Box The Rhynie Chert a window on earliest land life

Rhynie is a remote village in northeast Scotland consisting of only 50 or so houses; the bus stops there once a day. In 1914, Dr William Mackie, a physician, found traces of plant fossils in some speckled black and white chert rocks. He cut thin sections and took his specimens to Glasgow, where Robert Kidston, the foremost expert in Britain on floras of the Carboniferous, confirmed that the chert contained nearly perfectly preserved plants. Kidston, together with William Lang, Professor of Botany at the University of Manchester, England, published a classic series of monographs (Kidston & Lang 1917-1921) in which they presented superb photographs of microscopic sections through the Rhynie Chert plants. These publications established the Rhynie Chert as one of the oldest land-based ecosystems on Earth.

The Rhynie fossils include the remains of seven vascular land plants, as well as algae, fungi, one species of lichen and bacteria, as well as at least six groups of terrestrial and freshwater arthropods. What is amazing is the quality of preservation: every cell and fine detail can be seen, as if frozen in an instant and preserved forever (see Fig. 3.8a).

The Rhynie ecosystem was no towering forest. If you went for a stroll in Scotland in the Early Devonian, the green rim of plants probably did not extend far from the sides of ponds and rivers, and the tallest plants would have barely brushed your knees (Fig. 18.8). To see anything, you would have to go down on your hands and knees, and peer at the stems through a magnifying glass. Most of the taller plants had smooth stems, and branched simply in two, with knob-like sporangia at the tops of their stems - just larger examples of rhyniopsid plants like the Silurian Cooksonia (see Fig. 18.7a-d). Asteroxylon, a relative of Zosterophyllum (see Fig. 18.7e), had small scale-like leaves growing up from the stem. Microscopic cross-sections of these plants show they had simple vascular canals, stomata and terrestrial spores. Between the plants crept spider-like trigonotarbids and insectlike arthropods, and some of these are even found within cavities in the plant stems. There were crustaceans in the warm pools.

These discoveries show how extraordinary the preservation is in the Rhynie Chert. The fossils are silicified through having been flooded by silica-rich waters from nearby hot springs. Recent work (Trewin & Rice 2004) has confirmed that Scotland in the Early Devonian was an actively volcanic zone, perhaps related to Caledonian tectonic activity associated with the closure of the Iapetus Ocean (see pp. 45-8). Rhynie in the Early Devonian was like Yellowstone National Park today, with hot geysers erupting and immersing vegetation in silica-rich waters at a temperature of 35°C - an ecosystem frozen (or boiled) in time.

Read more about the Rhynie Chert in the volume by Trewin and Rice (2004), and on web links from http://www.blackwellpublishing.com/paleobiology/.

Rhynie Chert Reconstruction

Figure 18.8 Reconstructed scene in the Early Devonian Rhyme ecosystem showing the commonest vascular plants Rhynia and Asteroxylon in the foreground, and a selection of small arthropods that lived in the water and in and on the plants (scale bars, 100 |im). (Drawing by Simon Powell, based on information from Nigel Trewin.)

Figure 18.8 Reconstructed scene in the Early Devonian Rhyme ecosystem showing the commonest vascular plants Rhynia and Asteroxylon in the foreground, and a selection of small arthropods that lived in the water and in and on the plants (scale bars, 100 |im). (Drawing by Simon Powell, based on information from Nigel Trewin.)

resprout from underground rhizomes. They grew in incredibly dense, bamboo-like thickets with locally more than 10 trees per square meter. One form, Calamites (Fig. 18.10a), reached nearly 20 m in height, but shows the jointed stems and whorls of leaves at the nodes typical of modern smaller horsetails; the trunk of Calamites generally arose from a massive underground rhizome. Horsetail leaves formed radiating bunches at nodes along the side branch (Fig. 18.10b), and there were usually two types of cones, some bearing megaspores (Fig. 18.10c), and others bearing microspores. The giant horsetails disappeared at the end of the Pennsylvanian, as did the arborescent lycopsids. Some modest tree-like forms up to 2 m tall existed in the Permian and Triassic, but later horsetails were mainly small plants living in damp boggy areas.

Ferns - fronds or leaves?

Ferns are familiar plants today, typically with long fronds, each composed of feathery side branches that uncurl as they develop. Various fern-like plants are known in the Devonian and Carboniferous, and undisputed ferns are known in abundance from the Carboniferous onwards. As with the lycopsids and horsetails, some of the Carboniferous ferns, like Psaronius (Fig. 18.11) were tree-like. The fronds were borne on a vertical trunk, and they show all the features of their smaller modern tropical relatives. Other Carbonifer-

Figure 18.9 Reconstructing the arborescent lycopsid Lepidodendron, a 50 m-tall tree from the Carboniferous coal forests of Europe and North America. No complete specimen has ever been found, but complete root systems, Stigmaria, and logs from the tree trunk are relatively common. The details of the texture of the bark, branches, leaves, cones, spores and seeds are restored from isolated finds.

Figure 18.9 Reconstructing the arborescent lycopsid Lepidodendron, a 50 m-tall tree from the Carboniferous coal forests of Europe and North America. No complete specimen has ever been found, but complete root systems, Stigmaria, and logs from the tree trunk are relatively common. The details of the texture of the bark, branches, leaves, cones, spores and seeds are restored from isolated finds.

ous and Permian ferns were smaller herbaceous plants.

Ferns today are generally low-growing herbaceous plants, common in many environments. Ferns are resilient plants. After the huge eruption of Mount St Helens in 1980, the first living things to appear through the thick layers of ash were ferns. Their fronds had been burned to the ground, and yet somehow they were not killed, and they uncurled through the ash to begin the greening of the Washington State landscape within weeks of the eruption.

Is the fern frond a leaf or a branch? Technically, it is a branch, and the individual small green structures along each part of the frond are leaves. So, each frond is made from many small leaves. Leaves are common in more advanced plants, and they may have arisen by fusion of the small leaflets of fern-like plants

Figure 18.10 Giant Carboniferous horsetails: (a) Calamites, a 10 m-tall tree; (b) Annularia, portion of a terminal shoot bearing 10 mm-long leaves; and (c) Palaeostachya, diagrammatic cross-section of a cone-like structure, 15 mm in diameter, bearing small numbers of megaspores. (Based on Thomas & Spicer 1987.)

Figure 18.10 Giant Carboniferous horsetails: (a) Calamites, a 10 m-tall tree; (b) Annularia, portion of a terminal shoot bearing 10 mm-long leaves; and (c) Palaeostachya, diagrammatic cross-section of a cone-like structure, 15 mm in diameter, bearing small numbers of megaspores. (Based on Thomas & Spicer 1987.)

to provide a typical leaf, an efficient broad photosynthesizing structure that can turn to face maximally toward the sun.

The ferns showed a second burst of evolutionary radiation during the Jurassic and Cretaceous, and they are the dominant plants in some Jurassic floras. Again, there were treelike forms, as well as the more familiar low-growing ferns seen today.

Archaeopteris: the missing link?_

One of the greatest developments in the evolution of plants was the seed, a key feature of the dominant modern plant groups, the gymnosperms and angiosperms. All the other plant groups considered so far - bryophytes, rhyniopsids, clubmosses, horsetails and ferns - lack seeds. Gymnosperms and angiosperms also show an advance in their woody tissues that permits the growth of very large trees; their lignified tracheids, vascular canals, can develop in a secondary system. The extraordinary early tree Archaeopteris from the Mid to Late Devonian, seems to represent a

Carboniferous Tree Fern

Figure 18.11 The tree fern Psaronius, a 10 m-tall fern from the Pennsylvanian of North America. (Based on Morgan 1959.)

integument archegonia

Figure 18.11 The tree fern Psaronius, a 10 m-tall fern from the Pennsylvanian of North America. (Based on Morgan 1959.)

half-way stage. It looked superficially like a tree fern, but its trunk showed the development of secondary woody tissues and growth rings, as seen in modern conifers.

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