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261 7.1. Stratigraphic table of the earlier parts of the Carboniferous with localities and their faunas indicated. Stratigraphic information from Gradstein et al. (2004), locality and taxa from Smithson (1985a), and other sources cited in the text. 7 Emerging into the Carboniferous: The First Phase At the end of the Devonian, a major extinction event hit most groups of vertebrates, both marine and nonmarine. Although an earlier extinction event at the Frasnian–Famennian boundary has been recognized for many years, it appears to have affected invertebrates, especially marine ones, with most vertebrate groups essentially passing through it unscathed. By contrast, a massive vertebrate faunal turnover at the end of the Devonian, associated with the geological phenomenon known as the Hangenberg event, saw the extinction of many groups of vertebrates such as placoderms, and most acanthodians and sarcopterygians (Sallan and Coates 2010). Of those acanthodians and sarcopterygians that did survive, most were represented by only a remnant of their former populations , and these too eventually became extinct. After the extinction, a few groups notably survived well. These included the ray-finned fishes, which had begun their radiation in the Late Devonian but which expanded greatly in numbers, species, and niches in the Early Carboniferous . The chondrichthyans, although they had been persistently present through the Devonian, again became more widespread and numerous, particularly later in the Early Carboniferous. Finally, the tetrapods really began their radiation at this stage. From this point on, the multidigited forms from the Devonian were rare or absent, and five-digited forms became dominant. It was during this period that the advent of tetrapods onto the land really began, with their radiation into a wealth of body forms, niches, and families, among them fully terrestrial forms that were the forerunners of modern amniotes. Regrettably, the early parts of the period have a poor fossil record, not only for tetrapods but for plants and invertebrates too. By the time tetrapod fossils become at all common, during the later parts, they had already acquired many of the characteristics that mark them as terrestrial tetrapods, which makes understanding this sequence of events difficult. To place the tetrapods in context, it is necessary to survey what is known of their contemporaries in the plant and invertebrate world because it was these that paved the way for the tetrapods to follow. The Carboniferous period is usually divided into two phases, called the Early and Late Carboniferous in Europe, and the Mississippian and Pennsylvanian in North America. Although the boundaries between these divisions have in the past not been considered equivalent on each side of the Atlantic, this seems to have been an effect of incomplete stratigraphic correlation, and more recently, these two boundaries have The Carboniferous World Gaining Ground 262 been reconciled. The terms “Mississippian” and “Pennsylvanian” are now used for European sequences as well (Heckel and Clayton 2006). One way to remember which term equates to Early and which to Late Carboniferous is to realize that M precedes P in the alphabet, indicating that Mississippian precedes Pennsylvanian in the stratigraphic sequence. For the current discussion, because ages rather than rock sequences are usually being described, the terms Early and Late Carboniferous convey the sense of relative time more immediately. In turn, each of the two parts of the Carboniferous is further divided into stages, with the Early Carboniferous consisting of an earlier Tournaisian and a later Viséan stage. Among vertebrate paleontologists, the names widely used for the Late Carboniferous are Namurian for the earliest part, Westphalian for the middle, and Stephanian for the latest. Because of more detailed work by sedimentologists and stratigraphers, these names are no longer universally accepted because they are imprecise . For example, the earliest part of the Namurian is now considered to be part of the Early Carboniferous, while parts of the Stephanian may fall within the Permian. The older names are nonetheless convenient to use for the study of tetrapods because they seem, by and large, to coincide with changes to the faunas. Subdivisions divide some of these stages, applicable in broad geographical areas, to give more precision to the relative dating of faunas. As far as plants are concerned, the Namurian-stage flora shows more similarities to the Early Carboniferous than to the Westphalian and Stephanian. Figure 7.1 sets out a simplified stratigraphical table of the Early Carboniferous and indicates the localities at which some the animals described in this chapter are found. Biogeography, Paleoecology, and Coal Much of the Carboniferous was a...

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