Early Flowers and Angiosperm Evolution Else Marie Friis, Peter R

Cambridge University Press 978-0-521-59283-3 - Early Flowers and Angiosperm Evolution Else Marie Friis, Peter R. Crane and Kaj Raunsgaard Pedersen Excerpt More information 1 Introduction to angiosperms The phylogenetic diversification and ecological radiation of marine habitat (e.g. Zostera). As a result of their ecological angiosperms (flowering plants) that took place in the Early dominance angiosperms account for the majority of terres- Cretaceous, between about 135 and 65 million years ago, trial primary productivity, play a significant role in major was one of the major biotic upheavals in the history of life. geochemical cycles and influence atmospheric composition It had dramatic consequences for the composition and and climate. They comprise the autotrophic foundation on subsequent evolution of terrestrial ecosystems. Ancient which almost all terrestrial ecosystems are built. Mesozoic vegetation, which was dominated by ferns, con- The diversity of angiosperms also includes great variety ifers, ginkgos and cycads, as well as Bennettitales and other in physiology and biochemistry. Most angiosperms are groups of extinct seed plants, was eventually almost autotrophs, but there are also diverse parasites and sapro- entirely replaced by more modern ecosystems dominated phytes. Some angiosperms supplement their intake of by angiosperms. Since the Early Cretaceous, high diversifi- nitrogen through trapping and digesting animals. A much cation rates have generated more than 350 000 extant angio- larger number are involved in elaborate symbiotic associ- sperm species. Today there are more living species of ations with a wide range of fungi and bacteria. Several angiosperms than all other groups of land plants combined. groups of angiosperms have evolved modifications to the In their rise to ecological dominance angiosperms have basic processes of photosynthesis, which confer significant exhibited extraordinary developmental and evolutionary ecological advantages in arid or hot environments. Angio- plasticity. This has resulted in overwhelming morpho- sperms also produce a bewildering variety of biochemical logical diversity and a great variety of adaptive types. compounds, many of which appear to be important in Angiosperms are far more diverse in vegetative form and interactions with other organisms ranging from fungi to in the structure of their reproductive organs than any other vertebrates. As food, as a source of raw materials, and as group of land plants. key participants in global ecological systems, angiosperms In growth habit angiosperms range from minute are fundamental to human life and survival. free-floating aquatics less than 1 mm long (Wolffia) and moss-like plants of fast-flowing water (Podostemaceae), to herbs, epiphytes, lianas, shrubs and tall trees. The massive 1.1 PHYLOGENETIC POSITION Eucalyptus trees of Tasmania reach heights of more than OF ANGIOSPERMS 100 m. In floral morphology the structural diversity of 1.1.1 Anthophytes and alternative patterns angiosperms ranges from the minute staminate flowers of Hedyosmum, which comprise only a single stamen, to the For almost 150 years, attempts to understand the origin giant blossoms of Rafflesia, up to 90 cm in diameter, or to and diversification of flowering plants were hindered by the slender flowers of Aristolochia that may be up to 1 m uncertain relationships among the vast array of extant long (Endress, 1994b). angiosperms, as well as the apparently insuperable mor- Structural diversity is matched by ecological diversity: phological ‘gap’ between angiosperms and other seed angiosperms occupy an astonishing range of habitats plants (gymnosperms). More generally, progress in study- from deserts to freshwater swamps, and from tropical rain ing the patterns and larger-scale processes of biological forests to the Antarctic Peninsula. Except for boreal conifer evolution was retarded by the absence of an appropriate forests and moss–lichen tundra, angiosperms dominate the methodological framework in which to develop, and choose vegetation of all the major terrestrial biomes. They are also among, competing phylogenetic hypotheses. As a result, it the only group of land plants that have reinvaded the was difficult to evaluate the great variety of ideas that 1 © in this web service Cambridge University Press www.cambridge.org Cambridge University Press 978-0-521-59283-3 - Early Flowers and Angiosperm Evolution Else Marie Friis, Peter R. Crane and Kaj Raunsgaard Pedersen Excerpt More information 2 Introduction to angiosperms had been published on angiosperm origins, and that had implicated almost all groups of fossil and living gymnosperms as potential angiosperm ancestors (Crane, 1985a). The role of the plant fossil record in contributing to an understanding of angiosperm origins and early evolution was also uncertain. Some authors assumed that palaeobotanical data were either unavailable or uninformative as a source of useful information bearing on angiosperm evolution (e.g. Stebbins, 1974); others argued that only the fossil record could provide reliable data for interpreting angiosperm evolution (e.g. Hughes, 1976, 1994). Over the past few decades the development of phylogenetic systematics (cladistics) has stimulated intense dis- cussion of the philosophical and methodological bases for reconstructing phylogenetic patterns. Many of the crucial theoretical and other issues that formerly blocked progress Figure 1.1 Cladistic relationships among land plants in phylogenetics have now been wholly or partly resolved, (embryophytes) showing the nested positions of three clearing the way for new research that has made rapid monophyletic groups: vascular plants (tracheophytes), seed plants progress. The development of computer software for (spermatophytes) and flowering plants (angiosperms). Under this powerful numerical cladistic analyses of large complex interpretation bryophytes, pteridophytes and gymnosperms are datasets, as well as techniques for rapidly amplifying and defined by exclusion (paraphyletic). For example, gymnosperms sequencing nucleic acids, have also reactivated interest in are those seed plants that are not angiosperms. Adapted from phylogenetic reconstruction as a key component of modern Crane (1985a). evolutionary biology and as one of the central goals of contemporary plant systematics. In the process, the nature that include a significant representation of extinct seed of what the palaeobotanical record can, and cannot, con- plants also recognise that the seed ferns (Pteridospermales) tribute to studies of angiosperm evolution has been clari- as traditionally defined constitute a highly unnatural assem- fied (Crane et al., 2004). blage of seed plants of diverse relationships (Crane, 1985a). The diversity of land plants can be viewed as a nested Most studies also confirm the Gnetales as a monophyletic set of four major groups: land plants (embryophytes), group (but see Nixon et al., 1994). vascular plants (tracheophytes), seed plants (spermato- Morphology-based phylogenetic analyses of extant land phytes) and flowering plants (angiosperms) (Figure 1.1). plants have generally placed the Gnetales as the closest In this context, and in cladistic terms, resolving the ques- living relatives to angiosperms (Figure 1.2), and analyses tion of angiosperm origin requires recognising and defining incorporating extinct plants show that among fossils the major groups of seed plants, determining their phylo- the Bennettitales are also closely related (Figure 1.3), genetic interrelationships, and thus establishing the group supporting earlier hypotheses of angiosperm and seed with which flowering plants share a most recent common plant relationships (Bessey, 1897, 1915; Hallier, 1901; ancestor. Arber and Parkin, 1907). The angiosperms, Gnetales and Since the early 1980s (Hill and Crane, 1982; Crane, Bennettitales, sometimes together with the extinct Pentoxy- 1985a) a series of cladistic analyses using parsimony lon plant, form a monophyletic group, usually referred to have been conducted to investigate the phylogenetic pos- as the anthophytes (e.g. Doyle and Donoghue, 1986), in ition of angiosperms among living and fossil seed plants recognition of their shared possession of flower-like repro- based on syntheses of mainly morphological data (Doyle ductive structures (Figure 1.3). An area of disagreement and Donoghue, 1986, 1992, 1993; Loconte and Stevenson, among these cladistic analyses based on morphological data 1990;Nixonet al., 1994; Rothwell and Serbet, 1994). All of concerns the resolution of relationships among anthophytes these studies identify angiosperms as a strongly supported (Figure 1.3). This disagreement arises mainly as a result clade, and indeed as one of the most strongly supported of uncertainties over the homologies among seed plant monophyletic groups in the plant kingdom. Cladistic studies reproductive structures, including angiosperm flowers. © in this web service Cambridge University Press www.cambridge.org Cambridge University Press 978-0-521-59283-3 - Early Flowers and Angiosperm Evolution Else Marie Friis, Peter R. Crane and Kaj Raunsgaard Pedersen Excerpt More information 1.1 Phylogenetic position of angiosperms 3 LAND PLANTS with angiosperms and also with each other. These difficul- VASCULAR PLANTS ties are further exacerbated by imperfect knowledge of GYMNOSPERMS important extant and extinct plants. There is still much ANTHOPHYTES more work to be done to develop a more satisfactory LiverwortsHornworts Mosses LycopsidsFerns,

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