One of the major predictions of neodarwinian theory concerns the existence of 'transitional forms'. These are organisms that must have developed, according to evolution, as one major group evolved into another. Transitional forms would stand morphologically (i.e. in body form) between the ancestral and descendant groups. They would also be expected to lie between their ancestors and descendants in the vertical sequence of rock layers. Since every group of organisms is thought to have evolved from an earlier group, transitional forms between groups ought to be common. However, convincing examples of such transitional forms are rare; there are far fewer than evolutionary theory would predict. This seems especially true of the most easily preserved groups in the fossil record – shallow marine invertebrates such as molluscs and brachiopods. From an evolutionary perspective this is counterintuitive because these are precisely the groups that would be expected to yield the most convincing series of transitional forms.
Nevertheless, there are some fossil organisms, mostly among the vertebrates, which are claimed to be transitional forms. Among them are the fossils found in Upper Devonian sediments (conventionally dated to around 359-385 million years ago) which are said to document the evolution of the first tetrapods (i.e. vertebrates with four limbs) from fish-like ancestors. The Devonian tetrapods, represented by creatures such as Acanthostega and Ichthyostega, are one-metre-long aquatic animals thought to have lived a predatory lifestyle in weed-infested shallow water. Evolutionists have sought their ancestry among the lobe-finned fishes which form a major part of the fish faunas of Devonian sediments. One of these, a fish called Elpistostege, is thought to have been very tetrapod-like in overall body shape, although its remains are very incomplete and its anatomy is poorly known. Complete specimens of a similar lobe-fin, called Panderichthys, however, confirm the tetrapod-like impression. Panderichthys had a superficially crocodile-like skull with dorsally-placed eyes, a straight tail, and a slightly flattened body without dorsal or anal fins. Furthermore, like tetrapods, but unlike all other fishes, both Elpistostege and Panderichthys had frontal bones in the skull roof.
In 2006, the evolutionary transition between fish and tetrapods again came to prominence when a new fossil lobe-fin called Tiktaalik was described in the journal Nature (Daeschler et al. 2006). The name Tiktaalik means "large freshwater fish" in the traditional language of the Nunavut region of Arctic Canada where its remains were found. Several specimens, some extremely well preserved in three dimensions and with parts of the skeleton still articulated, were discovered in a 15-cm siltstone layer in the middle part of a rock unit called the Fram Formation. Upon reconstruction, the specimens revealed a creature that was, in effect, "a better-preserved version of Elpistostege" (Ahlberg and Clack 2006, p.748). Like its previously-discovered counterparts, Tiktaalik had a flattened body, gills, bony scales, and fins with fin rays. However, Tiktaalik had a longer snout, larger ribs and a wider spiracle (small gill slit), and lacked a bony gill cover. Tiktaalik also displayed some features of the neck and the fin skeleton that suggested that its body could be supported in shallow water or on land. In other words, Tiktaalik seemed to be even more tetrapod-like than Panderichthys or Elpistostege. Neil Shubin, a member of the team that discovered Tiktaalik, commented: "We describe this as a 'fishopod': part fish, part tetrapod" (Holmes 2006). What is more, Tiktaalik was found in sediments that are dated as older than those containing Acanthostega and Ichthyostega, but younger than those containing Panderichthys; it apparently lies between its presumed ancestors and descendants in the geological sequence. It is not hard to see why this discovery was regarded as important confirmatory evidence of the hypothesized evolutionary transition between the lobe-finned fishes and their tetrapod descendants.
How, then, should skeptics of neodarwinism evaluate the status of Tiktaalik? While this creature does seem to be a morphological and stratigraphic intermediate, comprising a strange mixture of fish-like features and tetrapod-like features, it should be noted that many organisms, both fossil and living, exhibit a similar mosaic distribution of character traits. A living example is the platypus, which has features of both mammals (hair, milk production) and reptiles (egg-laying). The late Stephen Jay Gould called such organisms "mosaic forms" or "chimeras" (Wise 1994, p.227). When these mosaic forms occur in the right part of the geological record, they are often interpreted as evolutionary intermediates linking major groups.
However, the mosaic pattern also poses a problem for evolutionary scenarios because it often makes it difficult to identify organisms or groups of organisms that possess the 'right' combination of characters to be considered part of an evolutionary lineage. Consider the tetrapod-like lobe-fins Panderichthys and Elpistostege. Despite their appearance, these fish have some unique characters (such as the design of the vertebrae) that rule them out as tetrapod ancestors. At best, evolutionists can only claim that they are a model of the kind of fish that must have served as that ancestor. The same problem is encountered with the Devonian tetrapods. For example, Ichthyostega is described as "a very strange animal, and parts of it are like no other known tetrapod or fish" (Clack 2002, p.115). Similarly, the shoulder girdles of the Devonian tetrapods "are not obviously halfway in structure between those of fishes and those of later tetrapods but have some unique and some unexpected features" (Clack 2002, p.42). Another example is Livoniana, a so-called 'near tetrapod' known from two lower jaw fragments. It possesses a curious mixture of fish-like and tetrapod-like characteristics, but it also has up to five rows of teeth, a feature not seen either in the fishes from which it is thought to be descended nor the tetrapods into which it is said to be evolving (Ahlberg et al.2000). A similar observation can be made with Tiktaalik, which has an unossified backbone with an unusually large number of vertebrae – more than in its presumed ancestors or descendants (Daeschler et al. 2006, p.761).
It is also worth noting that one of the key morphological transitions between fish and tetrapods – the purported change from paired fins to limbs with digits – remains undocumented by fossils. One of the most striking features of Tiktaalik is the structural arrangement of its pectoral fin, in which the individual bony elements are so well preserved that detailed studies were able to elucidate how it might have moved during life (Shubin et al. 2006). It appears that Tiktaalik had a sturdy, though highly flexible, fin with 'wrist-like' and 'elbow-like' joints that would have allowed the animal to prop itself up on a substrate. Nevertheless, Tiktaalik is still clearly a fish with fins and fin rays – it does not possess the fingers and toes that mark out the tetrapods. As Ahlberg and Clack (2006, p.748) write in their commentary in Nature: Although these small distal bones bear some resemblance to tetrapod digits in terms of their function and range of movement, they are still very much components of a fin. There remains a large morphological gap between them and digits as seen in, for example, Acanthostega: if the digits evolved from these distal bones, the process must have involved considerable developmental repatterning.
From a design perspective, Tiktaalik appears beautifully constructed for its life in an aquatic habitat where the water was not deep enough to support the body. Many of the 'fish-like tetrapods' and 'tetrapod-like fish' found in Devonian sediments, Tiktaalik included, are thought to have been ambush predators lurking among the tangled weeds and roots of shallow pools and other similar environments. They were therefore equipped with characteristics appropriate to that habitat (e.g. crocodile-like morphology with dorsally placed eyes, limbs and tails made for swimming, internal gills, lateral line systems) that gave them the ability to function both in the water and, to some extent, on land. In this context, their morphologically intermediate characters can be understood as a highly efficient design for life in the marginal aquatic ecosystems in which they lived.
Summary
Tiktaalik is a lobe-finned fish with an unusual combination of characters shared with other lobe-fins and aquatic tetrapods of the Upper Devonian. However, it also had some unique characters not found in its presumed ancestors or descendants. Although evolutionists have interpreted Tiktaalik as a transitional form, it was clearly a fish with paired fins and its discovery sheds little light on the origin of limbs bearing digits.
References
Ahlberg, P.E. and Clack, J.A. (2006). 'A firm step from water to land', Nature, Vol. 440, pp.747-749.
Ahlberg, P.E., Luksevics, E., Mark-Kurik, E. (2000). 'A near-tetrapod from the Baltic Middle Devonian', Palaeontology, Vol. 43, pp.533-548.
Clack, J.A. (2002). Gaining Ground: The Origin and Evolution of Tetrapods, Indiana University Press, Bloomington, 2002.
Daeschler, E.B., Shubin, N.H. and Jenkins Jr, F.A. (2006). 'A Devonian tetrapod-like fish and the evolution of the tetrapod body plan', Nature, Vol. 440, pp.757-763.
Holmes, B. (2006). 'The fish that headed for land', New Scientist, No. 2546, p.14.
Shubin, N.H., Daeschler, E.B. and Jenkins Jr, F.A. (2006). 'The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb', Nature, Vol. 440, pp.764-771.
Wise, K.P. (1994). 'The origin of life's major groups', pp.211-234 in: Moreland, J.P. (editor), The Creation Hypothesis: Scientific Evidence for an Intelligent Designer, InterVarsity Press, Downers Grove.