Abstract
The Lower Mississippian (Tournaisian) Ballagan Formation in SE Scotland yields tetrapod fossils that provide fresh insights into the critical period when these animals first moved onto land. The key to understanding the palaeoenvironments where they lived is a detailed analysis of the sedimentary architecture of this formation, one of the thickest and most completely documented examples of a coastal floodplain and marginal marine succession from this important transitional time anywhere in the world. Palaeosols are abundant, providing a unique insight into the early Carboniferous habitats and climate.More than 200 separate palaeosols are described from three sections through the formation. The palaeosols range in thickness from 0.02 to 1.85m and are diverse: most are Entisols and Inceptisols (63%), indicating relatively brief periods of soil development. Gleyed Inseptisols and Vertisols are less common (37%). Vertisols are the thickest palaeosols (up to 185cm) in the Ballagan Formation and have common vertic cracks. Roots are abundant through all the palaeosols, from shallow mats and thin hair-like traces to sporadic thicker root traces typical of arborescent lycopods.Geochemical, isotope and clay mineralogical analyses of the palaeosols indicate a range in soil alkalinity and amount of water logging. Estimates of mean annual rainfall from palaeosol compositions are 1000–1500mm per year. The high mean annual rainfall and variable soil alkalinities contrast markedly with dry periods that developed deep penetrating cracks and evaporite deposits. It is concluded that during the early Carboniferous, this region experienced a sharply contrasting seasonal climate and that the floodplain hosted a mosaic of closely juxtaposed but distinct habitats in which the tetrapods lived. The diversification of coastal floodplain environments identified here may link to the evolution and movement of tetrapods into the terrestrial realm.
Highlights
The terrestrialization of vertebrates is one of the most important events in the evolution of life on Earth
We describe and interpret for the first time the palaeosols found in the Ballagan Formation, in order to understand the range of habitats available to the earliest terrestrial tetrapods
The simplified interpretation of USDA Soil Taxonomy can be used to relate the pedotypes found in the Ballagan Formation to modern soil types
Summary
In Poland, the occurrence of Mid Devonian tetrapod trackways suggests this change may have been driven by the development of novel habitats such as woodlands (Retallack, 2011) and intertidal and lagoonal environments (Niedzwiedzki et al, 2010). These early forms of limbed vertebrates have been shown to be mainly aquatic or semi-aquatic, with primitive features (Clack, 2009; Pierce et al, 2012; Smithson et al, 2012) and the trackways were formed underwater (Narkiewicz et al, 2015).
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