Eusthenopteron (fish) ⇒ Panderichthys (fish) ⇒ Acanthostega (tetrapod)

The origins of animal classes and phyla prior to the mid Palaeozoic, including all fish classes, are obscure. By the time animals appeared as fossils, nine-tenths of preserved geological time had passed. The evolution that led to this final chapter of their story was all unseen.

Half way through the Palaeozoic it is alleged that a select group of fish evolved into tetrapods, animals with limbs and feet. ‘Tetrapods’ rather than ‘amphibians’ is the preferred term, because contrary to the expected Darwinian story, it has become apparent that the first four-footed animals acquired their feet while still living in the water.

Background: fish living and extinct

Fish take so many wonderful forms that it is difficult to untangle their relationships, and schemes for classifying them have changed over the years, but currently living fish are divided into 5 classes:

• Hagfish (Myxini)
• Lampreys (Petromyzontida)
• Cartilaginous fish (Chondrichthyes)
• Ray-finned fish (Actinopterygii)
• Lobe-finned fish (Sarcopterygii)

We will assume for the sake of argument (and it seems likely) that each of these classes is monophyletic, i.e. they each stem from a common ancestor.

Classes subdivide into orders. The hagfish and lamprey classes consist of only 1 order. The cartilaginous fish comprise 14 orders: chimaeras, sharks of various kind, skates, sawfish and rays. The ray-finned fish comprise 44 orders. Extremely diverse both in form and habitat, they now make up the vast majority of fish in the sea. The lobe-finned fish consist of 2 orders, coelacanths and lungfish.

In addition there are several extinct classes, notably:

• Conodonts
• Pteraspidomorphs (including the heterostracans)
• Anaspida
• Osteostracomorphs
• Placoderms
• Acanthodians

The general evolutionary scheme proposed for fish is that those with jaws evolved from those without jaws and, somewhat in parallel, fish gradually evolved fins.

From earliest to latest, the first fossil occurrences of these ten classes, some living, some extinct, are as follows (dates in millions of years, radioisotope dating):

The periods shown are not necessarily when the class originated. Isolated teeth, scales and bone fragments often occur somewhat earlier – shark scales, for example, are known from the Late Ordovician. Not all fish have teeth, scales or bone, and it should be remembered that jawed bony fish are easier to fossilise than jawless, cartilaginous fish (such as lampreys). The earliest fish are certainly jawless. Jawed fish did not become numerically dominant until the second half of the Palaeozoic.

The pattern of rare fragments followed by more common, better preserved remains reflects a world where fish were initially few and far between, and where the still unstable habitats on the continental margins tended to erode and destroy fragile carcasses. The colonising fauna came from the open sea, where the floors beneath them were hot, basaltic wastelands offering little by way of habitat and nutrition. Their food had to come from algae and other kinds of plankton, so that they had no use for teeth and jaws. Now, however, near-shore environments were becoming more habitable, and morphologically more complex forms were developing in tandem, capable of nibbling seaweeds, filtering sediment, cracking open shells and indeed eating other fish. Brackish and freshwater environments were beginning to host a range of plant and invertebrate life as well as fish, and as the available niches became more various, so the fish moving into the new estuaries and lagoons became more specialised.

Genetic programs are so fantastically complex that no transformation can be ruled out as impossible. Jawed fish seem to have evolved from jawless fish, and there is no reason why one subgroup of jawed fish might not have undergone a no less radical transformation into amphibian tetrapods. Nonetheless, one cannot just assume that this is what happened, on the grounds that it must have. The question is, does the fossil evidence indicate that it did?

The class that presents the smallest apparent gap between fish and tetrapod, at the appropriate time, is the lobe fins. They had fleshy pectoral and pelvic appendages that, at least superficially, suggest an analogy with tetrapod limbs, and they were, in the Devonian, the most diverse fish group, offering a range of forms from which to choose the most tetrapod-like. Eusthenopteron and Panderichthys were two such fish:

Eusthenopteron is sometimes figured as crawling out of the mud of a Devonian lake, apparently with the intention of finding another pool to swim in. … Because Eusthenopteron was once cast in the role of the “ancestor” of tetrapods, tetrapodlike behavior was attributed to it. … However, taking the whole morphology of the fish, with its streamlined torpedo shape, and dorsal, anal, and pelvic fins place near the back of the body, it seems that the lifestyle of Eusthenopteron was much more like that of a modern pike (Esox), a fully aquatic lurking predator.

Jennifer Clack, Gaining Ground, p. 63 (2002)

When there were no better candidates, Eusthenopteron was regarded as well on the way to becoming a tetrapod, and was duly portrayed as such in illustrations. Now a more rounded view can be taken and it is no longer seen as a convincing intermediate. ‘Palaeontologists didn’t previously have a decent fossil representing the intermediate between finned fish and four-footed land animals,’ admitted Bob Holmes in the New Scientist. That was before the discovery of Tiktaalik.

Since the announcement of its discovery in 2006, Tiktaalik has supplanted Panderichthys as the candidate best qualified to stand as intermediate. Thus Panderichthys has been relegated to a side branch, while an unknown predecessor is thought to have given rise to Tiktaalik and Elpistostege. A close relative then gave rise to Acanthostega and Ichthyostega:

The merits of Tiktaalik are discussed in a separate article, the main points being:

• The characteristics shared by the Devonian tetrapods and certain lobe fins were those that equipped them for a similar kind of environment.

• The first known tetrapods with limbs and digits lived wholly (Acanthostega) or primarily (Ichthyostega) in the water – the ‘acquisition’ of such appendages does not support the story of how ‘our fishy ancestors began hauling themselves onto dry land’. Their splayed limbs were designed for paddling, not walking.

• Acanthostega and Ichthyostega did not, as predicted of the first tetrapods, have five fingers and five toes. They had seven or eight.

• There is a large morphological gap between Tiktaalik and the first tetrapods.

• Trackways attesting the existence of tetrapods substantially predate Acanthostega and Ichthyostega. They are about the same age as Panderichthys, so Panderichthys cannot be ancestral to the first tetrapods.

• Acanthostega and Ichthyostega are both dead-ends and therefore cannot illuminate the story of how any tetrapods but themselves evolved from fish.

Pectoral limb series

Despite much searching, the pelvic (rear) fin bones of Tiktaalik have not been located, so the case for its representing a step towards terrestrial locomotion has to rest on the pectoral fins. The following diagram is from the Nature report on those fins (Shubin et al 2006). Even within the sarcopterygians (lobe-finned fish) there is no smooth progression of increasingly tetrapod-like forms, still less such a progression across the divide from sarcopterygian to tetrapod. As the ‘News & Views’ commentary put it, ‘There remains a large morphological gap between them and digits as seen in, for example, Acanthostega.’

In technical terms, the unjointed fin rays in Tiktaalik are reduced, the radials have expanded to a proximal, intermediate and distal series (as with Sauripterus), and there are multiple transverse joints in the distal fin (again as with Sauripterus). The fin retains a mosaic of features seen in basal species, including a central axis of enlarged endochondral bones (cf. Glyptolepis). Overall, Tiktaalik is more similar to Eusthenopteron in fin morphology than to Panderichthys.

One does not need a palaeontology degree to judge the strength of the evidence. In October 2005 eleven parents in Pennsylvania brought the now famous action Kitzmiller v. Dover against a requirement to teach pupils Intelligent Design alongside Darwin’s theory. Kevin Padian, Professor of Integrative Biology at the University of California, was called in as expert witness. This was shortly before Tiktaalik entered the story. One of his key evidences was the transition from Eusthenopteron to Ichthyostega, illustrated with the following diagram:

(A click on the image will give a larger view, and there are transcripts of what was said on the web.) As the court studied the slide, he commented:

“There really is no dispute about the fact that these are, in fact, the precursors of limbs that we see in animals today, the same kinds of structures, the humerus here in yellow, the radius, and ulna, which are, I guess, in green, and then some of the features that become parts of the hand and the other digits in a darker color there. You can also see that in the course of evolution, animals that begin having eight digits, such as Acanthostega here, reduce to seven digits, to six digits, and to five digits. I don’t know how we could find anything more in the way of transitional forms or features unless we went to six and three-quarters or five and a half digits.”

Whether Padian’s comments can be regarded as an objective assessment is very doubtful. As every palaeontologist knows, the occurrence of ‘the same kinds of structures’ is not necessarily an argument for common ancestry, for identical or closely similar structures often turn up in phylogenetic trees where the common ancestor does not have that structure (a phenomenon known as ‘convergent’ or ‘parallel’ evolution).

Secondly, although Padian placed Panderichthys after Eusthenopteron, in the stratigraphic record they are contemporaries. Acanthostega, Ichthyostega and Tulerpeton are also contemporaries. It was misleading to presume a linear evolutionary relationship and then present that presumption as evidence of evolution.

Finally, there is no evidence for progressive reduction from 8 digits to 5. As mentioned, Acanthostega, Ichthyostega and Tulerpeton were all contemporaries. Panderichthys, like Tiktaalik, had none. So the sequence is: zero digits, then simultaneously 6-8 digits in the aquatic tetrapods, which did not use them for manipulation or walking but for paddling (their feet being probably webbed), then 5 digits in the terrestrial tetrapods. Before these fossils came to light, evolutionists expected the earliest tetrapods to have 5 digits, not 8, for that was the number living tetrapods had. So strong was this expectation that for many years the palaeontologist who discovered Ichthyostega, Erik Jarvik, made out that Ichthyostega actually had only 5.

Pelvic limb series

Padian’s testimony in relation to the pelvic limb series was also misleading. Here is the slide:

And here is the commentary:

We can see, moving from Eusthenopteron up through Acanthostega and Ichthyostega, that, in fact, you can go from small, unattached hind limbs and hipbones to become somewhat larger as you can see in Acanthostega and attached to the backbone by what we call a sacral rib. Our sacroiliac is the human equivalent of that. And as you can see in Ichthyostega and other animals, it gets even larger, expanded and attached to the backbone as these animals begin to use their limbs more in support of the skeleton.

What Padian failed to mention was that, as in modern lobe fins, the fin bones of Eusthen- opteron were not attached to the backbone, whereas those of all tetrapods were. To fill the gap he included Panderichthys ‘in press’ in his diagram, referring to a forthcoming paper by Catherine Boisvert, but saying nothing about what that paper would reveal. Here is Boisvert herself:

One of the most marked transformations in the vertebrate transition to land was that of fins to limbs. This transformation involved not only the generation of morpho- logical novelties (digits, sacrum) but also a shift in locomotory dominance from the pectoral to the pelvic appendage. Despite its importance, the transformation from pelvic fin to hindlimb is the least studied and least well-documented part of this transformation, which is bracketed by the osteolepiform Eusthenopteron and the early tetrapods Ichthyostega and Acanthostega, but is not directly illuminated by any intermediate form. … Here, I present the only known articulated pelvic fin endoskeleton and associated partial pelvis of Panderichthys. The pelvic girdle is even less tetrapod-like than that of the osteolepiform Eusthenopteron, but the pelvic fin endoskeleton shares derived characteristics with basal tetrapods despite being more primitive than the pectoral fin of Panderichthys.

Not exactly a ringing endorsement of Padian’s view! The pelvic girdle was small – smaller relative to body size than that of Eusthenopteron and half that of Acanthostega. Most of the transformations in the pelvic appendage that accompanied the fish to tetrapod transition, she surmised, must have taken place between Panderichthys and Acanthostega, i.e. precisely in the fish-to-tetrapod evidence gap. And there was no evidence of the pelvic girdle being attached to the backbone.

Another view

The phrase ‘tetrapod-like’ implies that we are seeing something evolving towards the tetrapod state, but the significance of such features could be ecological rather than evolutionary: they were what an animal needed in order to exploit the new shallow-water ecosystems of the Late Devonian. The floodplains were transitional environments between sea and land, and it would be surprising if some fish exploiting them – amongst the huge range of fish forms that arose in the Devonian – did not share some features with the tetrapods exploiting them. The convergences are sufficiently understandable in terms of functionality, and suggest that the genetic programs which produced the innovations had the capacity to adapt to the new environments; they were engineered with the future environments already in mind. Evolution of this kind would be quite distinct from the fumbling-in-the-dark scenario of random mutations, and biologists are now beginning to get glimpses of how, genetically, this might have occurred. So far as fossils are concerned, the still sharp divide between lobe-fins and tetrapods can be seen as marking the limit of the program’s ability to innovate in that direction.

Unfortunately, the National Academy of Sciences is intent on blurring the differences. Tiktaalik, it says, had both the ‘features of fish (scales and fins) and features of land-dwellers (simple lungs, flexible neck, and fins modified to support its weight). The bones in the limbs of this fossil, named Tiktaalik, resemble the bones in the limbs of land-dwelling animals today.’ This is not an accurate statement. Lungs, simple or otherwise, were not mentioned in the scientific report. A mobile neck (‘flexible’ is too strong) was cursorily inferred, on the basis that the pectoral girdle was not attached to the skull, but while this quite possibly helped the animal to raise its head above water and target prey by the water’s edge, it was not a uniquely tetrapod feature. The closely related 2-metre fish Mandageria also had a mobile neck (a good example of ‘parallel evolution’), just like the modern eel catfish does. And Tiktaalik’s fins were not attached to its backbone, so they had only limited ability to support its weight. Its rear fins, to judge from its nearest relatives, were probably incapable of uplift. Acanthostega’s splayed, paddle-like limbs were also unsuited to supporting its body.

Verdict? Panderichthys, Eusthenopteron and Tiktaalik were highly specialised fish close to the acme of sarcopterygian diversification. During the Devonian, lobe fins exploited the full range of marine environments, from deep sea to shallow estuaries, and the whole question of whether lobe fins evolved into tetrapods needs to be understood against this background. Until such time as fossils turn up to bridge the divide between these three contemporaries and the later Acanthostega, Ichthyostega and Tulerpeton, which were also highly diverse when they made their appearance, we have no basis for linking them. Bearing in mind that tetrapods were leaving fossil tracks even before Tiktaalik, it is difficult to see on what basis an objective scientist could even ‘predict’ such a series.