The spread of vascular plants into estuaries, river plains and deltas established habitats not only for a great range of invertebrates but also for freshwater fish and animals with more amphibian lifestyles. Some of these were tetrapods. The term ‘tetrapod’ designates an animal with four legs, on the basis that the acquisition of four legs was one of the key events in the history of life. The first tetrapods in the fossil record were predators, attracted to the animals that had entered these habitats before them. Having held on to life in what can have been far from ideal circumstances, they appear in places as far apart as Australia and North America, in close association, for the most part, with the plants, arthropods and fish that made up their natural world.

Only when there was sufficient shelter and humidity under the plant canopy and sufficient invertebrates to supply them with food is it likely that vertebrates would have begun to explore the terrestrial environment.

Jennifer A. Clack, Gaining Ground, p 96 (2002).

As we shall see, the environment that the first tetrapods explored was generally aquatic rather than terrestrial, but it was certainly influenced by the developing terrestrial world. That said, a new environment cannot of itself call forth the novelties that might enable it to be exploited. The evolution postulated in the standard interpretation depends on chance. It has to work by genetic mutations that little by little, and at every point along the way, produce innovations in body and behaviour that happen to confer an advantage. The existence of a new habitat has no effect on the likelihood of the right mutations occurring. It cannot climb ‘Mount Improbable’ simply ‘because it is there’.

Jawed fish can be classified into two big groups, those with a cartilage skeleton (such as sharks) and those with a bony skeleton. The bony fish in turn are classified into those with ray fins and those with lobe fins. In the Devonian the dominant bony fish were the lobe-fins, and it is from them that tetrapods are supposed to have evolved. Lobe-fins still exist today, in the form of the coelacanth and two families of lungfish; other lobe-fin families went extinct.

The origins of these groups are obscure, whether it be questions of how the jawed fish evolved from the jawless fish, the bony fish from the cartilaginous fish, or the lobe-fins and ray-fins from some earlier ancestor. The lungfish, for example, were from their first appearance both highly specialised organisms and surprisingly diverse (seven families). Today they comprise only two families.

Since they still inhabit the world, lungfish can be studied for what they are rather than for what they might have become. They live in lakes, rivers and wetlands. In addition to gills they have modified swim bladders (so-called ‘lungs’) which can be used for absorbing oxygen gulped from the air. Because they live in areas subject to periodic drying, African lungfish obtain most of their oxygen in this way. During times of drought they bury themselves in the mud and aestivate, in a state of suspended animation. Fossilised burrows from the early Carboniferous, Kentucky, show that this behaviour is ancient. By contrast, Australian lungfish live in areas which are wet all the time, and obtain most of their oxygen from the water.

Lungfish are the only group of fish to possess a type of circulatory system characteristic of some tetrapods. In their fellow lobe-fins, the coelacanths, the system is that typical of other fish. Nonetheless, while it used to be thought that lungfish gave rise to tetrapods, this view has been abandoned in favour of descent from other lobe-fins. Lungfish are therefore left to illustrate the point that some fish seem well designed for a mode of life intermediate between the aquatic and the amphibian, and are under no compulsion to choose between them. Interestingly they have the largest genomes in the entire living world, with one species Protopterus aethiopicus weighing in at a staggering 133 billion DNA base pairs (estimated). Human beings, by contrast, have around 3 billion.

Panderichthys – the nearest thing (before Tiktaalik) to a missing link

Amongst the other Devonian lobe-fins that are sufficiently well preserved to permit conclusions, the most eligible candidate to serve as an ancestor of tetrapods, until very recently, was Panderichthys. Over 1 metre long, the fish had eyes near the top of its skull, gills, a straight tail fin (like some lungfish and like the tetrapod Acanthostega) and tetrapod-like jaws. An external nostril (spiracle) in the same position as in the early tetrapods, on top of the skull, may have enabled the fish to inhale water while lying on the seabed and so avoid gulping in grit through the mouth, as similarly positioned nostrils enable sharks and rays to do.

Panderichthys differed from other lobe-fins in having just two pairs of fins: one pair at the front (the pectoral fins) and another at the back (the pelvic fins). While this arrangement is analogous to that of tetrapod limbs, the presumed loss of the dorsal and anal fins possessed, for example, by the next closest candidate, Eusthenopteron, seems to have been abrupt, and in some ways the front fins of Panderichthys appear less tetrapod-like than those of Eusthenopteron (Clack, p. 159). Unlike either Eusthenopteron or the early tetrapods it also lacked an iliac process.

The early tetrapods were mostly ‘rear-wheel drive’ animals with larger hind limbs than fore limbs. By contrast, the fins of their presumed ancestors tended to be larger in the front than in the rear. In the case of Panderichthys this difference is pronounced. Evidence recently analysed has shown that the pelvic fin is ‘more primitive’ (evolutionarily more ancient) than the front fin and the pelvic girdle ‘even less tetrapod-like’ than that of Eusthenopteron (Catherine Boisvert, 2005).

No fossils have ever been found showing a structure intermediate between fins and feet. In technical language Boisvert enumerates the ‘radical changes’ that have not been documented by fossils:

The pelvic girdle became a weight-bearing structure by evolution of an ischium, a full mesio-ventral contact between the two sides of the girdle, an ilium, and a contact between the vertebral column and the girdle through a sacral rib. Fore and hind-limbs shifted laterally by reorientation of the glenoid and the acetabulum. The pectoral girdle became detached from the skull by loss of the extrascapulars, posttemporal and supracleithrum, and became adapted for limb support and muscle insertion by enlargement of the scapulo-coracoid. Lepidotrichia [rays around the fins] were lost and digits were gained. The proportions of the limb elements changed by elongation of the humerus and (more strongly) femur relative to the ulna+radius and fibula+tibia, and equalization of the lengths of the radius+ulna, and tibia+fibula, by shortening of the radius and tibia. The postaxial processes of the ulnare and the fibula were lost, and the radius and ulna, as well as the tibia and fibula were realigned to be parallel rather than diverging. In the course of this transition, there was a shift in locomotory dominance from the forelimb to the hindlimb, which was first demonstrated by Acanthostega and Ichthyostega.

All this is imagined to have happened in 5 million years or less (after Panderichthys and before a fragmentary fossil called Elginerpeton). A big gap in morphology is exacerbated by a miniscule gap in time.

The same is true of parts of the body other than those involved in locomotion. As Per Ahlberg recently let on in regard to the transition as a whole, there is ‘a big gap in the fossil record between the most evolved fish and the earliest amphibians’ – notwithstanding his claiming, in the same breath, to have filled it by the discovery of a fragmentary jaw. Unlike the jaws of tetrapods or other lobe-finned fish, the jaw supported multiple rows of teeth, and came from rocks dated millions of years before the presumed transition between Panderichthys and the first tetrapod. Indeed more than one Panderichthys was found in the same rocks, as well as a coelacanth.

In some respects Tiktaalik roseae, a Panderichthys-like fish whose discovery was announced to the world on 6 April 2006, has supplanted Panderichthys in the story of how fish crawled out of the water. Its evolutionary significance is discussed elsewhere on this site.

Acanthostega and Ichthyostega – the first well-preserved tetrapods

No fewer than ten tetrapod genera are known from the Late Devonian. They are diverse from their first appearance, and almost as different from each other as they are from their presumed closest ancestors. For example, apart from similarities attributed to their ultimate common ancestry, Acanthostega and Ichthyostega ‘have almost nothing in common’ (Clack, p 121) and therefore, although contemporary, cannot be closely related to each other. Some were mainly aquatic in lifestyle, others, as evidenced for example by trackways in southwest Ireland, more amphibian.

Acanthostega (which means ’spine armour’, referring to certain features of its skull) was entirely aquatic. Its gill skeleton was fish-like and most closely resembled that of the Australian lungfish, which uses gills for taking oxygen from the water and ‘lungs’ for breathing air. Most probably the limb joints were not weight-bearing, and the digits – eight of them, not five as had long been expected of a tetrapod ancestor – were linked by webbing. The hind limbs functioned as paddles, pushing towards the rear. All in all, the animal presents a mosaic of primitive and ‘derived’ (i.e. newly evolved) features, prompting some authorities to suggest that it descended from a more terrestrial tetrapod that subsequently reverted back to the water! Were this true, it would narrow the time gap between Panderichthys and this first tetrapod down to zero.

Ichthyostega, stocky and heavily built, was ‘a very strange animal, and parts of it are like no other known tetrapod or fish’ (Clack, p 115). Among its many unique features were a narrow braincase, massive shoulders, and broad, overlapping ribs. With regard to the ribs, ‘there is more contrast in rib morphology between Acanthostega and Ichthyostega than there is between any other two Paleozoic tetrapods’ (p 313). The ribcage may have had some role in breathing, and in storing air during long periods under water.

Unusually for a tetrapod, the hindlimbs were diminutive compared with the forelimbs. The hindlimbs were paddle-like, as with Acanthostega, and ended in seven digits, two more than the world had been told about prior to 1990 and one less than Acanthostega had. Along with aspects of the vertebral column, the limb anatomy suggests that if it moved on land at all, Ichthyostegawould have moved like a seal, first arching its back, then simultaneously advancing both forelimbs, and finally bringing up the rest of its body. This would have been quite unlike the sinuous, side-to-side motion of fishes swimming in water. The possibility that it spent any time on land is speculation, however. Its fish-like tail, paddle-like hindlimbs, deeply grooved gill bars, highly specialised ear for hearing underwater, and lateral line system (a network of nerves which detected changes in water pressure) all indicate an aquatic existence. Its sharp teeth suggest a diet of fish and marine or freshwater invertebrates.

Owing to its specialisations Ichthyostega is considered to be a ‘sidebranch’ of the tetrapod family tree rather than a direct ancestor; it was a short-lived evolutionary ‘experiment’, a ‘dead-end’. The tetrapods that led to reptiles must therefore have predated Ichthyostega and remain to be discovered. For much the same reason Panderichthys and Acanthostega are also thought to represent sidebranches. Palaeontologists cannot point to any fossils that have a truly transitional status. The transition, Robert Carroll supposes, ‘did not occur within a single lineage, but encompassed a series of radiations.’ There was a diversity of intermediate forms, because the ‘transition occurred within an extensive wetlands ecosystem that included a variety of distinct habitats’.

Designed for life in the shallows

Those habitats are indeed a key part of the story, a story that has changed radically in recent years. The possession of lobe fins, it turns out, had nothing to do with being ‘adapted’ for life in the shallows. Many lobe-finned fish lived in deep water, as the coelacanth does today. Some modern ray-finned fish, such as frogfish, have fleshy fins not unlike lobe fins with rays not unlike digits, which they use for walking along the sea bottom. The diversity of modern fishes and ancient tetrapods seems purposely to subvert any attempt to construct a story where limbs and digits are acquired in the course of ‘conquering the land’. We are no longer to imagine fish crawling on their fleshy fins out of ponds that had dried out under the midday sun, in search of deeper ponds elsewhere, and discovering that they could, after all, survive on the land. Limbs and digits were acquired while the animals concerned were still adapted for life in the water. Acanthostega, to judge from its anatomy, had no thoughts of venturing onto the land. If Ichthyostega did (which is far from certain), it moved like a seal, not a reptile. And as every palaeontologist knows, seals are not examples of animals evolving from life in the sea to life on land – quite the reverse.

The characteristics which the Devonian tetrapods had in common with certain lobe-fins were those which suggest that they were designed for a similar kind of life, predators lurking in the shallows. Their flat skulls had an opening on top that may have enabled air to be inhaled while the rest of the head was submerged and out of sight, though Panderichthys seems to have been more of a bottom-dweller and used it only for inhaling water. The habitats of tetrapods were various kinds of wetland, which were ‘clogged’ (if that is not putting it too strongly) with vegetation. ‘Plant-clogged waterways made weight-bearing fins, and eventually limbs, useful for getting around,’ suggests Neil Shubin, imagining the weight-bearing fins that are not there in the fossil record. While Ichthyostega in some respects resembled a seal, others had limbs arranged more like a crocodile’s or newt’s, with the shoulders facing sideways and the arms projecting out from the trunk at right angles. Again, there is no evidence that crocodiles or newts are in evolutionary transition, and there is no reason why the early tetrapod evidence need be so interpreted.

No reason (leaving ideological commitment aside) except one: the tetrapods, displaying a heterogeneous assortment of features – primitive, derived, even unique, highly specialised features – nonetheless appear at the right time. They are preceded by lobe-finned fish and they are followed by four-footed, fully terrestrial reptiles. Between the most tetrapod-like fish and the first actual tetrapod there is a temporal gap, and while it is uncomfortably small, at least there is a gap. Whatever the difficulties surrounding the evidence when considered close-up, the sequence is of a kind that might be expected, given that life started in the sea. Creationists may dwell on the difficulties and emphasise the differences in anatomy rather than the similarities, but their critique offers no credible alternative. An evolutionist explanation for the sequence has therefore seemed the only one possible, winning by default.

Palaeontologists do not study body fossils in isolation; they study them in relation to their ecological environments. And it is a remarkable fact that, almost as soon as new environments present themselves, there are almost always animals to exploit them, turning up, apparently, out of nowhere. Because of the overall order of appearance – a recolonisation sequence which in some respects mimics the expected evolutionary sequence – Darwinians interpret the phenomenon as animals becoming adapted to these environments through natural selection, as if the existence of a new habitat of itself produced the variations on which selection could act. The tacit logic is that the overall order of appearance supports the view that life originated from the sea, and since evolution by natural selection is the ‘only game in town’, it must have been the mechanism for the evolution presumed to have taken place. The large morphological gaps are interpreted as accidents of an incomplete record.

But even the argument that the fossils appear in the right order no longer seems available. In 1995 Iwan Stössel reported several trackways made by tetrapods in the Valentia Slate Formation of southwest Ireland, and these, it was subsequently established, were of the same age as Panderichthys. When several years later Jennifer Clack discussed them, she was clearly unaware of the dating work, for she supposed that they were significantly younger. Convinced that the body fossil evidence showed that tetrapods came into being midway through the Frasnian, she ventured the opinion that ‘tracks made by a terrestrial tetrapod are unlikely to be found before the late Frasnian’.

That the tracks were those of a tetrapod was evident from the difference in size between the front and the hind foot, and from the way the angle of stride varied as the animal moved. Two of the trackways included sinuous drag marks left by its belly, but whether the animal could walk clear of the ground without the support of water is unknown, since, like crocodiles, which also sometimes crawl on their bellies, it may have been capable of more than one type of gait. The environment was not one choked with vegetation, but an apparently barren floodplain.

Once one has stripped away the Darwinian language, the story actually told is that of a world going through stages of ecological recovery. From a distance, recolonisation can appear like evolution, because the organisms concerned do not appear all at once, but progressively, and in the process they diversify into new species. As animals suited to exploiting the waters of the coastal margins, the Devonian tetrapods were an ecological rather than evolutionary step up – rare in the fossil record because they were rare, at this early stage, in fact. It is not necessary to choose an explanation of the succession which endows Nature with supernatural powers of creation.

The power of the paradigm

Evolutionists, however, seem unable to reason outside their theory. In practice, research takes place in a one-party state, and the unspoken but accepted rules are: If an analysis supports Darwinian doctrine, it may be accepted; if it does not, an explanation must be found for why it does not. It is a game where ‘heads I win, tails you lose’, a logically closed system in which it is not permitted to view the sudden loss of anal and dorsal fins, the morphological gap between Panderichthys and Acanthostega, the fact that Acanthostega cannot have been the direct descendant of Panderichthys, the narrowness of the temporal gap between them, the fact that the earliest tetrapod limbs and digits were designed for swimming rather than walking, the diversity of the earliest tetrapods, and their polydactyly all as evidence against the theory – as evidence tantamount to falsification of the theory. The problems attending the appearance of diverse kinds of arthropods, all with diverse kinds of legs, are even worse. How did the trilobites, eurypterids, horseshoe crabs, centipedes, millipedes, spiders, insects and so on acquire their legs? One does not even ask, for there are no plausible antecedents.

Numerous disparate organisms, moreover, have to be interpreted as having evolved already far past the ‘primitive tetrapod’ stage immediately after the presumed transformation of fish into tetrapods. One example is the lepospondyls (click on diagram left), quite small animals that are ‘highly derived when they first appear in the fossil record, with no plausible intermediates between them and any other groups’ (Carroll 2001). Although they share some features, they are also a disparate group in relation to each other. Perhaps most anomalous are the snake-like aïstopods, which appeared in the early Carboniferous and had neither limbs nor limb girdles. On the evidence of the skull, vertebrae and ribs, they are classified as tetrapods, so they are presumed to have completely lost their limbs after only just acquiring them. Even supposing that this did happen, it is difficult to see how it could have happened in the interval between the evolution of the first lepospondyl, some time after the first tetrapods, and the first aïstopod in the Visean. The aïstopods may have been aquatic. The Visean tetrapod Crassigyrinus certainly was, and is believed to have somehow retraced the evolutionary journey of its terrestrial ancestors back to the sea – without needing to lose its limbs. Bizarre indeed!