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472 FOSSIL VERTEBRATES/Palaeozoic Non-Amniote Tetrapods Figure Map of the world showing the distribution of Devonian tetrapods worldwide Other Devonian tetrapods discovered over the last 10 years or so include Hynerpeton, known from a lower jaw and pectoral girdle material, and Densignathus, known from a lower jaw, both from Pennsylvania, USA Ventastega, represented by excellent skull and pectoral girdle material, is from Latvia and, most recently, new taxa have been found in Greenland (as yet un-named), China (Sinostega) and Belgium (un-named) Metaxygnathus, from New South Wales, Australia, has been reconfirmed as a tetrapod in the light of work on Acanthostega Ventastega is the best known of these and, interestingly, although it shows many similarities to Acanthostega, it also shows some more primitive and some unique features Figure shows the distribution of Devonian tetrapod finds worldwide Tetrapods of the Early Carboniferous After the end of the Devonian, there is a long hiatus in the fossil record of tetrapods, informally known as ‘Romer’s Gap’ (Figure 5) There may have been a mass extinction that correlates with the end-Devonian, but, whether sudden or gradual, there appears to have been a climate change affecting plants and animals alike for the next 15–20 million years Unfortunately, it was during this period that tetrapods became fully terrestrial, but evidence to document this crucial period in the history of life on Earth is extremely scarce Only a few specimens represent the Tournaisian and early Vise´ an epochs in tetrapod history, whereas the record of the late Vise´ an and Namurian is now much fuller than it was even two decades ago Pederpes finneyae is the only articulated tetrapod specimen currently known from Romer’s Gap (Figures and 6) It was found near Dumbarton in Scotland, in marginal marine cementstone sequences that have not otherwise yielded many fossils It shows the earliest evidence of a hind limb that was used for walking and that was functionally pentadactyl, although what little is known of its fore limb suggests that it might have had relict accessory toes like those of the Devonian forms Its closest relative, called Whatcheeria deltae, is from the Vise´ an of Iowa, USA It is also possibly related to a mid-Vise´ an form, Ossinodus, from Queensland, Australia, and another possibly Tournaisian or Vise´ an form from Northern Ireland They each show some primitive features, but share some more derived ones with later Carboniferous forms, and appear to form a clade (or possibly a grade) of Early Carboniferous forms that were widely distributed in time and space Apart from these, a few isolated limb and girdle elements, and some large footprints from Horton Bluff in Nova Scotia, are all that represent the Tournaisian and early parts of the Vise´ an The tetrapods that have been discovered in the period following Romer’s Gap show a range of sizes and body forms greater than that seen in almost any vertebrate group before (Figures and 6) Certainly the tetrapods of the Late Devonian are quite conservative in shape and size – all are around a metre long with flattened and approximately spade-shaped heads Those of the mid-Carboniferous range from mouse-sized to several metres in length with evidence of diversity in locomotory and feeding adaptations
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