Other Devonian tetrapods discovered over the last10 years or so include Hynerpeton, known from a lower jaw and pectoral girdle material, and Den-signathus, known from a lower jaw, both f
Trang 1Other Devonian tetrapods discovered over the last
10 years or so include Hynerpeton, known from a
lower jaw and pectoral girdle material, and
Den-signathus, known from a lower jaw, both from
Penn-sylvania, USA Ventastega, represented by excellent
skull and pectoral girdle material, is from Latvia and,
most recently, new taxa have been found in
Green-land (as yet un-named), China (Sinostega) and
Bel-gium (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
fea-tures Figure 4 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
Tournai-sian 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 ( Fig-ures 5 and 6) It was found near Dumbarton in Scot-land, 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, al-though 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 Hor-ton 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 5 and 6) Certainly the tetrapods of the Late Devonian are quite conser-vative 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 Figure 4 Map of the world showing the distribution of Devonian tetrapods worldwide.
472 FOSSIL VERTEBRATES/Palaeozoic Non-Amniote Tetrapods