Homospory to heterospory
Evidence from the fossil plant record indicates that plants were producing sporangia yielding two kinds of spores Figure 4.8 from the early Devonian Emsian, 400Ma onwards. These include megaspores that were between approximately 150 and 200 um in diameter and microspores that were usually lt 50 im Traverse, 1988 . The transition from plants that were homosporous one spore size to heterosporous two spores sizes is considered one of the most important evolutionary trends in the development of...
Aglaophyton major
The only record of Aglaophyton major Figure 3.13 is found in the Rhynie Chert, Scotland, and is dated to approximately 400 Ma early Devonian, Emsian Rice, 1994 . It was morphologically more complex than Cooksonia, and consisted of branched aerial stems that grew up to 20 cm tal , rising from a horizontal rhizome Fdwards, 1986 Bell, 1992 . As in Cooksonia, the sporangia were located at the tips of the stems, and there was a well-defined cuticle with stomata in the aerial parts of the plant. A...
Red algae
Figure 3.24 Suggested molecular lineage between bryophtes and tracheophytes after Qiu et ah, 1998 . In evolutionary terms, the land-plant lineage then split between the liverworts and all other land plants Kenrick and Crane. 1997 . A second split then occurred between the hornwort lineage and all other land plants, and finally a split between the mosses and the two tracheophyte clades lycophytes and the euphyllophytes . This interpretation therefore suggests that mosses should bear the closest...
Eustele
A third stele type, termed the eustele Figure 4.5c , first emerged in the fossil record in the Givetian 380 Ma . This stele appears to be the most complex of the three structures in that it was composed of distinct strands of phloem and xylem, separated by parenchymal tissue. These strands were arranged either in a circle or throughout the ground tissue Taylor and Taylor, 1993 . The eustele first occurred in the progymnosperms the precursor to the seed plants . Evidence from the geological...
Advanced vascular system stelar evolution
In order to attain greater height, one of the most important adaptations would have been the development of the central conducting cy'inder the stele for the effective transport of water and nutrients around the plant Stewart and Roth well, 1993 Taylor and Taylor, 1993 Niklas, 1997 . Evidence from the geological record suggests that the stele of plants became progressively more complex with time and that by the late Devonian 374Ma at least three different stele types were apparent.
Megaphylls
These are leaves associated with stems that have either a siphonostele or eustele, and are attached to the stem by a petiole Figure 4.7b . Their evolution is thought to be closely linked to the three-dimensional vegetative branching pattern i.e. branches with no sporangia on them of the earliest vascular plants Figure 4.7 a Formation of microphylis according to i the 'Enation theory' Bower, 1935 , which proposed that these 'stem-hugging leaves' evolved from the outgrowth of vascular tissue into...
34 Evolutionary trends green algae to land plants
Geological evidence suggests that by 400 million years ago plant evolution had progressed from multicellular eukaryotic organisms that were reliant upon an aquatic medium for survival e.g. green algae , to vascular land plants permanently adapted to a water-deficient habitat. However, what is not so clear from the geological record is the evolutionary pathway that led from the algae to Table 3.1 Classification of the division Chlorophyta green algae in the subkingdom Algae from Bell, 1992 the...
Contents
1 The evolutionary record and methods of reconstruction 1.2 The geological timescale 3 1.3 Method- of reconstruction 6 2.1 The earliest environments 18 2.2 Accumulation of organic material and formation of the first cell 20 2.3 The first prokaryotes the geological evidence 23 2.4 Evolution of the eukaryotes 29 2.5 Possible triggering mechanisms of eukaryotic evolution 37 3.1 Environmental changes during the Cambrian and Ordovician 3.2 Fossil evidence for plant terrestrialization 48 3.3 Examples...
36 Biogeographical distribution of the earliest land plants in the late
A framework detailing the various stages towards the colonization of land by plants Gray, 1993 Kenrick and Crane, 1997a,b Figure 3.18 suggests that three epochs can be recognized in the spore record. The first is termed the Eoembryophytic epoch i.e. emphasizing the role of the bryophytes and tracheophytes in the early stages of terrestrialization . It is dated from mid-Ordovician to early Silurian 476-432 Ma and represents the time in the fossil record when there is evidence for spore tetrads...
35 Evolutionary trends nonvascular to vascular plants
As more fossil evidence comes to light, it is becoming apparent that the record of the earliest plants is highly biased towards vascular plants tracheophytes . There are few macrofossil records of early non-vascular land plants bryophytes , although, as described above, there are 'disarticulated' microfossils to suggest tneii presence, such as unlignifted tubes, rhizoids, and tetrahedral spores. This biased record is certainly due in part to the poor preservation potential of nonvascular plants...
Anchoring mechanisms
Standing upright was not the only structural problem associated with the terrestrialization process. Early land plants also needed to acquire a system to anchor them to the ground and enable them to obtain the mineral elements necessary for nutrition and water from soils. The earliest unequivocal fossil evidence for rooting systems comes from plant and root traces preserved in palaeosols dating from around 408 million years ago early Devonian . These traces indicate dichotomous roots between...
Molecular evidence
Molecular systematics also appears to support the suggestion that Charophyceae are the closest in evolutionary terms to land plants e.g. Manhart and Palmer, 1990 Chapman and Buchheim, 1991 . One example of this type of approach is work based on an examination of group II introns non-coding DNA thai interrupts short sections of coding DNA found in the tRNA genes of all land-plant chloroplast DNA Manhart and Palmer, 1990 . Previous work had indicated that all algae and eubacteria had...
Cooksonia
Early plants that are classified as Cooksonia Figure 3.12 occur in large numbers of fossil localities in North America and Europe. The earliest examples of Cooksonia have been found in deposits from Ireland dating to early Silurian age Wenlockian, 428Ma Edwards et al., 1983 . They consist of a simple Figure 3.12 Photograph of fossil Cooksonia alongside sketch to indicate main features photograph by H. Barks courtesy of P. Gesel sketch redrawn from Bell, 1992 . Figure 3.12 Photograph of fossil...
Ecological evidence
As a group, green algae have a wide ecological tolerance, with the ability to exist in a large range of habitats, extending from the subtidal zones of seas, to fresh water, damp soils, the surfaces of leaves, and, in symbiosis with fungi, to the harshest environments on Earth. Before the advent of ultrastructural or molecular techniques, it was these ecological tolerances that were most often cited as evidence for an evolutionary link to land plants. These observations still hold true but, with...
Morphological evidence
Certain morphological similarities between some species of green algae and vascular plants have also been used to suggest a different evolutionary pathway between these two groups of organisms. For example, at the whole-organism level it has been recognized that various groups of algae are structurally similar to land plants, consisting of a multicellular plant body thallus composed of both prostrate and erect components Stewart and Rothwell, 1993 Taylor and Taylor, 1993 . One example that has...
Psilophyton dawsonii
Psilophyton dawsonii Figure 3.17 has been identified at a number of fossil localities dating back to approximately 395 million years ago Banks et al., 1975 Edwards and Fanning, 1985 . It is known to have grown to a height of approximately 60 cm, with greatly branched fertile and vegetative lateral branches growing out from a central stem. The fertile branches typically dichotomized up to six times before terminating in clusters of approximately 32 sporangia Figure 3.17 Bell, 1992 Taylor and...
25 Vascular Plants
Figure 2.12 Perpendicular cross-section of these fossil filaments has revealed stacked disc-shaped cells in a relatively transparent enveloping sheath scale bar, 25 nm , indicating close morphological similarity to Batigiomorpha pubescens photograph N. Butterfield . Figure 2.13 a Transverse cross-section of the fossil himgiophyte showing wedge-shaped cells arranged axially around a central core photograph N. Butterfield . b Transverse cross-section of an extant species of Bangia, showing eight...
Mechanical support
With the loss of support provided by water, another adaptation necessary in the process of terrestrialization was a means of staying upright. Although the mechanical strength of lignin would have provided some support Taylor and Taylor, 1993 , calculations on stem diameter suggest that structural organization was also important Niklas, 1986, 1994, 1997 . Calculations on the biomechanics of early terrestrial vegetation have indicated that although there is no single 'optimal' plant morphology,...
Rhynia gwynnevaughanii
Numerous permineralized specimens o Rhynia gwynne-vaughanii Figure 3.14 Edwards, 1980,1986 dating back to approximately 400 million years ago Rice, 1994 have been recovered from the Rhynie Chert in Scotland. These fossils record a plant growing to approximately 18 cm tall with simple dichotomizing stems arising from a rhizome bearing thread-like rhizoids Figure 3.14 . The stems were 2-3 cm in diameter Taylor and Taylor, 1993 and were vascularized with helical S-type elements Kenrick and Crane,...
21 The earliest environments
Geological evidence indicates that the continental crust started to form by 4200Ma and that by 1900Ma a large, single, lens-shaped body had amassed Goodwin, 1991 Condie, 1997 . This supercontinent has been named 'Rodinia' and is thought to have been located around the equatorial belt, in roughly the same position as present-day Africa Briggs, 1995 . Initial splitting up of Rodinia occurred from approximately 1000 Ma Figure 2.1 , resulting in three major parts, Laurasia consisting of North...
Fossil evidence ColeochaeteParka
Fossil evidence would also appear to support the theory that members of the Charophyceae may have provided an evolutionary link between green algae and land plants. In particular there is evidence for a late Siluriaa early Devonian terrestrial thalloid plant 415-395 Ma that bears a remarkable morphological similarity to the thalloid form of Coleochaete. This fossil plant, called Parka dccipiens Figure 3.23 , was between 0.5 and 7.0 cm in diameter and composed of cells arranged in a...
Zosterophyllum divaricatum
Zosterophyllum divaricatum Figure 3.15 is one of a number of early fossil plants dating back to approximately 400 million years ago showing the presence of sporangia borne laterally along the stem, or attached to it by a short branch Gensel, 1982 Gensel and Andrews, 1987 . These sporangia were borne on short stalks and usually in two rows orientated to one side of the axis Figure 3.13 Reconstruction of fossil Aglaophyton major redrawn from Bell, 1992 . Figure 3.13 Reconstruction of fossil...
Baragwanathia longifolia
Baragwanathia longifolia Figure 3.16 has been identified from a number of Australian sites which was part of the Gondwanaland supercontinent Figure 3.16 . They were composed of a robust stem that was approximately 1-2 cm in diameter and thickly covered with long, slender leaves Gensel and Andrews, 1987 . Fossil evidence shows that the stems were extremely long, extending out from a rhizomatous base up to 1 m. The stems contained tracheids with annular or helical thickenings, some with a G-type...
The colonization of land
The colonization of the land by plants was one of the most significant evolutionary events in the history of the planet. Before colonization could occur, however, major changes were necessary, both to the environment and to early plants, in order to enable growth outside of an aquatic environment. By the early Silurian 430Ma plants that were permanently adapted to a terrestrial or water-deficient habitat land plants had evolved. This chapter examines the environmental changes leading up to and...
24 Evolution of the eukaryotes
Eukaryotes differ from prokaryotes in that they have a membrane-bound nucleus in which the DNA is contained, organelles including mitochondria, integrated multicellularity, and sexual reproduction sometimes Figure 2.10 . They constitute the three major groups of multicellular organisms plants, animals, and fungi , along with many groups of the Protista, including species of red, green, and brown algae Lipps, 1993a . Because of the diversity and importance of eukar-yotic organisms to life on...
The evolutionary record and methods of reconstruction
Historically the study of evolution has concentrated almost exclusively upon the animal record. If plants are mentioned it is only in passing and usually to reconstruct the environment in which to view the animal record. Yet from the earliest fossil records plants have provided an equally diverse and interesting picture. The earliest fossil photosynthesizers date back to approximately 3500 million years ago Ma early Archaean and some o - the earliest eukaryotes were plants, namely red and green...