Plants were beginning to stabilise terrestrial environments, and their spread 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 – animals with four legs. The first tetrapods were predators, attracted to the animals that had entered these habitats before them. Having held on to life in what were far from ideal circumstances, they appeared in places as far apart as Australia and North America, in close association 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).

Environments were diversifying, and animals were diversifying in tandem, availing themselves of the new ecospace; in non-technical language, they were being fruitful and multiplying. But the world first explored by the tetrapods was aquatic, albeit influenced by the developing terrestrial world, and they were invading these brackish to freshwater margins because their prey was invading them. Far from being pioneers, they were following where fish had gone before.

Almost all modern fish have jaws, and jawed fish can be classified into two main groups, those with a cartilage skeleton (such as sharks) and those with a bony skeleton. Bony fish in turn group into those with ray fins and those with lobe fins. At these levels of classification, fish origins are obscure, whether it be how jawed fish evolved from jawless, the bony fish from the cartilaginous, or the lobe-fins and ray-fins from some ancestor that was neither one nor the other. They were diversifying and multiplying too fast for the fossil record to keep up with them, and the bounds of evolution are consequently impossible to draw.

The Devonian is called ‘the age of fishes’ because this is when a great variety of fish appeared – at least 70 families. The dominant group was the lobe-fins, from which tetrapods purportedly evolved. Lobe-fins still exist in the form of coelacanths and lungfish, but they are no longer dominant. ‘The age of fishes’ was itself short-lived. Massive perturbations at the end of the Devonian tore into the oceanic world, rendering 75% of fish families extinct.

Lungfish

The theory of evolution requires that all organisms be related to each other. Initially, since no better candidates were on offer, tetrapod ancestry was traced back to lungfish. The combination of fish with lungs seemed an obvious intermediate stage. However, the natural world has not proved so simple. Air breathing has arisen independently numerous times in the course of fish evolution, and the view that lungfish went on to become committed land-dwellers has been dropped. Indeed, they are more likely to be characterised as ‘living fossils’ – animals that have changed little for tens, even hundreds, of millions of years.

There are now just three genera of lungfish, one occurring in parts of Africa, one in South America, and one in Australia. They live in a variety of habitats, such as lakes, rivers and wetlands, and their breathing apparatus varies accordingly. In addition to gills for breathing under water they have highly modified swim bladders (‘lungs’) with which they absorb oxygen gulped from the air. The most proficient air-breathers are the African lungfish. They have overcome the hazards of drought conditions not by evolving legs and abandoning water altogether – or permanently returning to the water – but by developing a cardio-pulmonary system that minimises loss of oxygen to the environment and instinctively burying themselves in the mud, where, sometimes for months, they lie cocooned in a state of suspended animation. Fossilised burrows from the early Carboniferous show that this behaviour is ancient. Australian lungfish, by contrast, live in areas which are wet all the time, are less well-adapted to air-breathing, and obtain most of their oxygen from the water. The earliest lungfish were completely marine, so the innovation of specialised lungs must have arisen during the Devonian, not long after they first appeared in the fossil record.

The transformation of swim-bladders into lungs, concomitant changes in heart design and blood circulation, and the ability to slow down metabolic rate to a trickle are surely no less remarkable innovations than the alleged transformation of fins into legs. Although not seen in the fossil record, the innovations seem to have occurred very quickly. As soon as lungfish were numerous enough to impact the record, at the beginning of the Devonian, their forms were already highly distinctive. They reached their peak of diversity in the mid to late Devonian. By the Permian the number of genera had declined by over 70%, after which they ceased to innovate.

Hundreds of millions of years of stasis since then (assuming the chronology is valid) shows that fish on the interface between sea and land are not necessarily in transition from one to the other. Despite its hazards, the intermediate zone might be what they were designed for – even if they had been fully marine initially. In terms of their genetic make-up there is good reason to think of ‘design’. Despite representing an evolutionary dead-end, lungfish have the second largest genomes in the animal world, with one species Protopterus aethiopicus weighing in at a staggering 133 billion DNA base pairs (estimated). The largest genomes of all are those of some amoebas! 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 for an ancestor of tetrapods, until recently, was Panderichthys. Over 1 metre long, the fish had eyes near the top of its skull, gills, and a straight tail fin (like some lungfish and like the tetrapod Acanthostega). It also, like the tetrapods, had on top of the skull an external nostril, or spiracle, which may have enabled it to inhale water while lying on the seabed and so avoid gulping in grit through the mouth; sharks and rays have similarly positioned nostrils.

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 very small gap in time.

Partly for this reason, Tiktaalik roseae, a fish whose discovery was announced to the world on 6 April 2006, has supplanted Panderichthys in the story of how vertebrates crawled out of the water. In the paper describing the new fossil it is now admitted that Panderichthys possessed ‘relatively few’ tetrapod-like features. Tiktaalik’s 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. 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.

Ichthyostega – comprising a unique mixture of features, with massive shoulders and seven hind digits – had an amphibian life style. Acanthostega, its late Devonian contemporary, was wholly aquatic, its limbs functioning as paddles.

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 tetrapod ancestors – were linked by webbing. The hind limbs functioned as paddles, pushing towards the rear. All in all, the animal was a mosaic of primitive and derived features, prompting some authorities to suggest that its lineage represented a reversion back to the water by more terrestrial ancestors! In that case, the time gap during which Panderichthys might have evolved into the first tetrapod would be zero.

Ichthyostega, stocky and heavily built, is ‘a very strange animal, and parts of it are like no other known tetrapod or fish’ (Clack, p 115). Among its many unique features are 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. If it moved on land at all, Ichthyostega would have moved like a seal, arching its back, then 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. Other aspects of its anatomy – 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) – indicate that it led a predominantly aquatic existence. Its sharp teeth suggest a diet of fish and marine or freshwater invertebrates. Only in adulthood, when the still developing musculature of its forelimbs enabled it to lift at least the front part of its body off the ground, is it likely to have spent any time on land.

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. Panderichthys and Acanthostega are also thought to represent sidebranches. There are no 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 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 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 coelacanths do today. Some modern ray-finned fish, such as frogfish, have fleshy fins with rays not unlike lobe fins with 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’. Gone is the picture of fish crawling on their fleshy fins out of ponds that had dried out under the tropical sun, in search of deeper ponds, and discovering that after all they could survive on land. (In reality, of course, fish stranded in desiccated pools simply die.) 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 such as flattened skulls and dorsally placed eyes, were those which suggest they were designed for a similar kind of life, predators lurking in the shallows. The habitats of tetrapods were various kinds of wetland, fringed here and there with vegetation. 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 at right angles. Just as crocodiles or newts are not in evolutionary transition, so there is no reason why the early tetrapods need be thus interpreted.

No reason (ideological commitment aside) except one: the tetrapods nonetheless appear at the right time. They are preceded by lobe-finned fish and they are followed by four-footed, fully terrestrial reptiles. While the time gap between the most tetrapod-like fish and the first actual tetrapod is uncomfortably small, at least there is a gap.

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 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

Palaeontologists, 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 is acceptable; 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 – 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 dare even to ask.

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!