Devonian tetrapod habitats indicated by palaeosols in Pennsylvania Journal of the Geological Society service Email alerting

Late Devonian (Famennian) tetrapods from the Duncannon Member of the Catskill Formation, near Hyner (Pennsylvania, USA) have been found within palaeosols that offer a new line of evidence for their ancient environments. Hyner has yielded at least two fossil tetrapod species, Hynerpeton basseti and Densignathus rowei. Subhumid Vertisol palaeosols and semiarid Aridisol palaeosols alternate at Hyner, as in other Catskill Formation sedimentary cycles attributed to Milankovitch-eccentricity periods (100 ka). Tetrapods were found in palaeosols of subhumid, not semiarid, phases of 100 ka palaeoclimate cycles. In addition, palaeosols at Hyner record an unusually warm and wet climatic episode compared with other palaeosols of the Catskill Formation, and correlated with the European Annulata black shale event. Tetrapod bones are cracked and disarticulated in red palaeosols of an oxbow lake margin, as if victims of dry-season death, decay, and subaerial weathering. Vegetation-choked swales and floodplains would have afforded shelter from 3–4 m long predatory fish (Hyneria lindae) of open water, and offered food of large dead fish, small live fish, arachnids, myriapods and scorpions. If these early tetrapods lived on land at all, it was during wet seasons of feeding and breeding, rather than fatally dry seasons. Supplementary material: Chemical data are available at http://www.geolsoc. org.uk/SUP18373. For many years, the ‘desert pond’ scenario for evolution of amphibians from fish postulated that limbs evolved from fins adapted to overland escape from shrinking ponds in dry climates or seasons (Romer 1933). Others have argued that desert ponds selected for evolution of limbs to dig aestivation burrows (Orton 1954) or clamber over a crowd of tadpoles (Warburton & Denman 1961). Subsequently discovered aquatic limbs and tails in Devonian tetrapods were interpreted as adaptations for reproduction and feeding in shallow water, rather than on land (Clack 2002). Other biotic models of selection argue for evolution of limbs and air-breathing to escape marine or lagoonal anoxia (Inger 1957; Clack 2007) or predation (Goin & Goin 1956). Testing such hypotheses requires not only tetrapod fossils close to the fish–amphibian transition and information on their adaptive features (Clack 2002; Long & Gordon 2004) but also independent evidence for their palaeoenvironment, such as palaeosols. Palaeosols reveal formative factors of past soils, including climate, vegetation, palaeotopography, parent materials and time for formation (Retallack 2001). Palaeosols can be correlated to establish geological age from regionally significant attributes, such as degree of development (Rutter et al. 2006). Climatic variables such as mean annual precipitation also can be inferred from the degree of nutrient leaching from clayey subsurfaces of palaeosols (Sheldon et al. 2002), and from the depth of carbonate nodules in the profile (Retallack 2005). Palaeosols are also by definition in the place they formed, and so reveal the distribution of ancient ecosystems represented by root traces and burrows within ancient sedimentary environments (Retallack et al. 2000). We have studied palaeosols near Hyner, Pennsylvania (Fig. 1), as evidence for habitats of the fossil tetrapods found there (Daeschler et al. 1994; Daeschler 2000a; Shubin et al. 2004; Clack 2006). We also contrast other palaeosols in Pennsylvania lacking tetrapods, representing other palaeoenvironments and palaeoclimates available during the Late Devonian. Geological setting At least two species of early tetrapods and a variety of other fossil animals and plants (Table 1) have been found in a road cut near Hyner, in Clinton County, Pennsylvania (Fig. 1). These fossiliferous red and grey siltstones are within the Duncannon Member of the Catskill Formation (Woodrow et al. 1995). The Duncannon Member is widespread in Pennsylvania, although some authorities (Harper 1999), but not all (Cotter et al. 1993; Cotter & Driese 1998), merge it with underlying Sherman Creek Member into the Buddys Run Member at Selinsgrove, also reported here (Fig. 1c). A geological age of late Fammenian (late Fa2c), roughly 363–366 Ma (Gradstein et al. 2004), is suggested for the Hyner tetrapods by fossil spores of the VH (Apiculatisporites verrucosa–Vallatisporites hystricosus) zone (Marshall et al. 1999; Traverse 2003), consistent with this stratigraphic level (Sevon & Woodrow 1985; Harper 1999). The sandstones at Hyner have been interpreted as former channels of large rivers, which drained high Acadian mountains (Woodrow et al. 1995) of a long southern promontory to Euramerica (Fig. 2). The most fossiliferous strata are within a broad swale-like feature filled with red and grey siltstone (see photomosaics of outcrop and panel diagrams of Woodrow et al. 1995; Cressler 2006), interpreted as a seasonally inundated oxbow lake formed in an abandoned meander of a large river (Woodrow et al. 1995), or as one of several swales within the ‘avulsion belt’ (proximal floodplain) of a meandering stream (R.L. Slingerland, pers. comm.). Materials and methods Geological sections at the Hyner locality were measured using the method of eye-heights, in which the Brunton compass is used as a levelling device (Compton 1985), but detailed sections of palaeosols were measured using a metric flexible tape. The Fig. 1. Measured sections (a–c), their correlation (d) and location (e) in Pennsylvania: (a) fossil tetrapod locality in a road cut north of state highway 120, 3 km west of Hyner, Clinton County (41.298538N, 77.819168W); (b) correlative deep-calcic paleosols on state highway 322, 4 km NW of Newport, Perry County (40.505708N, 77.127668W); (c) near-complete section of Catskill Formation along US highway 11/15 south from 4 km south of Selinsgrove, Snyder County (40.762168N, 76.864558W); (d) correlation of deep-calcic (.650 mm) palaeosols in the Selinsgrove section with anoxic events in a composite section of Vogelsberg, Köstenhof and Drever quarries of Germany (Buggisch & Joachimski 2006); (e) locations and Devonian outcrop in Pennsylvania. Levels A–H in (c) are deep-calcic palaeosol intervals correlated with marine black shale events in (d). G. J. RETALLACK ET AL . 1144