japonica Ogura from the Cretaceous of Japan shows well-preserved internal structure that provides evidence for a better understanding of the morphological architecture of the cycadeoidalean plant. Structural details of the cone were confirmed. The ovule has an integument enclosing a free nucellus and a thin outer envelope. Both repro- ductive and vegetative structures support the medullosan affinity of Cycadeoidales. The cone is interpreted as a compressed fertile shoot. Axillary cones characterizing some Cretaceous genera such as Cycadeoidea and Monanthesia consist of a lateral shoot subtended by a frond that is the first leaf of the cone shoot itself. The origin of axillary buds in the Cycadeoidales is discussed. Heterochrony may have mediated the morphological changes that resulted in the establishment of the Cycadeoidales. Key words : Cycadeoidales-- Evolution -- Gymno- sperms -- Medullosales -- Morphology Cycadeoidales (Bennettitales) are Mesozoic Gymnosperms that appeared in the Late Triassic and became extinct at the end of the Cretaceous. Due to earlier extensive studies, Cycadeoidales represent one of the best-known taxa of Mesozoic plants. They are vegetatively similar to Cycadales but their reproductive organs that are reproductive cones contrast markedly and makes them appear distantly related to the Cycadales. The presence of axillary cones in some Cretaceous forms such as Cycadeoidea and Monanthesia poses a question concerning their morphological origin (Delevoryas 1968ab). A possible ancestor of the Cycadeoidales can be found in the Paleozoic Medullosales, based mainly on vegeta- tive similarities in wood structure and vascular bundle branching patterns (Stewart and Rothwel11993, Taylor and Taylor 1993). However, there is a large morphological gap between the two fossil groups. In the Cyca- deoidales, the stem is well-differentiated anatomically from the leaf which is basically a pinnate frond. The stem is eustelic producing solid leaf traces in dense spirals with leaf gaps. The medullosan stem is polystelic and less-differentiated anatomically from the pinnately compound frond-like phyllophore which has a large num- ber of traces. Traces derive sympodially from the stem vascular bundles without forming gaps (Stewart and Rothwell 1993). The medullosans also lack reproductive cones. These modifications must have caused by changes of genes that regulated the ontogeny at the shoot apex. Though the Medullosales and Cycadeoidales are dis- tantly located in recent cladistic analyses (Crane 1985, 1988, Doyle and Donoghue 1986), the Medullosales are the most probable Paleozoic ancestor of the Cycadeoidales based on anatomical studies. Detailed comparative morphological studies of the two groups are necessary to clarify how the cycadeoidalean apomorphies have evolved. Some studies of the cones propose the Cycadeoidales as a probable angiosperm ancestor (Crane 1985, 1986). Such discussions concerning the origin and evolution of Cycadeoidales, however, need affirmative morphological evidence based on well-preserved specimens. The recent discovery of beautifully preserved permineraliza- tions of cycadeoidalean plants from the Cretaceous of Japan has provided evidence for undrestanding better the morphology of their vegetative and reproductive organs. In this paper I strengthen the medullosan affinity of Cycadeoidales and try to reconstruct the morphological evolution of the cycadeoidalean cones in relation to the modification of vegetative structures. An ontogenetic pathway that may be responsible for the establishment of cycadeoidalean morphology also is considered. Materials and Methods Two fructifications and one stem from the Cretaceous of Japan, identified as belonging to the Cycadeoidales, provide morphological information that allows compari- sons with earlier described cycadeoidalean fossils and provides for further discussions (Table 1). The fructifica- tion of Cycadeoidella japonica Ogura (Ogura 1930) was preliminarily described by Nishida (1991). The other spec- imens were not specifically identified because of their incomplete preservation. All the specimens will be fully described elsewhere. The specimens are all calcified so that peel sections were made using hydrochloric acid (Joy et al. 1954) The peel micropreparations and original speci- mens are housed in the Laboratory of Phylogenetic Botany, Faculty of Science, Chiba University. The acqui- sition numbers are shown in Table 1.