Cercidiphyllum and Fossil Allies: Morphological Interpretation and General Problems of Plant Evolution And Development

Cercidiphyllum is a relict angiosperm bringing to us a flavor of Cretaceous Period. Its reproductive morphology was interpreted, in the spirit of the dominant evolutionary paradigm, as inflorescences of reduced flowers represented by solitary pistils and groups of stamens. Evolutionary significance of Cercidiphyllum has long been anticipated despite the unpersuasive morphological interpretation and irrelevant paleobotanical evidence. This work was initially intended for paleobotanists who willingly compare their fossil material with the living Cercidiphyllum, using schematic descriptions and illustrations of traditional plant morphology. The idea behind this book was to provide an adequately illustrated material for such comparisons. Yet it turned out that there are more things in Cercidiphyllum than are dreamed of in our traditional plant morphology. Some of morphological findings of this study, such as the replacement of the floral structure by leafy shoots or the subtending bract-leaf conversion are relevant to the experimental "evo-devo" studies and bear on general problems of angiosperm evolution. In Cercidiphyllum, the vegetative body is partly or mostly produced in the reproductive line, suggesting a neotenic ancestral form, in which the vegetative development was drastically reduced relative to reproductive domain. Later in evolutionary history, the vegetative body was rebuilt in the lineage on the way to arboreal forms, with foliation of the floral receptacles and the carpel-leaf convergence as side effects of the process. Neoteny, or precocious fertility, might have required developmental machinery with ambivalent regulatory gene expressions in the foliar and floral primordia, invoking two opposite evolutionary tendencies: (1) reduction and elimination of sterile organs from the apical zone and (2) secondary foliation of floral structures. The ambiguous floral morphology of Cercidiphyllum is here interpreted as a long-standing balance of these conflicting tendencies. With regard to this model, the fossil record of Cercidiphyllum-like plants appears meaningful, although still incomplete. The weedy Early Cretaceous ranunculids, including forms with a few basal leaves, represent the neotenic stage, also recognizable in the evolution of their contemporaneous anthognetophytes. Transitional floral morphologies link such forms to the later appearing cercidiphyllids, platanoids and trochodendrocarpoids, the vegetative features of which were formed at the next, amplification stage, represented by the Eocercidiphyllites plant from the mid- Cretaceous of the Negev, and were conserved through the later evolutionary developments.

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