Peter R. Crane

Angiosperm Diversification and Paleolatitudinal Gradients in Cretaceous Floristic Diversity

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(∼20�N to 20�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(∼50�N to 20�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.

The Origins of angiosperms and their biological consequences

List of contributors Preface 1. Introduction to angiosperms E. M. Friis, W. G. Chaloner and P. R. Crane 2. The origin of angiosperms J. A. Doyle and M. J. Donoghue 3. Global palaeogeography and palaeoclimate of the Late Cretaceous and Early Tertiary Judith Totman Parrish 4. Mid-Cretaceous to Early Tertiary vegetation and climate: evidence from fossil leaves and woods G. R. Upchurch, Jr and J. A. Wolfe 5. Vegetational consequences of angiosperm diversification P. R. Crane 6. Time of appearance of floral features E. M. Friis and W. L. Crepet 7. The evolution of insect pollination in angiosperms W. L. Crepet and E. M. Friis 8. Interactions of angiosperms and herbivorous tetrapods through time S. L. Wing and B. H. Tiffney 9. Dinosaurs and land plants M. J. Coe, D. L. Dilcher, J. O. Farlow, D. M. Jarzen and D. A. Russell 10. Vegetational and mammalian faunal changes in the Early Tertiary of southern England M. E. Collinson and J. J. Hooker Classification of plants and animals Stratigraphic table Glossary Index.

Rate heterogeneity among lineages of tracheophytes: Integration of molecular and fossil data and evidence for molecular living fossils

Many efforts to date evolutionary divergences by using a molecular clock have yielded age estimates that are grossly inconsistent with the paleontological evidence. Such discrepancies often are attributed to the inadequacy of the fossil record, but many potential sources of error can affect molecular-based estimates. In this study, we minimize the potential error caused by inaccurate topology and uncertain calibration times by using a well-supported tree, multiple genes, and multiple well-substantiated dates to explore the correspondence between the fossil record and molecular-based age estimates for major clades of tracheophytes. Age estimates varied because of gene effects, codon position, lineage effects, method of inferring branch lengths, and whether or not rate constancy was assumed. However, even methods designed to ameliorate the effects of rate heterogeneity among lineages could not accommodate the substantially slower rates observed in Marattia + Angiopteris and in the tree ferns. Both of these clades of ferns have undergone dramatic decelerations in their rates of molecular evolution and are “molecular living fossils,” consistent with their relative morphological stasis for the past 165–200 million years. Similar discrepancies between the fossil record and molecular-based age estimates noted in other studies may also be explained in part by violations of rate constancy among lineages.

The 2010 challenge: data availability, information needs and extraterrestrial insights.

At the 2002 Johannesburg World Summit on Sustainable Development, 190 countries endorsed a commitment to achieve, by 2010, a significant reduction of the current rate of biodiversity loss at the global, regional and national levels. A wide range of approaches is available to the monitoring of progress towards this objective. The strengths and weaknesses of many of these approaches are considered, with special attention being given to the proposed and existing indicators described in the other papers in this issue. Recommendations are made about the development of indicators. Most existing and proposed indicators use data collected for other purposes, which may be unrepresentative. In the short term, much remains to be done in expanding the databases and improving the statistical techniques that underpin these indicators to minimize potential biases. In the longer term, indicators based on unrepresentative data should be replaced with equivalents based on carefully designed sampling programmes. Many proposed and existing indicators do not connect clearly with human welfare and they are unlikely to engage the interest of governments, businesses and the public until they do so. The extent to which the indicators already proposed by parties to the Convention on Biological Diversity are sufficient is explored by reference to the advice an imaginary scientific consultant from another planet might give. This exercise reveals that the range of taxa and biomes covered by existing indicators is incomplete compared with the knowledge we need to protect our interests. More fundamentally, our understanding of the mechanisms linking together the status of biodiversity, Earth system processes, human decisions and actions, and ecosystem services impacting human welfare is still too crude to allow us to infer reliably that actions taken to conserve biodiversity and protect ecosystem services are well chosen and effectively implemented. The involvement of social and Earth system scientists, as well as biologists, in collaborative research programmes to build and parameterize models of the Earth system to elucidate these mechanisms is a high priority.

Phase-contrast X-ray microtomography links Cretaceous seeds with Gnetales and Bennettitales

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.

Comparing palynological abundance and diversity: implications for biotic replacement during the Cretaceous angiosperm radiation

The Cretaceous radiation of angiosperms initiated a major reorganization of terrestrial plant communities as dominance by pteridophytic and gymnospermic groups eventually gave way to dominance by angiosperms. Previously, patterns of biotic replacement have been assessed using measures based on taxonomic diversity data. However, using measures of both abundance and diversity to investigate replacement patterns provides more information about macroecological change in the fossil record than either can provide alone. Analyses of an updated and expanded database of North American palynological samples from Cretaceous sediments document a rapid increase in angiosperm diversity and abundance within individual fossil palynofloras (represent- ing local/subregional vegetation). New analyses of floristic diversity patterns support previous results and indicate that the decline of free-sporing plants is more pronounced than the decline of gymnosperms. In contrast, analyses of abundance data appear to show that the decline of gym- nosperms is far more pronounced than the decline of free-sporing plants. Detailed examination of both data sets segregated by paleolatitude shows that this apparent contradiction reflects biogeo- graphical differences in the patterns of vegetational change (e.g., free-sporing plants declined in abundance at lower latitudes) as well as sampling bias (e.g., greater sampling in the northern region in the Late Cretaceous). Analyses accounting for these biases support the conclusion that as an- giosperms radiated, free-sporing plants rather than gymnosperms (in this case, mainly conifers) experienced the most pronounced decline. A thorough understanding of the Cretaceous radiation of angiosperms will require both abundance and diversity data. It also will require expanding the analyses presented here into other geographic regions as well as sampling more completely at all spatial scales.

Hemisphere-scale differences in conifer evolutionary dynamics

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.

Mapping tree density at a global scale

The global extent and distribution of forest trees is central to our understanding of the terrestrial biosphere. We provide the first spatially continuous map of forest tree density at a global scale. This map reveals that the global number of trees is approximately 3.04 trillion, an order of magnitude higher than the previous estimate. Of these trees, approximately 1.30 trillion exist in tropical and subtropical forests, with 0.74 trillion in boreal regions and 0.66 trillion in temperate regions. Biome-level trends in tree density demonstrate the importance of climate and topography in controlling local tree densities at finer scales, as well as the overwhelming effect of humans across most of the world. Based on our projected tree densities, we estimate that over 15 billion trees are cut down each year, and the global number of trees has fallen by approximately 46% since the start of human civilization.

Reproductive Structure and Organization of Basal Angiosperms from the Early Cretaceous (Barremian or Aptian) of Western Portugal

A survey of five mesofossil floras from the Early Cretaceous (Barremian or Aptian) of Portugal documents considerable taxonomic and structural diversity among early angiosperms. Currently ca. 140–150 different angiosperm taxa have been identified in these floras, of which ca. 85% are taxa at the magnoliid grade—perhaps also with some basal monocots. Evidence is beginning to emerge that those lineages identified as basal among angiosperms in phylogenetic analyses (recently referred to as the ANITA grade of basal magnoliids, including Amborellaceae, Nymphaeales, Illiciales, Trimeniaceae, and Austrobaileyaceae) were already well represented in the Early Cretaceous, at which time they were probably more diverse than they are today. Many of the fossil angiosperms from the Portuguese floras cannot be assigned to any existing group, and there is clear evidence of significant extinction.

Patterns of floral evolution in the early diversification of non-magnoliid dicotyledons (eudicots)

Recent cladistic analyses of angiosperms based on both morphological and molecular sequence data recognize a major clade of dicotyledons defined by triaperturate or triaperturate-derived pollen (non-magnoliids/eudicots). Evidence from morphology, as well as the atpB and rbcL genes (cpDNA), indicates that extant Ranunculidae (e.g., Papaverales, Lardizabalaceae, Berberidaceae, Menispermaceae, Ranunculaceae) as well as “lower” Hamamelididae [e.g., Eupteleaceae (allied to Ranunculidae), Hamamelidaceae, Myrothamnaceae, Platanaceae, Trochodendraceae] and several other families (e.g., Gunneraceae, Nelumbonaceae, Proteaceae, Sabiaceae) are basal in this group. The earliest records of diagnostic eudicot pollen are of mid-late Barremian age (c. 126myr BP) and by around the latest Albian (c. 97 myr BP) several basal eudicot groups (e.g., Trochodendrales, Platanaceae, Buxaceae, and perhaps Circaeasteraceae, Myrothamnaceae, and Nelumbonaceae) are recognizable in the fossil record. Possible Hamamelidaceae and perhaps Proteaceae are present by the Turonian (c. 90 myr BP). Among basal eudicots, flowers are generally bisexual although unisexual flowers are also common. In some groups (e.g., Myrothamnaceae, Buxaceae, certain Berberidaceae), delimitation of the flower is not always clear and there is a more or less gradual transition between tepals and inflorescence bracts. Plasticity in floral form at this level of angiosperm evolution is predominantly encompassed by dimerous and trimerous cyclic floral organization and transitions from one to the other are common. Spiral floral phyllotaxis of numerous stamens and carpels is more or less restricted to the Ranunculaceae. The basic condition of the perianth in eudicots appears to lack differentiation into sepals and petals, and petals appear to have arisen independently numerous times from stamens. Based on the generality of its systematic distribution, cyclic floral architecture is probably basic for eudicots as a whole, and at this level of angiosperm evolution flowers with numerous, helically-arranged stamens and/or carpels (e.g., many Ranunculaceae) almost certainly reflect processes of secondary multiplication that have occurred independently many times.