Exquisite preservation: two examples from the plant fossil record

Willis, K.J., McElwain, J.C. The evolution of plants

At a time when the popular perception of paleontology is dominated by images of dinosaurs and other spectacular vertebrates, or the mysteries surrounding the Cambrian “explosion” of animal life, it is perhaps not surprising that the rich and informative fossil record of plants has scarcely made an impact on the public consciousness. In reality, as one would expect from those organisms that comprise the bulk of the biological material in terrestrial ecosystems, the fossil record of plants is extensive (Stewart and Rothwell, 1993). Leaves, wood fragments, pollen grains, spores, fruits, seeds and other plant parts are the most common fossils in rocks deposited in ancient flood plains, lakes and many other environments - and they are often exquisitely preserved. This excellent fossil record provides important information about the ecology of ancient terrestrial ecosystems. The quality of the plant fossil record also makes paleobotanical data highly informative about the historical pattern of plant evolution. It is this pattern, and its congruence with patterns in the characters of living and fossil plants — as summarized in a classification — that is the focus of this chapter.

The role of development in plant phylogeny: A paleobotanical perspective

At a time when the popular perception of paleontology is dominated by images of dinosaurs and other spectacular vertebrates, or the mysteries surrounding the Cambrian “explosion” of animal life, it is perhaps not surprising that the rich and informative fossil record of plants has scarcely made an impact on the public consciousness. In reality, as one would expect from those organisms that comprise the bulk of the biological material in terrestrial ecosystems, the fossil record of plants is extensive (Stewart and Rothwell, 1993). Leaves, wood fragments, pollen grains, spores, fruits, seeds, and other plant parts are the most common fossils in rocks deposited in ancient flood plains, lakes, and many other environments—and they are often exquisitely preserved. This excellent fossil record provides important information about the ecology of ancient terrestrial ecosystems. The quality of the plant fossil record also makes paleobotanical data highly informative about the historical pattern of plant evolution. It is this pattern, and its congruence with patterns in the characters of living and fossil plants—as summarized in a classification—that is the focus of this chapter.

Cascales-Minana, B. & Cleal, C.J. 2011: Plant fossil record and survival analyses. Lethaia, Vol. 45, pp. 71–82. Survival analysis is a classic palaeobiological method widely used on the animal fossil record. This study reports the first application of survivorship analyses on the plant fossil record from a global viewpoint and provides a new comparative approach of this methodology. The results reveal three important plant extinction events in the history of plant life at a global scale. The results also clearly suggest that the origination events are more intensive than extinction processes and that the origination moment of several lineages of vascular plants is an important factor that conditions their longevity. This study supports the general idea that vascular plants tended to be less affected by the environmental changes that caused mass extinction in other groups of organisms. □Extinction events, fossil record, survival patterns, taxonomic survivorship curves, vascular plants.

Plant evolution viewed through a functional and paleoclimatic prism

Reconstruction of palaeobiomes, ancient communities that exhibit a physiognomic and functional structure controlled by their environment, depends on proxies from different disciplines. Based on terrestrial mammal fossils, the late Miocene vegetation from China to the eastern Mediterranean and East Africa has been reconstructed as a single cohesive biome with increasingly arid conditions, with modern African savannahs the surviving remnant. Here, we test this reconstruction using plant fossils spanning 14-4 million years ago from sites in Greece, Bulgaria, Turkey, the Tian Shan Mountains and Baode County in China, and East Africa. The western Eurasian sites had a continuous forest cover of deciduous or evergreen angiosperms and gymnosperms, with 15% of 1,602 fossil occurrences representing conifers, which were present at all but one of the sites. Raup-Crick analyses reveal high floristic similarity between coeval eastern Mediterranean and Chinese sites, and low similarity between Eurasian and African sites. The disagreement between plant-based reconstructions, which imply that late Miocene western Eurasia was covered by evergreen needleleaf forests and mixed forests, and mammal-based reconstructions, which imply a savannah biome, throws into doubt the approach of inferring Miocene precipitation and open savannah habitats solely from mammalian dental traits. Organismal communities are constantly changing in their species composition, and neither animal nor plant traits by themselves are sufficient to infer entire ancient biomes. The plant fossil record, however, unambiguously rejects the existence of a cohesive savannah biome from eastern Asia to northeast Africa.

Palaeobotanical experiences of plant diversity in deep time. 1: How well can we identify past plant diversity in the fossil record?

The fossil plant record and global climatic change

Ecosystems are the products of regional biotic history, shaped by environmental changes that have occurred over thousands of years. Accordingly, ecological changes take place at many timescales, but perhaps none is more significant than the truly long-term scale of centuries and millennia, for it is at these timescales that ecosystems form, break apart, and reform in new configurations. This is certainly true in the alpine regions, where glaciations have dominated the landscape for perhaps 90% of the last 2.5 million years (Elias 1996a). In the alpine tundra zone, the periods between ice ages have been relatively brief (10,000–15,000 years), whereas glaciations have been long (90,000–100,000 years). Glacial ice has been the dominant force in shaping alpine landscapes. Glacial climate has been the filter through which the alpine biota has had to pass repeatedly in the Pleistocene. This chapter discusses climatic events during the last 25,000 years (figure 18.1). At the beginning of this interval, temperatures cooled throughout most of the Northern Hemisphere, culminating in the last glacial maximum (LGM), about 20,000–18,000 yr b.p. (radiocarbon years before present). The Laurentide and Cordilleran ice sheets advanced southward, covering most of Canada and the northern tier of the United States. Glaciers also crept down from mountaintops to fill high valleys in the Rocky Mountains. In the Southern Rockies, the alpine tundra zone crept downslope into what is now the subalpine, beyond the reach of the relatively small montane glaciers. By about 14,000 yr b.p., the glacier margins began to recede, leading eventually to the postglacial environments of the Holocene. It is now becoming apparent that the climate changes that drove these events were surprisingly rapid and intense. This chapter examines the evidence for these climatic changes and the biotic response to them in the alpine zone of Colorado. To reconstruct the environmental changes of this period, we must rely on proxy data, that is, the fossil record of plants and animals, combined with geologic evidence, such as the age and location of glacial moraines in mountain valleys. As of this writing, the principal biological proxy data that have been studied in the Rocky Mountains are fossil pollen and insects.

Before the advent of grasses, dry continental interiors would have been vegetated by other kinds of plants. What these plants were like is difficult to say because well drained soils of desert regions preserve plant material only under exceptional circumstances (Retallack 1984a), such as the urine-impregnated middens of packrats (Neotoma spp.). There are few lakes and streams in deserts where plant fossils could be preserved and oases are surrounded by a lush growth of local plants completely different from those found in the desert beyond. Hence, the fossil record of plants in dry regions is both meager and biased. Some indications of ancient aridland ecosystems are provided by fossil root traces (Loope 1988), bones (Olson 1985), and burrows (Olson & Bolles 1975, Smith 1987) in calcareous red paleosols. Large plants and animals play a conspicuous role in desert ecosystems, but productivity is limited and there is much bare earth exposed. Aridisols forming in such environments do not appear greatly different from calcareous paleosols as old as 1900 million years (Campbell & Cecile 1981). For many millions of years after the mid-Paleozoic advent of trees, forests may have graded out through dry woodland into scrubby desert vegetation in dry continental interiors.

William G. Chaloner, widely known as Bill, was a world leader in the study of plant fossils. He was a pioneer in the development of palaeopalynology and helped integrate studies of macroscopic plant fossils with investigations of fossil pollen and spores. His early work expanded our understanding of Carboniferous coal-forming plants and vegetation and his investigations on the changing distributions of fossil plants contributed to improved knowledge of biogeographic patterns during the late Paleozoic, including the concept of continental drift before it was widely accepted. Bill's relentless creativity demonstrated how the fossil record of plants could be exploited to reconstruct ancient climates. He also recognized that because certain structural features of fossil leaves directly record levels of the greenhouse gas carbon dioxide they are valuable ‘biosensors’ of ancient atmospheric composition allowing investigation of the link between the greenhouse effect and past warm climates. With his clear and critical mind and grasp of diverse subject matter, Bill was adept at distilling disparate information into a coherent and understandable whole. An engaging communicator, and an unfailingly supportive mentor, he inspired many young scientists during his five decades of service to colleges of the University of London.

Can atmospheric composition influence plant fossil preservation potential via changes in leaf mass per area? A new hypothesis based on simulated palaeoatmosphere experiments.

This chapter describes ‘the big five’-the mass extinction events viewed observed in the marine faunal record, -comparing the nature and magnitude of these events to changes apparent in the plant fossil record. It examines the evidence for mass extinction in the plant fossil record, particularly to the physiological characteristics that enable plants to be resilient to the types of environmental change associated with mass extinction events. It also cites examples of the long-term persistence of various families in the plant fossil record and the implications of this in terms of understanding the driving mechanisms behind plant evolution. The chapter explains mass extinctions that are characterized by the relatively rapid extinction of groups of organisms. It points out the reasons for the lack of mass extinction in the fossil plant record relatesd to the mechanisms that enable plants to withstand a certain amount of environmental stress.

This chapter outlines several of the main evolutionary theories that are part of the current and lively debate and examines them in light of the plant fossil record. It analyszes evidence from the plant fossil record that indicates a broadening spectrum of diversity and morphological and developmental complexity through time. It also cites suggestions in the plant fossil record concerning the major evolutionary change and innovation on the broadest scale, which concentrated into relatively short intervals in geological time. The chapter discusses biological interactions which played a role in terrestrial diversification and which have been responsible for the decline in species diversity of incumbent groups or clades of plants as new ones evolved. The chapter identifies potential drivers of the macroevolutionary pattern, such as plate movements, changes in land mass area, climate, and alterations to atmospheric concentrations and chemistry.