Biogeochemical evidence for the presence of the angiosperm molecular fossil oleanane in Paleozoic and Mesozoic non-angiospermous fossils

摘要

Recent molecular phylogenetic and molecular clock data both suggest a pre-Mesozoic age for the divergence of the angiosperm lineage from other seed plants, greatly predating the confirmed fossil record of the angiosperm crown group. In addition, molecular phylogenetic studies have not supported the morphologically based conclusion that gnetophytes are the extant sister group to angiosperms. We examine these relationships and divergence ages by using a novel approach of examining the presence of oleanane. This includes the development of methods using zeolites to preferentially reduce hopanes that can co-elute with oleanane. The presence of this molecular fossil strongly correlates with angiosperm diversification; in its functionalized form, along with its triterpenoid precursors, it is found in many living angiosperms. Our data show that among non-angiosperm seed plants examined thus far, oleanane is found only in fossil Cretaceous Bennettitales and Permian Gigantopteridales, both of which share characteristics with angiosperms. Previous morphological phylogenetic results indicate Bermettitales Could be a sister group to or member of the angiosperm stem lineage, and results of our preliminary phytogenetic analysis including the Gigantopteridales suggests the same. Our data, based on a new pyrolysis method to treat living species, support previous research indicating that oleanane and its precursors are absent in living gnetophytes. If oleanane originated once in seed plants then the angiosperm stem lineage would have diverged from other seed plant lineages by the late Paleozoic.