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569 Land Plants as a Source of Food and Environment in the Age of Dinosaurs Bruce H. Tiffney Plants in a book on dinosaurs? Not as out of place as you might think. Plants are autotrophs (self-feeders), organisms that are able to capture the sun’s energy directly. By contrast, dinosaurs, like all animals, are heterotrophs (other feeders), organisms that have to feed on other organisms in order to live. Since plants lie at the base of the food chain, they have had an immense influence on the evolution of both herbivores and carnivores in Earth history . The size of the available plants, their rate of growth, their ability to recover from damage, the rate at which they reproduce, their abundance in the environment, and the digestibility of their leaves and reproductive organs all combine to influence the amount of energy that herbivores can draw from them. As these features of plants change through evolutionary time, so also will the nature of the herbivore and carnivore communities dependent upon them. Additionally, plants are important to animals in that they define the environment within which animals live. By example, a forest forms a barrier to large animals and is difficult for them to pass through. In contrast, smaller animals perceive the forest as a three-dimensional habitat, and have the option to move vertically as well as horizontally within it. Conversely, the two-dimensional surface of an open “grassland” allows free motion of large animals but limits the options for small animals. Only by burrowing can smaller animals create a three-dimensional environment in open country. This is a common solution for mammals, but among the dinosaurs, only a few birds were small enough to effectively explore it. The interaction of plants and animals is not a one-way street. Herbivores have had an important influence on plant evolution by their choice of food, the volume that they consume, and the frequency and duration of their feeding. Excessive herbivory can destroy environments and place selective pressure on plants, possibly even leading to the extinction of existing lineages or the evolution of new ones. In this chapter we will survey (1) the major groups of plants available to dinosaurs as food, and (2) the possible effects of this interaction between plants and dinosaurs on both groups. In looking at this interaction, we will view plants from the perspective of an herbivorous dinosaur, examining those features of plants that influence their quality as food. Similarly, we will look at herbivorous dinosaurs as plant-consuming machines. Readers should be aware of two features. First, this chapter is brief, and presents generalized information for which specific exceptions are often known to exist. Second, the study of the evolutionary interaction of herbivores and the plants they fed upon in the fossil record is in its infancy. Thus, many 27 Bruce H. Tiffney 570 of the interpretations presented in the second portion of this chapter are hypotheses open to test and refutation. Dinosaurs radiated into a world already populated by two major kinds of land plants, the pteridophytes and the gymnosperms, and were joined by a third kind, the angiosperms, in the mid-Cretaceous (Figs. 27.1 and 27.5). All three groupings are vascular plants, that is, they possess conducting tissue that transports water and food products within the plant body. This is in contrast to the algae (seaweeds), which are almost entirely aquatic, or to the bryophytes (mosses and associates), which are terrestrial but are restricted in their size and significance by the lack of vascular tissue. We will look at the characteristics of the three vascular plant groups in turn, and then at their distribution in space and time during the Mesozoic. We will not discuss terrestrial algae or bryophytes, as they did not form an important part of the diet of herbivorous dinosaurs. What is the source of the following information? Where fossil plants are similar to living ones, we can infer the biology of the fossil from that of the living counterpart, recognizing that the extant plant cannot be a perfect proxy for the fossil. However, in many cases there are no close living relatives, and inferences must be made from the circumstantial evidence provided by the anatomy and morphology of the fossil, and their parallels in plants of the present day. Further insight may be gained from the depositional situation within which the fossil was found. Such evidence allows us to erect a model of Mesozoic ecosystems, but with the...

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