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321 9 The Late Carboniferous: Expanding Horizons During the Late Carboniferous, the continents, which had slowly moved southward through the Devonian and Early Carboniferous, changed direction and began to rotate. Gondwana and Euramerica gradually collided, initiating the formation of the supercontinent Pangaea. The world’s vegetation had differentiated into continental regions so that, for example, the Gondwana flora became quite distinct from those of Euramerica and of what are now China and Siberia. At this time, Euramerica, positioned in the tropics, was covered by a vast swamp forest, while to the north and south of it, evaporite deposits speak of arid climates (Milner 1993a). The forest has provided an excellent fossil record of both plants and animals. There was a huge increase in diversity not only of plants but also of invertebrates and vertebrates. Partly this can be explained by the quality of the fossil record and the distribution of fossil collectors, but the increase of taxonomic diversity, body forms, and ecologies among the animals parallels but lags behind that among the plants, suggesting that the phenomenon is real, not simply an artifact. In the Late Carboniferous , more orders and classes of plants were contributing more diversity to the flora than at any other time in Earth’s history. The swamp forests of the Late Carboniferous were not completely heterogeneous. Greb et al. (2006) describe and illustrate at least six different types of habitat, each with a different complement of floral types, varying according to distance from water margins, depth of standing water, depth of decaying vegetation (mires), and so on. Most of the plants from this time are those of the lowland swamps, but a few localities preserve more upland floras showing adaptations to drier conditions. The dominant plants were still lycopsids in the form of lepidodendrons. These grew into huge trees up to 30 meters tall. They had a life history like no modern plants. In their prereproductive phase, they consisted of a single polelike trunk that, at a certain point in their growth, sprouted a crown of branches bearing conelike fruiting bodies. The branches consisted of thickened woody tissue, making them unappetizing to arthropod herbivores, but the cones containing a rich filling of spores would have been appreciated. Some of them were eaten by the large Arthropleura type of millipedes (see Chapter 7 and below). Probably only flying insects could reach the cones borne in the tops of the trees until these were shed (DiMichele and Hook 1992). Figure 9.1 shows a selection of Late Carboniferous plants. Late Carboniferous/ Early Permian Biogeography and Paleoecology Gaining Ground 322 The lycopsid trunk was formed of a relatively thin but tough outer casing, while the inner tissue was soft and open, unlike the structure of woody trees today. When a lycopsid died, the inner material rotted faster than the outer, sometimes leaving a tall hollow cylinder standing upright after the rest of the plant decayed. This construction is important for vertebrate paleontologists because it has led to the occasional and unusual preservation of some small terrestrially adapted animals, in particular in Joggins, Nova Scotia. Pteridosperms, or seed ferns, were diverse during the Late Carboniferous , and some reached heights of 6–10 meters. Individual leaves could be several meters long, and when they died, the fronds remained on the trees, drooping around the trunk in a skirt that would have provided important refuges for small arthropods or tetrapods (DiMichele and Hook 1992). As with the lepidodendroids, the stems and branches offered tough food for herbivores, but the fruiting bodies, once the protective outer coat had been penetrated, would have provided a rich source of food. As well as trees, scrambling or vinelike habits were common among pteridosperms. True ferns were also part of the flora, low in numbers but high in diversity. Both low-growing ground-covering forms and tree ferns were present, as well as climbing and vinelike forms. Their leaves were soft and might have been eaten by herbivores, but today’s ferns produce toxins to deter this. Perhaps this ability evolved first in the Carboniferous with the first radiation of herbivorous insects. Sphenopsids, relatives of modern horsetails, provided major groundcovering forms in the sphenophylls, while the calamites formed trees with trunks up to half a meter in diameter. The sphenophylls formed thickets with branches linked by hooks and barbs, much like brambles do today. The undergrowth would have been impenetrable to most large animals, but it formed cover for smaller ones. Cordaites (pronounced Cor-DAY-tees) evolved...

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