JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Missouri Botanical Garden Press is collaborating with JSTOR to digitize, preserve and extend access to Annals of the Missouri Botanical Garden. ABSTRACT Principles of phylogenetic analysis (cladistics) are introduced with an examination of relationships among extant genera of Gnetales. The Gnetales can be supported as a monophyletic group, with Gnetum and Welwitschia more closely related to each other than either is to Ephedra. Characters of the progymnosperm Archaeopteris and 19 extinct and extant seed plant taxa are then reviewed as a basis for a cladistic analysis of their interrelationships. The seed plant taxa included are: Bennettitales are also presented. Results suggest that seed plants are a monophyletic group, and Lyginopteris is resolved as the sister taxon to all other seed plants considered. The cordaites, Cordaixylon and Mesoxylon, along with Lebachia, extant conifers, and Ginkgo constitute a monophyletic group. Pentoxylon is the sister taxon to Bennettitales, and the Gnetales are the sister group to angiosperms. Together the Gnetales plus the angiosperms form the sister group to the Bennettitales plus Pentoxylon. If the outer integument of bennettitalean and angiosperm ovules is interpreted as homologous with the "cupule" of Caytonia and corystosperms, then the corystosperms are resolved as the sister group to the Bennettitales plus Pentoxylon plus Gnetales plus angiosperm clade. Under this interpretation all the seed plant taxa considered except Lyginopteris, cycads, and medullosans are part of a single clade in which flattened seeds and saccate pollen are primitive. The principal difficulties with the cladistic analysis concern necessary inferences on unknown characters in certain plants, and the current absence of a large base of comparative data. Results of the analysis suggest that the seed ferns as currently circumscribed are not a meaningful group for phylogenetic purposes and permit an evaluation of the possible phylogenetic position of Eremopteris, Nystroemia, Spermopteris, Phasmatocycas, Vojnovskiales, Leptostrobus, and several other enigmatic groups of plant fossils. Comparison of the phylogenetic analysis with previous theories of angiosperm origin shows that it reconciles the ideas of Arber and Parkin, that Bennettitales and Gnetales are closely related to flowering plants, with more recent hypotheses that invoke Caytonia and corystosperms as highly relevant to the angiosperm problem. The results suggest that increased understanding of Triassic Bennettitales, Gnetales, and corystosperms will be of maximum interest in further elucidating the phylogenetic relationships of flowering plants.' This paper is dedicated to the late Dr. P. D. W. Barnard, who...

Fundamental differences in the distribution of oceans and landmasses in the Northern and Southern Hemispheres potentially impact patterns of biological diversity in the two areas. The evolutionary history of conifers provides an opportunity to explore these dynamics, because the majority of extant conifer species belong to lineages that have been broadly confined to the Northern or Southern Hemisphere during the Cenozoic. Incorporating genetic information with a critical review of fossil evidence, we developed an age-calibrated phylogeny sampling ∼80% of living conifer species. Most extant conifer species diverged recently during the Neogene within clades that generally were established during the later Mesozoic, but lineages that diversified mainly in the Southern Hemisphere show a significantly older distribution of divergence ages than their counterparts in the Northern Hemisphere. Our tree topology and divergence times also are best fit by diversification models in which Northern Hemisphere conifer lineages have higher rates of species turnover than Southern Hemisphere lineages. The abundance of recent divergences in northern clades may reflect complex patterns of migration and range shifts during climatic cycles over the later Neogene leading to elevated rates of speciation and extinction, whereas the scattered persistence of mild, wetter habitats in the Southern Hemisphere may have favored the survival of older lineages.

The latitudinally diachronous appearance of angiosperm pollen during the Cretaceous is well documented, but the subsequent diversification and accompanying significant changes in floristic dominance have not been assessed quantitatively for a wide range of paleolatitudes. Trend surfaces fitted to within-palynoflora diversity data from 1125 pollen and spore assemblages show that angiosperms first become floristically prominent in low paleolatitude areas( approximately 20 degrees N to 20 degrees S). Non-magnoliid dicotyledons show a similar but slightly delayed pattern of increase and are the principal component of angiosperm diversity from all areas sampled. Monocotyledons and magnoliid dicotyledons are significant primarily in low to middle paleolatitude palynofloras( approximately 50 degrees N to 20 degrees S) during the latest Cretaceous. As angiosperms become increasingly prevalent the importance of most non-angiosperm taxa either decreases or remains unchanged. The only apparent exception is a striking increase in gnetalean diversity concurrent with the initial angiosperm diversification at low paleolatitudes.

Over the past 25 years the discovery and study of Cretaceous plant mesofossils has yielded diverse and exquisitely preserved fossil flowers that have revolutionized our knowledge of early angiosperms, but remains of other seed plants in the same mesofossil assemblages have so far received little attention. These fossils, typically only a few millimetres long, have often been charred in natural fires and preserve both three-dimensional morphology and cellular detail. Here we use phase-contrast-enhanced synchrotron-radiation X-ray tomographic microscopy to clarify the structure of small charcoalified gymnosperm seeds from the Early Cretaceous of Portugal and North America. The new information links these seeds to Gnetales (including Erdtmanithecales, a putatively closely related fossil group), and to Bennettitales--important extinct Mesozoic seed plants with cycad-like leaves and flower-like reproductive structures. The results suggest that the distinctive seed architecture of Gnetales, Erdtmanithecales and Bennettitales defines a clade containing these taxa. This has significant consequences for hypotheses of seed plant phylogeny by providing support for key elements of the controversial anthophyte hypothesis, which links angiosperms, Bennettitales and Gnetales.