The Origin of the Tetrapod Stem Group – Climate Change in The Devonian and breathing

Australia: The Land Where Time Began

The Origin of the Tetrapod Stem Group – Climate Change in The Devonian and breathing

The stem group of tetrapods diversified in the Middle to Late Devonian (Givetian - Famennian stages) about 285-365 Ma. According to Clack¹ a reasonable consensus has been reached concerning the relationships between the known taxa that represent this radiation which allows the interpretation of the order of appearance of characters of the tetrapods to be possible. What is interpreted as a progressive increase in the spiracular chamber and its opening to the outside world is seen in the fish that are immediately related to the earliest limbed tetrapods, which is inferred here to be an increased capacity for breathing air. According to Clack most early bony fish are believed to have had lungs which most likely were ventilated by gulping air which is suggested to have possibly brought about the facultative capacity for breathing air from the atmosphere, the tetrapod stem group then exploiting it its greatest degree. Not only freshwater forms have these adaptations, it has been shown that they were also present in estuarine and marginal marine forms. It is suggested by estimates of atmospheric oxygen levels during the Givetian and Frasnian periods oxygen levels were unprecedentedly low, while at the same time the diversification of plants was at its most rapid level, which was contributing to the changes of the character of the landscape and, via soils, soluble nutrients and plant matter that was decaying, to the development of anoxia in all water systems. In at least 2 lobed-finned groups, the co-occurrence of these events on a global scale may explain the evolution of the adaptations for breathing air that contributed directly to the rise of the tetrapod stem group. Low levels of atmospheric oxygen is not considered to be a causal factor in the lack of fossils that document the early evolution of tetrapods in the Early Carboniferous, in contrast to recent studies.

Introduction

Knowledge of the “fish - tetrapod transition” – the diversification of the tetrapod stem group, the sequence and timing of the events have been augmented in recent years by finds of tetrapods from the Devonian (Ahlberg, 1991a, 1995; Ahlberg et al., 1994; Daeschler et al., 1994; Ahlberg & Clack, 1998; Lebedev, 2004) and their immediate stem group (Daeschler et al., 2006; Shubin et al., 2006). A result of this is that attempts can now be made to answer the questions such as where and when tetrapods (vertebrates with legs) evolved, and the order in which tetrapod characters arose, and how their evolution took place (Clack, 2002a, 2005). Knowledge of the environments they inhabited has also increased (Lebedev, 2004; Clack, 2006). A question that is more difficult to answer, though is commonly asked, is why the transitional forms made the first move towards life on land. Attempts to answer this question have recently involved reconstructions of the composition of the atmosphere in the Devonian, and the global temperatures and the distribution of continents at that time (Berner, 1999, 2006; Scotese, 2002; Berner et al., 2003). Combination of these with recent assessments of vegetation cover, the effect of this cover on rates of sedimentation, the production and erosion of soils (Algeo et al., 1995, 2001; Algeo & Scheckler, 1998), and when the invertebrate fauna are taken into account (Shear & Selden, 2001), has led to the generation of some new suggestions that might explain some of the events leading to the evolution of air breathing and ultimately to tetrapods that lived on the land.

There are 2 aspects of the morphology of tetrapods [i.e., stem plus crown groups of tetrapods (Ahlberg, 1991b)] that includes the early limbed forms, are of particular significance, the region of the spiracle and the hyoid arch, and the pectoral limb. It is also possible that changes seen in these regions among the tetrapodomorphs could be related. The spiracular region is interpreted as being associated intimately with breathing; It is known that the spiracular chamber became enlarged in the tetrapod stem group, and it has been suggested that the increasing size and complexity of the pectoral fin/limb skeleton function as support for the head and body anterior end as the animals emerged partly from the water (Shubin et al., 2004, 2006).

In this article Clack¹ reviews the spiracular region and the hyoid arch across the basal tetrapodomorph fish to limbed tetrapod transition with the aim of showing the increasing capacity of the spiracular chamber and increasing breadth of the spiracular notch apparent among them, suggesting the best explanation could be as an elaboration of an air-breathing mechanism. Information from other studies of environments, atmospheric composition, soils, sediments, plants, and invertebrates which suggests a new scenario for the evolution of air-breathing in the tetrapod stem group and related forms, such as the dipnoans, in the Devonian. The subsequent tetrapod evolution in the Carboniferous, which was a time when air-breathing became the major mode of breathing, is also considered, though to a lesser extent.

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