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Ancient carbon cycles and carbon-dioxide
Fossil plant data are proving to be invaluable for estimating past carbon-dioxide levels and providing insights on the functioning of ancient carbon cycles. Paleontological data for the diversity of marine animals and land plants was integrated elegantly with a concurrent measure of stable carbon-isotope fractionation for the last 400 million years (D.H. Rothman, Proceedings of the National Academy of Sciences, USA, v. 98, p. 4305). Rothman's study suggests that long-term fluctuations of carbon dioxide levels can be linked to complementary changes in the biological and fluid reservoirs of carbon, and illustrates the value of the paleontological records of biodiversity for estimating fluctuations of ancient of carbon dioxide levels.
Paleobotanists continue to use analyses of stomatal density (SD) or stomatal index (SI) from fossil leaves as indicators of paleoatmospheric carbon dioxide concentration. The reliability of these methods was assessed by D.L. Royer (Review of Palaeobotany and Palynology, v.114, p.1) and SD-based estimations, which are strongly influenced by environmental stresses, were found likely to be less accurate than SI-based carbon-dioxide reconstructions. Furthermore, Royer's work casts some doubt on the belief that stomata are unable to respond to carbon-dioxide concentrations above present-day levels. Clearly paleobotany will play a significant role in interpreting not only plant responses to changes of carbon dioxide levels in the past but in predicting the consequences of future fluctuations in carbon dioxide levels.
Once again, the year 2001 found many paleobotanists focusing on plant-insect interactions. The role of paleobotany in helping to elucidate interactions within paleocommunities was well illustrated by several contributions made in a topical session entitled "New Frontiers in the Fossil Record of Insects and Terrestrial Arthropods" during the Annual Meeting of the Geological Society of America in Boston. Sara Lubkin (Cornell University) reported on a well-preserved Turonian-age insect assemblage associated with a diverse array of angiosperm flowers. This material has the potential to provide a valuable glimpse of plant-insect interactions during the Cretaceous. During the same session, Conrad Labandeira (National Museum of Natural History) presented presence-absence data for 51 plant-insect associations on 13,441 fossil plant specimens from the K/T boundary in southwestern North Dakota. Labandeira's data show a primary and simultaneous demise of both plants and their herbivores at the K/T boundary. This is in marked contrast to the Paleocene-Eocene boundary that is characterized by no net loss of associations.
Wilf and colleagues (Proceedings of the National Academy of Sciences, USA, v. 98, p. 6221) examined insect damage on fossil leaves from the central Rocky Mountains and documented herbivore responses to regional climate and vegetation change during the late Paleocene through the middle Eocene. The significance of this study is that on a regional level it appears that climate and plant defense had linked effects on herbivory. In general, a significant component of insect species avoided evergreen taxa and favored more palatable, short-lived foliage. Very few botanists will argue that insects were not a driving force influencing plant evolution. Ongoing paleobotanical work will continue to assess insect and plant interactions in the context of both paleoecological and plant evolutionary studies.
Early land plants and the
Recent studies have extended the record of evidence for early terrestrial plants back to the Ordovician and have provided new information about Devonian plant systematics and diversity. A fine synthesis of our current knowledge of early land plants in the context of chemical, geological, and physical data is presented through a series of 13 papers published in Plants Invade the Land (Columbia University Press, 2001), edited by P. Gensel (USA) and D. Edwards (UK). An entire issue of Review of Palaeobotany and Palynology (v. 116, edited by P. Gerrienne and C. Berry) was dedicated to recent studies of Devonian plants. Papers included in the issue greatly expand the geographic range of Devonian floras and include reports from Argentina, Brazil, Morocco, and China. Noteworthy contributions include Roth-Nebelsick's innovative approach to modeling heat transfer in rhyniophytic plants (p. 109) and Hueber's classification of the enigmatic Prototaxites in the Kingdom Fungi (p. 123).
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