William G. Chaloner

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.

Evolution of leaf-form in land plants linked to atmospheric CO2 decline in the Late Palaeozoic era

The widespread appearance of megaphyll leaves, with their branched veins and planate form, did not occur until the close of the Devonian period at about 360 Myr ago. This happened about 40 Myr after simple leafless vascular plants first colonized the land in the Late Silurian/Early Devonian, but the reason for the slow emergence of this common feature of present-day plants is presently unresolved. Here we show, in a series of quantitative analyses using fossil leaf characters and biophysical principles, that the delay was causally linked with a 90% drop in atmospheric pCO2 during the Late Palaeozoic era. In contrast to simulations for a typical Early Devonian land plant, possessing few stomata on leafless stems, those for a planate leaf with the same stomatal characteristics indicate that it would have suffered lethal overheating, because of greater interception of solar energy and low transpiration. When planate leaves first appeared in the Late Devonian and subsequently diversified in the Carboniferous period, they possessed substantially higher stomatal densities. This observation is consistent with the effects of the pCO2 on stomatal development and suggests that the evolution of planate leaves could only have occurred after an increase in stomatal density, allowing higher transpiration rates that were sufficient to maintain cool and viable leaf temperatures.

Three growth in the Mesozoic and Early Tertiary and the reconstruction of palaeoclimates

Abstract Evidence from the distribution and characteristics of fossil wood in the Mesozoic and Early Tertiary indicates that a much warmer global climate prevailed in those times. There appears to have been a broad zone of largely non-seasonal climate stretching from about 32° N to 32° S (palaeolatitudes). In addition to this low-latitude zone, forest growth extended into very high palaeolatitudes where trees cannot grow at the present day. A number of theories have been proposed to account for the palaeoclimate responsible for this distribution of forests. Most notable have been those involving changes in the amount of carbon dioxide in the atmosphere, in the positions of the continents or in the obliquity of the earths axis of rotation. Evidence from fossil forests indicates that a combination of the effects of an increased quantity of carbon dioxide in the atmosphere and the palaeopositions of the continents in the Mesozoic and Early Tertiary appears at the moment to be the simplest explanation for the climate of those geological times, without the need to invoke axial movement.

Assessing the potential for the stomatal characters of extant and fossil Ginkgo leaves to signal atmospheric CO2 change

The stomatal density and index of fossil Ginkgo leaves (Early Jurassic to Early Cretaceous) have been investigated to test whether these plant fossils provide evidence for CO(2)-rich atmosphere in the Mesozoic. We first assessed five sources of natural variation in the stomatal density and index of extant Gingko biloba leaves: (1) timing of leaf maturation, (2) young vs. fully developed leaves, (3) short shoots vs. long shoots, (4) position in the canopy, and (5) male vs. female trees. Our analysis indicated that some significant differences in leaf stomatal density and index were evident arising from these considerations. However, this variability was considerably less than the difference in leaf stomatal density and index between modern and fossil samples, with the stomatal index of four species of Mesozoic Ginkgo (G. coriacea, G. huttoni, G. yimaensis, and G. obrutschewii) 60-40% lower than the modern values recorded in this study for extant G. biloba. Calculated as stomatal ratios (the stomatal index of the fossil leaves relative to the modern value), the values generally tracked the CO(2) variations predicted by a long-term carbon cycle model confirming the utility of this plant group to provide a reasonable measure of ancient atmospheric CO(2) change.

Fossil charcoal as an indicator of palaeoatmospheric oxygen level

Charcoal results from incomplete combustion of plant material. It is produced naturally by wildfire, and being relatively tough and unbiodegradable it may be transported and incorporated into a variety of sedimentary environments. Wildfire requires adequate atmospheric oxygen for the combustion of plant fuel (wood, leaf litter). Fossil charcoal from the Devonian onwards suggests that the oxygen level in the atmosphere has not fallen below 13% in this interval. Further, the extreme flammability of even wet plant fuel at oxygen levels above 35% makes this the highest figure compatible with the occurrence of terrestrial vegetation.

Stomatal Density Responds to the Glacial Cycle of Environmental Change

Examining the response of stomatal density to past changes in atmospheric CO2 concentration helps us to understand how plants adapted to palaeoenvironmental change, and so helps in predicting their response to future global environmental change. Stomatal density is an important physiological parameter that underpins the productivity of terrestrial vegetation by affecting both rate of carbon uptake and water use efficiency. Previous work on temperate tree species showed a decrease in stomatal density in response to a 60 p. p. m. v. increase in atmospheric CO2 concentration over the past 200 years. However, such a short timespan largely excludes the genetic component of plant adaptation to change in CO2 concentration. We present the first record of stomatal density from fossil leaves extending over 140 ka. Our record for the arctic-alpine dwarf shrub Salix herbacea L. extends back to the penultimate glacial stage, spanning two intervals when atmospheric CO2 concentration was considerably lower (by ca. 170 p. p. m. v.) than at present. Our results demonstrate a decrease in stomatal density in response to long-term increases in atmospheric CO2 concentration, and are in accordance with previous short-term observations and experiments. This implies that relaxation of the stress imposed by low atmospheric CO2 concentration has enabled terrestrial plants to exhibit an adaptive response to the other limiting factor of water availability by reducing stomatal density and hence improving water use efficiency.

The Earliest Fossil Conifer from the Westphalian B of Yorkshire

Small charred (fusainized) leaves and leafy shoots, prepared by maceration of an Upper Carboniferous (Westphalian B) shale from Yorkshire are illustrated and described as Swillingtonia denticulata gen. et sp. nov. Examination by s. e. m. has yielded details of stomatal structure and distribution, and some internal anatomy. The uncompressed nature of the epidermal and hypodermal cells, with their open lumina, we attribute to the rigidity of the charcoalified cell walls, that result from exposure to wild fire before fossilization. Most of the leaves are triangular-lanceolate with a denticulate margin and single vascular strand; but a few of the leaves fork, with accompanying dichotomy of the vascular strand. The stomata are sunken and paracytic, and form two bands on the lower (abaxial) face. The leafy shoots show superficial similarity to both conifers and lycopods, but resemble most closely and significantly the conifer Lebachia Florin. Swillingtonia is accordingly assigned to the Lebachiaceae of the Coniferales, and constitutes the earliest record of a conifer.

The Composition of Sporopollenin and its use in Living and Fossil Plant Systematics

Abstract Previous investigations of sporopollenin using 13C Solid State Nuclear Magnetic Resonance have demonstrated differences between the major groups of plants in the composition of this acetolysisresistant biomacromolecule. The work presented here corroborates these results and suggests that sporopollenin obtained from seed megaspore-membranes differs slightly from that of pollen from the same plant group. Spectra obtained using NMR have been subjected to multivariate analysis. This approach has provided information which may be interpreted in phylogenetic terms. A number of fossil sporopollenins have also been investigated. These all show considerable degradation through diagenesis but retain certain characteristics of the original sporopollenin composition. Microspores and megaspores from the same species of Carboniferous arborescent lycopsid have also been investigated with a view to discerning any differences in composition that may exist between these sources of sporopollenin in the fossil recor...

The fossil plant record and global climatic change

Abstract The generally sedentary character of terrestrial plants gives them a special dependence on their adaptation to the climate under which they live. As a consequence, plants normally show structural adaptations which are characteristic of their habitat, and fossil plants constitute particularly sensitive palaeoenvironmental indicators. In Quaternary pollen analysis the assumption is generally made that the species recognised as pollen had the same climatic constraints as their present-day representatives. As we go back through Tertiary time, and extant species become progressively rarer, we seek the nearest living relatives of the plant fossils as a basis for palaeoclimatic interpretation. This approach relies on the accuracy of the taxonomic assignment of the fossil material. Various ‘non-taxonomic’ features of Tertiary fossils have also been used in attempts to read a ‘palaeoclimatic signal’, independent of the correctness of the taxonomic assignment. These include most notably leaf physiognomy, and growth responses to seasonality such as growth rings in fossil wood. When we look to Palaeozoic plants, even leaf physiognomic features are of limited value, but fossil plants of this age can still give us significant information about their palaeoenvironment. The presence of charcoal (fusain) produced by wildfire puts a constraint on the level of oxygen in the palaeoatmosphere. Stomatal density and index may be used to give a proxy measure of palaeo-CO 2 levels. The realisation of the link between the carbon-dioxide greenhouse phenomenon and climate makes the use of stomatal data from fossil plants of particular relevance to palaeoclimatic interpretation. Our results from a study of stomatal index in plants from the devonian to Permian interval are consistent with evidence from physical sources of major changes in global CO 2 levels through that period.

Stomatal Density as an Indicator of Atmospheric CO2 Concentration

Examining the response of stomatal densities to changing atmospheric CO2 concentrations is an important element in understanding whole plant responses because it effects both water use efficiency and rate of carbon uptake. These two physiological processes have a major influence on the productivity of vegetation. Methods and sources of material that may be used for making stomatal counts are reviewed. The response of stomatal density to changing concentrations of atmospheric CO2 are considered from four types of investigation: 1) field studies on living plants growing at different altitudes; 2) studies on plants grown in controlled environments; 3) historical trends detected from herbarium material and 4) comparisons of historical records with modern material. The results suggest conflicting responses which could, to some extent, be due to methodological differences between workers. The more fundamental questions about the variability of stomatal density within and between species, the action of CO2 in controlling stomatal development, and whether observed responses represent acclimation or genetic adaptation are also discussed.