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The Influence of Plants on the Evolution of Terrestrial Communities

by Dr. Bruce H. Tiffney
University of California, Santa Barbara

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Uniformitarianism, the concept that processes observed on the present day Earth may be used to interpret the past, is a useful scientific tool. However, all too often we assume that the world has always been like the present. This denies both evolution, which implies ever-increasing complexity, and the sequential evidence of the fossil record.

Vascular land plants provide food and moisture to the animals that consume them, and create the three-dimensional world in which terrestrial animals live. The complexity of modern biomes is the end product of many intermediate stages without modern equivalents: we live in a unique time.

The earliest land communities probably lived with the soil; surficial communities date from the Middle Silurian, involving low plants restricted to moist places. Concomitant animals included snails and various detritivores. From this beginning, more complex plants evolved to form three-dimensional lowland forest communities, inhabited by the first terrestrial tetrapods and many invertebrates. However the vertebrates were all carnivores, and the insects were just beginning to learn to eat plants.

It is only in the Permian that ecosystems like those of the modern day first appeared, but these systems had a brief success, lasting until the Middle Triassic extinctions which set the stage for the radiation of more drought tolerant gymnosperms, accompanied by huge vertebrate herbivores, the dinosaurs. This relationship prevailed until the Cretaceous-Tertiary extinction, when simultaneous radiations of birds, mammals, and angiosperms, modified by the climatic cooling of the later Tertiary and Quaternary, set the stage for the world as we know it.

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