Implications of fossil conifers for the phylogenetic relationships of living families

Fossils have played a central role in our understanding of the evolution of conifers. Interpretation of the seed cone as a compound strobilus and the homologies of the ovuliferous scales of modern conifers with the axillary dwarf shoot of Pennsylvanian forms are based on fossils. Similarly, early evolutionary trends involving the reduction, fusion, and planation of the fertile and sterile elements of the axillary dwarf shoot, leading to structures characteristic of modern families, are documented in Late Permian and Triassic conifers. However, a phylogeny elucidating the derivation of modern families from fossil forms based on shared derived features has been elusive. The present cladistic treatment using 11 characters of ovulate cones and one of pollen grains suggests three phylogenetic groups of Late Paleozoic conifers, represented loosely by the Emporicaceae, Utrechtiaceae, and Majonicaceae of Mapes and Rothwell. The Taxaceae appears to have diverged from ancestors within the Utrechtiaceae, whereas the other modern families owe their origins to the Majonicaceae. The origin of the Taxodiaceae appears to have been biphyletic.Taxodium, Cupressus andSciadopitys are strongly linked toDolmitia of the Majonicaceae, butCryptomeria, Cunninghamia andAraucaria are grouped together and diverge basal to the former taxa.Pinus branches from a position basal to the known genera of the Majonicaceae and all modern families except the Taxaceae.Podocarpus also diverges basal toMajonica but may share an ancestor with this genus;Cepahalotaxus diverges basal to theDolmitiaPseudovoltzia subclade but distal toMajonica. Similarly, the Cheirolepidiaceae originated from basal members of the Majonicaceae and shows no close phylogenetic relationship with any modern family. Except for a strong linkage betweenCycadocarpidium and theAraucariaCunninghamia subclade, genera of the Voltziaceae appear to have branched more or less independently from within the Majonicaceae and show no strong affinity with modern conifers. Thus differences between modern conifer families are due mainly to their divergence from different Paleozoic ancestors.