Aquatic tetrapods challenge the ‘drying pond’ hypothesis
Since 1990 our knowledge of ‘early’ tetrapods has been greatly expanded, with many new taxa being described. Fossil material is now known from Scotland, Greenland, Latvia, the USA, Australia, Russia, and China (Table 1).23–43 Furthermore, our understanding of the Greenland tetrapods has been revolutionized by the discovery of new material. As a consequence, a major re-evaluation of tetrapod origins has taken place, and almost every aspect of the ‘drying pond’ hypothesis has had to be discarded.
The fatal blow to the ‘drying pond’ hypothesis has been the realization that the Devonian tetrapods were predominantly aquatic in habit. 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 (Figure 1).44 This was a very surprising discovery because pentadactyly had been assumed to be the normal condition in ‘early’ tetrapods. Furthermore, the flattened bones and inflexible ankle of the hind limb suggests that it was more like the paddle of an elephant seal than the leg of a terrestrial animal.45 It appears that the earliest reconstruction of Ichthyostega as a creature at home in the water was more accurate than later ones portraying it on land.
Acanthostega is also much more completely known as a result of material collected by the 1987 expedition, including the first postcranial remains.47,48 It was a smaller animal than Ichthyostega and its teeth suggest that it had a different diet. Several articulated specimens were found in a single lens of rock, interpreted as a possible flash flood deposit.49 The remarkable preservation meant that some delicate structures, not often preserved in fossil tetrapods, are known in Acanthostega. The gill skeleton was fish-like50 and it has been suggested that Acanthostega had internal gills somewhat similar to those of the Australian lungfish (Neoceratodus). Acanthostega had a tail with fin rays, even larger than that of Ichthyostega (Figure 2). The fin rays also extended further beneath the tail, in similar fashion to those of a lungfish, suggesting that Acanthostega was a thoroughly aquatic creature. This conclusion is supported by the morphology of the fore and hind limbs which are difficult to interpret as load-bearing structures; rather, they appear to be designed for swimming. As with Ichthyostega, perhaps the most extraordinary feature was the number of digits. An articulated fore limb revealed eight digits in a paddle-like arrangement (Figure 3). Clack51 speculates that they may have been enclosed in some kind of webbing.
Most evolutionists had assumed that the origin of limbs with digits was synonymous with the vertebrate invasion of the land. This led to the popular ‘conquest of the land’ idea, typified by artistic reconstructions and museum displays of fish crawling out of Devonian pools. However, the latest thinking about the aquatic or semi-aquatic nature of the Devonian tetrapods has led modern-day evolutionists to reject this assumption. They now argue that the key tetrapod characters evolved for a shallow-water existence and were only later co-opted for terrestrial use. The new generation of Darwinists dismisses the ‘drying pond’ hypothesis as untestable story-telling, and increasingly relies on cladistics as an alternative framework for understanding the transition. The cladistic approach to the fish-tetrapod transition focuses on determining the sequence of acquisition of key tetrapod characteristics, from which inferences are drawn about the nature of the transition.53 We should recognize, however, that the cladistic methodology is inherently Darwinian and assumes from the outset the continuity of life. By its very nature, cladistics is insensitive to the discontinuities which creationists believe characterize living things.54
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