Devonian climate change, breathing, and the origin of the tetrapod stem group.

The diversification of the tetrapod stem group occurred during the late Middle through the Late Devonian, that is from the Givetian to Famennian stages about 385-365 million years ago. The relationships between the known taxa representing this radiation have currently reached a reasonable consensus so that interpretations of the order of appearance of tetrapod characters is possible. The immediate fish relatives of the earliest limbed tetrapods show what is interpreted as a progressive increase in the spiracular chamber and its opening to the outside. Here, this is inferred to be associated with an increased capacity for air-breathing. Lungs are thought to have been present in most early bony fishes, and were most likely ventilated by air-gulping. This could have brought about a facultative capacity for air-breathing, which the tetrapod stem group exploited to the greatest degree. These adaptations are shown not only in freshwater forms but also in estuarine and marginal marine forms. Estimates of oxygen levels during this period suggest that they were unprecedentedly low during the Givetian and Frasnian periods. At the same time, plant diversification was at its most rapid, changing the character of the landscape and contributing, via soils, soluble nutrients, and decaying plant matter, to anoxia in all water systems. The co-occurrence of these global events may explain the evolution of air-breathing adaptations in at least two lobe-finned groups, contributing directly to the rise of the tetrapod stem group. In contrast to recent studies, low atmospheric oxygen is not considered to be a causal factor in the lack of fossils documenting the evolution of Early Carboniferous tetrapods.