“DESCENT WITH MODIFICATION”, TRANSFORMATIONAL SERIES, AND PHYLOGENETIC ANALYSES TO INFER THE EVOLUTION OF MODERN CONIFER FAMILIES

Descent with Modification is perhaps Darwin's most insightful realization regarding biological evolution. This principle underlies the fundamental evolutionary paradigm of paleontology wherein transformational series of structures through time are employed to infer evolution and phylogenetic pattern. We apply these principles to the evolution of conifers. The origin of modern conifer families appears to have occurred during the Mesozoic, but more specific phylogenetic information has eluded detection because known Mesozoic conifer fossils are inadequate for developing whole-plant species concepts from which a large number of systematic characters can be scored. Phylogenetic analyses of living species infer that Pinaceae is the sister to all other living conifers, but Pinaceae does not appear in the fossil record until the Cretaceous, whereas representatives of some other families have been described from Triassic and Jurassic sediments. We tested the hypothesis that conifer seed cones display enough morphological/anatomical characters to be used as surrogates for whole plants in phylogenetic analyses. A matrix of 21 living genera from all families and 115 characters recovered the expected familial groupings of genera, thus demonstrating that characters of seed cones convey a strong phylogenetic signal and contribute strongly to their systematic distinction. With the addition of 13 extinct species ranging from Pennsylvanian to Eocene, preliminary analysis yielded a well resolved strict consensus tree in which Pennsylvanian-Triassic fossils are attached to the stem at the base of the tree and Mesozoic and living taxa form two clades with a topology that is surprisingly comparable to trees of living species. Cheirolepidiaceae + Pinaceae form a clade that is sister to a clade comprising all other living conifers. Moreover, the arrangement of taxa on the tree is highly congruent with the age of the fossils, thus demonstrating that seed cone characters have excellent potential for resolving the overall pattern of conifer phylogeny, for clarifying sister group relationships of Pinaceae, and for providing clade age dates for the origin of modern families.