The Proceedings of the Eighth International Conference on Creationism (2018)

long with flanged, imbricate ribs anterior to a more flexible lumbar region, an arrangement similar to that seen in the Carboniferous tetrapods Pederpes and Whatcheeria (Clack 2002a; Clack and Finney 2005; Lombard and Bolt 1995). The differentiation of the trunk into thoracic, lumbar, postsacral and caudal regions would have permitted dorsoventral flexion of the body, and a distinctive form of locomotion on land (Ahlberg et al. 2005a). New ichthyostegid material, including a well-preserved and articulated hind limb, collected by an expedition to East Greenland in 1987, revealed that Ichthyostega was polydactylous, with seven digits on the hind limb (Coates and Clack 1990). The pectoral and pelvic girdles are large and the hind limb paddle-like, with flattened bones and an inflexible ankle (Pierce et al. 2012). Fish-like characteristics of Ichthyostega include a lateral line system, a tail with bony fin rays and an ear region specialized for underwater hearing (Clack et al. 2003). Acanthostega is also much more completely known as a result of material collected by the 1987 expedition, including the first postcranial remains (Bendix-Almgreen et al. 1988; Bendix-Almgreen et al. 1990; Clack 1988). Several articulated specimens in a mass-death assemblage appear to represent juvenile Acanthostega with humeri displaying varying degrees of ossification in an ontogenetic series (Sanchez et al. 2016). The remarkable preservation also means that some delicate structures, not often preserved in fossil tetrapods, are known in Acanthostega . The braincase and ear region are tetrapod-like (Clack 1989; Clack 1994a; Clack 1994b; Clack 1998). However, the gill skeleton is fish-like, indicating that Acanthostega had internal gills somewhat similar to those of the Australian lungfish ( Neoceratodus ) (Coates and Clack 1991). Indeed, Acanthostega appears to have been more aquatic than Ichthyostega , with a longer tail and more numerous lepidotrichia (Coates 1996). Unlike Ichthyostega , the ribs are small and straight with little differentiation along the vertebral column, suggesting that its primary mode of locomotion was tail-propelled swimming. This conclusion is supported by the morphology of the fore and hind limbs, which are difficult to interpret as load-bearing structures. An articulated fore limb revealed that Acanthostega had eight digits arranged in a paddle-like fashion (Coates and Clack 1990). Since the discovery of Ichthyostega and Acanthostega , our knowledge of Devonian tetrapods has been greatly expanded, with many new taxa being described (Table 2). Thirteen genera are now known from Greenland, Scotland, Latvia, Russia, the USA, Australia and China, and there is additional unnamed material from the USA, Russia and Belgium (Olive et al. 2016). Like Ichthyostega and Acanthostega , these taxa display mosaic combinations of fish-like and tetrapod-like characters, along with some unique traits (such as polydactyly) not found in more ‘derived’ tetrapods. Furthermore, new discoveries are beginning to populate the previously depauperate interval covering the Tournaisian and most of the Viséan, a part of the Lower Carboniferous record known as ‘Romer’s Gap’ after the great vertebrate palaeontologist Alfred Sherwood Romer (Coates and Clack 1995). The diverse tetrapod assemblages of the upper Viséan include fully terrestrial forms with five or fewer digits, quite unlike the polydactylous, aquatic and semi-aquatic tetrapods of the Frasnian and Famennian. Until recently, however, the only tetrapod fossils from the intervening Tournaisian were isolated skeletal elements, trackways and a single articulated skeleton of the whatcheeriid Pederpes (Clack 2002a; Clack and Finney 2005; Smithson et al. 2012). However, Clack et al. (2016) have now described five new Tournaisian tetrapods from two localities ( Perittodus , Ossirarus , Diploradus , Koilops and Aytonerpeton ). Other taxonomically indeterminate taxa have also been recovered. In addition to body fossils, putative trackways of tetrapods have been documented from a number of Devonian localities in Australia, South America and Europe (Clack 1997; Lucas 2015; Table 3). The most securely identified are the Genoa River trackways in New South Wales, Australia (Warren and Wakefield 1972) and the Valentia Island trackways in southwestern Ireland (Stössel 1995; Stössel et al. 2016). Niedźwiedzki et al. (2010) described trackways in the Zachełmie Quarry in Poland that are Middle Eifelian in age, and thus predate the earliest tetrapod body fossils by 14 million years and the oldest elpistostegids by 5 million years (Narkiewicz and Narkiewicz 2015). Lucas (2015) argued that these ichnofossils did not have the diagnostic characteristics expected of Devonian tetrapod tracks and trackways and re- interpreted them as fish feeding traces/nests ( Piscichnus ). However, Qvarnström et al. (2018) have defended the tetrapod identification, based on the well-preserved morphology and new data indicating a Garner and Asher ◀ Devonian and Carboniferous tetrapodomorphs ▶ 2018 ICC 459 Taxon Stratigraphic unit Age Location Material Reference(s) Elpistostege Escuminac Fm Frasnian Quebec, Canada Partial dermal skull roofs and part of axial skeleton Schultze (1996); Schultze and Arsenault (1985); Westoll (1938) Tiktaalik Fram Fm Frasnian Nunavut Territory, Canada Multiple articulated specimens Daeschler et al. (2006); Downs et al. (2008); Shubin et al. (2006); Shubin et al. (2014) Panderichthys Gauja Fm Givetian Latvia and Estonia Complete specimens Ahlberg and Clack (1998); Ahlberg et al. (1996); Boisvert (2005); Boisvert (2009); Boisvert et al. (2008); Brazeau and Ahlberg (2006); Vorobyeva (1977); Vorobyeva (1980); Vorobyeva (1992); Vorobyeva (1995); Vorobyeva (2000); Vorobyeva and Kuznetsov (1992); Vorobyeva and Schultze (1991) Table 1. Devonian elpistostegids mentioned in this paper. Givetian is a subdivision of the Middle Devonian and Frasnian is a subdivision of the Upper Devonian. Fm = Formation.