A Natural History of the Central Appalachians

Chapter 02: History of the Flora and Fauna

CHAPTER 02 HISTORY OF FLORA AND FAUNA THE EARLIEST EVIDENCE OF LIFE ON THE EARTH is represented by fossils of bacteriumlike organisms that have been dated to about 3.5 billion years ago. These fossils occurred in rocks derived from sediments deposited in the shallow water of an early ocean, and for at least the first 2 billion years the history of life took place in the water. However, by at least 1.2 billion years ago, the first terrestrial algae are thought to have formed thin, scum-like mats on some fully exposed rock surfaces. These algae were microscopic (and largely unicellular) organisms, but they were the dominant plants on land for perhaps as much as three-quarters of a billion years. About 450 million years ago, during the latter part of the Ordovician, the first true macroscopic (and multicellular) land plants appeared. It seems that they would have been similar in appearance to some of the simple liverworts that survive today. These early multicellular land plants did not have specialized water-conducting (or vascular) tissue, which severely limited both their distribution and the size they could attain. The oldest known fossils of plants with vascular tissue date back to the Silurian, about 420 million years ago. Although the very earliest vascular plants were simple in structure, evolution in the group soon resulted in remarkable increases in size and complexity. Evidence of the level of diversification that had been achieved by the early Devonian (about 400 million years ago) is provided by an assemblage of fossils from the Rhynie chert in northern Scotland. These fossils, the most famous of which is Rhynia, are preserved in such extraordinary detail that even the structure of individual cells is apparent in some instances. By the middle of the Devonian, vascular plants had already acquired most of the features associated with present-day members of the group (e.g., roots, leaves, and secondary growth), and in the late Devonian (perhaps as early as 385 million years ago), the first tree-sized plants appeared. Fossils of Archaeopteris, an early tree with conifer-like wood, a trunk that could reach five feet in diameter, and branches that were flattened in one plane so as to resemble the fronds of ferns are known from the late Devonian in the Central Appalachians (fig. 9). It was thought that fossils of the flattened branches of Archaeopteris were from a large fern, and that fossils of the trunk belonged to an extinct conifer (assigned to the genus Callixylon), until 1960, when Charles Beck, a paleobotanist at the University of Michigan, discovered a fossil in which the two were attached and thus represented the same plant. As mentioned at the very beginning of this book, the landscape of the Central Appalachians about 300 million years ago during the latter part of the Carboniferous would have been totally unfamiliar to a modern observer. Anyone standing in the middle of the region would certainly have noticed the absence of any mountains, assuming that the surrounding plants did not block a clear view of the horizon. Moreover, because of the trees it seems likely that the modern observer would have wet feet from standing in the middle of what today would be called a swamp (a term often used for a forested wetland). As mentioned in chapter 1, these trees were very different from anything found in the Central Appalachians today, although a few non-tree representatives of the taxonomic groups to which they belonged survive as “living fossils” and can be quite common in some ecological situations. In addition to the trees, these ancient forests contained a number of other kinds of plants, although the overall diversity would have been much less than in most modern forests (fig. 10). COAL SWAMP FORESTS “Coal swamp forests,” or “coal forests,” are highly important in the context of the human history of the Central Appalachians, since they are the source of the large deposits of organic material that ultimately became converted into coal. Although the landscape upon which they formed apparently had little topographic relief, there were areas of higher ground along with poorly drained areas that had standing water. In addition, there must have been slowly flowing streams, ponds, and lakes of FIGURE 9 Archaeopteris, an early treelike plant with fernlike leaves 31 02 HISTORY OF THE FLORA AND FAUNA called leaf cushions, contained photosynthetic tissues, which means that the trunk of...