Evolution of plants

The evolution of plants has resulted in increasing levels of complexity, from the earliest algal mats, through bryophytes, lycopods, ferns to the complex gymnosperms and angiosperms of today. While many of the groups which appeared earlier continue to thrive, especially in the environments in which they evolved, for a time each new grade of organisation became more "successful" than its predecessors. Probably an algal scum formed on land 1,200 million years ago[citation needed]. In the Ordovician period, around 450 million years ago, the first land plants appeared.[1] These began to diversify in the late Silurian Period, around 420 million years ago, and the results of their diversification are displayed in remarkable detail in an early Devonian fossil assemblage from the Rhynie chert. This chert preserved early plants in cellular detail, petrified in volcanic springs.[2] By the middle of the Devonian Period, most of the features recognised in plants today are present, including roots, leaves and secondary wood; and, by late Devonian times, seeds had evolved.[3] Late Devonian plants had thereby reached a degree of sophistication that allowed them to form forests of tall trees. Evolutionary innovation continued into the Carboniferous period and is still ongoing today. Most plant groups were relatively unscathed by the Permo-Triassic extinction event, although the structures of communities changed. This may have set the scene for the appearance of the flowering plants in the Triassic (~200 million years ago), and their later diversification in the Cretaceous and Tertiary. The latest major group of plants to evolve were the grasses, which became important in the mid Tertiary, from around 40 million years ago. The grasses, as well as many other groups, evolved new mechanisms of metabolism to survive the low CO2 and warm, dry conditions of the tropics over the last 10 million years. The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.4 1.5 million years ago (mya), to the beginning of the Devonian Period, abo

t 419.2 3.2 mya (ICS, 2004[5]). As with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by several million years. The base of the Silurian is set at a major extinction event when 60% of marine species were wiped out. See Ordovician-Silurian extinction events. A significant evolutionary milestone during the Silurian was the appearance of jawed and bony fish. Life also began to appear on land in the form of small, moss-like, vascular plants which grew beside lakes, streams, and coastlines. However, terrestrial life would not greatly diversify and affect the landscape until the Devonian. The Ordovician ( /?rdv?n/) is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 485.4 1.9 to 443.4 1.5 million years ago (ICS, 2004[5]). It follows the Cambrian Period and is followed by the Silurian Period. The Ordovician, named after the Celtic tribe of the Ordovices, was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison, who were placing the same rock beds in northern Wales into the Cambrian and Silurian periods respectively. Lapworth, recognizing that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian periods, realized that they should be placed in a period of their own. While recognition of the distinct Ordovician Period was slow in the United Kingdom, other areas of the world accepted it quickly. It received international sanction in 1960, when it was adopted as an official period of the Paleozoic Era by the International Geological Congress. Life continued to flourish during the Ordovician as it did in the Cambrian, although the end of the period was marked by a significant mass extinction. Invertebrates, namely mollusks and arthropods, dominated the oceans. Fish, the world's first true vertebrates, continued to evolve, and those with jaws may have first appeared late in the period. Life had yet to diversify on land.