A transitional-combinational theory for the origin of angiosperms

The origin of the angiosperms has long remained problematical. Recent new discoveries of early angiosperm fossils and many new molecular phylogenies have helped to identify basal extant angiosperms and to infer their inter-relationships. Still unclear, however, are ties with gymnosperms, both living and fossil. In view of difficulty of finding ancestors for angiosperms, yet also considering their sudden appearance and explosive evolutionary success, a transitional-combinational theory is proposed. This theory suggests that angiosperms evolved slowly from seed ferns in the Jurassic beginning first with the carpel, followed later by double fertilization, and lastly by the appearance of flowers. These three fundamental transitions may have taken more than 100 million years to complete. The extant (or modem) angiosperms did not appear until Early Cretaceous, as attested by micro- and macrofossils, when the final combination of all three important angiosperm features occurred. This combination provided the opportunity for explosive evolutionary diversification, especially in response to selection from insect pollinators and predators, plus attendant modifications in compatibility and breeding systems. The transitional-combinational theory explains some of the discrepancy between fossil and molecular phylogenetic data, the latter of which suggests that angiosperms originated pre-Cretaceous. DNA (and protein) sequence data and oleananes reflect ancestry in an earlier transitional carpellate lineage (or lineages), much earlier than appearance of the final combination that includes floral and associated pollen modifications. The theory Suggests viewing all gymnosperms, other than extinct seed ferns from which carpels arose, as having had no direct phylogenetic connections to modem angiosperms.