Cilurian Plants Study Documents

CILURIAN PLANTS This chapter examines plant life and the geology of the Silurian period. The Silurians, from 440 to 400 million years ago, were named after the Silures, an ancient Celtic tribe of Wales, Great Britain. Many of today's mountain ranges were uplifted by the collision of the central Paleozoic continent. North America clashed with Eurasia to make Laurasia close the Sea of Iapetus during the Silurian.The collision formed a mountain belt on the periphery of the continent surrounding the ancient sea, resulting in highly folded rocks (Fig. 76). Evidence of widespread reef building by Silurian corals suggests the presence of warm, shallow seas with little variation in temperature. Tabulated corals, which consist of dense polygonal or spherical calcite cups or thecae, are another important group of reef builders. Rugose, or horn coral, so named because of its distinctive horn-like shape, is very abundant in the Silurian.They were the main coral builders of the late Paleozoic, eventually becoming extinct in the early Triassic. Finally, the creatures crawled out of the sea to eat them. AGE SEAWEED Like animals, plants do not appear in the fossil record as complex organisms until the late Precambrian or early Cambrian. The distinction between plants and animals is somewhat blurred in the obscure geological record of the past, when they shared many common characteristics. From humble beginnings as simple algae, single-celled plants were probably colonized for the same reasons unicellular animals grouped together, such as mutual support, division of labor, and protection. However, complex marine plants do not appear in the fossil record until the Cambrian, after which they evolved rapidly. Although the Cambrian has been referred to as the "age of seaweed," the geological record does not support this opinion with strong fossil evidence. Wellpreserved multicellular algae and various fossil spores found in late Precambrian and Cambrian sediments indicate the presence of complex marine plants. However, no other significant remains were found. Even as late as the Ordovician, plant fossils appear to consist almost entirely of algae, which probably formed stromatolite mounds and mats of algae similar to those on the coast today (Figure 77). Early seaweed is soft and can't stand it. Generally, they do not fossilize well. Seaweed-like plants grow halfsubmerged in estuaries and rivers. However, in order for plants to be completely shore- bound, they must reproduce completely from water. The first land plants achieved this function with spore sacs attached to the ends of simple branches. When the spores mature, they are thrown into the air and carried by the wind to a suitable location where they can grow into new plants. The first complex plants lived in shallow waters just below the surface, probably as protection against high levels of solar ultraviolet radiation. As the oxygen content of the atmosphere rises to near current levels, the upper stratospheric ozone layer filters out deadly ultraviolet light, allowing life to thrive on Earth's surface. As soon as the plant creeps ashore, the land is vast with dense forests. Before the invasion of true plants, slimy layers of photosynthetic cyanobacteria, or blue-green algae, may have inhabited the soil. Algae cover accelerates rock weathering and soil formation and nutrients needed for advanced plant life. Before the advent of land plants, microbial soils made the Earth more hospitable to life outside of water. Microorganisms may form dark, protruding soil, resembling mounds of viscous brown sugar scattered across the landscape. In this way, for about half a billion years, simple plants paved the way for more advanced vegetation. Before the arrival of terrestrial microbes, continents were too hot to support complex life forms. Early organisms played an important role in cooling the soil surface by attracting surplus atmospheric carbon dioxide for use in photosynthesis. This powerful loss of greenhouse gases cooled the climate and allowed higher life forms to populate the continents. Microbes also aid in weathering rocks into the soil, help prevent soil erosion, and provide more advanced plant nutrients needed for survival. The first land plants consisted of algae and plants such as seaweed. They are just below sea level in the shallow waters of the intertidal zone (Fig. 78). Primitive forms of mosses and lichens live on exposed surfaces. They are followed by small fern-like plants called psilophytes, or whipped ferns, the predecessors of trees. They were one of the first plants to live on land. This unpretentious plant lives half-submerged in the intertidal zone, lacks a root system and leaves, and is reproduced by throwing spores into the sea for dispersal. The most complex land plants grow less than an inch and resemble outdoor carpets covering the landscape. By the end of the Silurian, all major plant phyla already existed. Except for simple algae and bacteria, early land plants diverged into two large groups. Bryophytes, including lichens and liverworts, were the first plant phyla to become established on land. They have simple stems and leaves. However, they do not have true roots or a network of blood vessels to drain water to the higher extremities and are therefore necessary for living in a humid environment. They reproduce by spores carried by the wind for wide distribution. The earliest species occupied freshwater lakes in the late Precambrian. Pteridophytes, or ferns, were the first plant phylum to develop true roots, stems, and leaves. Some tropical ferns today grow to the size of trees as they did in the geological past. The whisking fern, which appeared in the late Silurian and became extinct towards the end of the Devonian, probably gave rise to the first club moss, horsetail and true fern. True ferns are the largest group both living and extinct and contribute greatly to Carboniferous coal deposits. Most are reproduced by spores, but extinct seed ferns produce seeds. Spermatophytes are higher plants that produce seeds and include gymnosperms and angiosperms. Gymnosperms are conifers that seed bare on open scales or cones. They range from Carboniferous to the present day and cover vast areas in harsh climates. Angiosperms are flowering plants, the seeds of which often develop in fruits. They come from Cretaceous and range in size from grasses to large trees. Higher plants are represented by about 270,000 modern species. The next major evolutionary stage was the development of vascular stems using channels to drain water from swamps or from nearby moist soil to plant extremities. The strengthening of the stem allows vascular plants to grow tall. Early club mosses, ferns, and horsetails were the first plants to use this system. As roots evolved, plants were able to survive entirely in dry soil by drawing water to their stalks from soil moistened by rain. At the end of the Silurian 400 million years ago, 2-inch-long plants had water-conducting vessels that allowed them to live completely outside of water. Lycopods, which included club mosses and scale trees (Fig. 79), were the first plants to develop true roots and leaves. The branches are arranged in a spiral, and the leaves are generally small. Spores are attached to modified leaves that become primitive cones. The scale tree, so named because the scars on its trunk resemble the scales of a large fish, grew upwards of 100 feet or taller and eventually became one of the dominant trees in the Paleozoic forest. Many plants are still quite primitive and confined to wet habitats. They have no roots or leaves and resemble branched sticks that creep on the ground. More advanced plants grow up to 10 feet tall and have a substantial root system that burrows into the soil 3 feet deep. Deep-rooted plants live along seasonally drying rivers, requiring them to tolerate prolonged dry conditions. Since the environment is very harsh, having large roots helps the plant survive in such severe conditions. Deep roots help control erosion, allowing the buildup of fertile soil in the soil, which promotes additional plant growth. The thriving vegetation also resulted in a substantial decrease in atmospheric carbon dioxide concentrations, turning the steaming greenhouse climate into another one hospitable to the first animals to replenish the soil. Plants share soil with various arthropods. They are joined by amphibian fish, which begin to make brief forays to the coast to prey on crustaceans and insects in abundance. Freshwater invertebrates and fish began to inhabit lakes and rivers. During their first 50 million years in dryland, plants showed increased diversity and complexity, including root systems, leaves, and reproductive organs that used seeds instead of spores. As true leaves evolved, plants developed various branching patterns to expose them to as much sunlight as possible to maximize photosynthesis. Competition for light was crucial to the evolution of land plants. Those who develop the most efficient branching patterns collect the most light and, therefore, are the most successful. As the plants grew larger, they progressed from random branching to branch tiers to achieve greater efficiency with a minimum of self-shade similar to present-day pines. This additional weight increases mechanical stress on the plant, requiring stronger branches to prevent broken limbs during storms. This innovation gave rise to the dominance of flora that exists today.