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 appear, nine-tenths of preserved geological time lies in the past, and the fossil record does almost nothing to illuminate their prehistory. The evolution that led to the final chapter of their story had already taken place, mysteriously out of sight.

The first transition considered to merit iconic status is when fish – a very select group of fish, for all the other groups remained fish – evolved into tetrapods, vertebrates with limbs and feet. ‘Amphibian’ is not the best term, for it has recently become apparent that the first tetrapods did not spend a significant time out of water.

The National Academy of Sciences has singled out the evidence for the transition for special emphasis. This more than anything, their updated version of Science, Evolution and Creationism tells us, is what convinces scientists that they descend from fish, and that the rest of society should be likewise persuaded.

Background: fish living and extinct

Fish take so many wonderful forms that it can be very difficult to untangle their relationships, and schemes for classifying them have changed a lot 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)

For the sake of argument we will assume that these classes are all monophyletic, i.e. their members are all related to each other.

Classes subdivide into orders. Hagfish and lampreys each consist of only 1 order and are therefore not a diverse group. The cartilaginous fish comprise 14 orders: chimaeras, sharks of various kinds, skates, sawfish and rays. The ray-finned fish comprise 44 orders; the vast majority of fish in the sea are therefore ray fins. The lobe-finned fish consist of 2 orders, coelacanths and lungfish.

In addition there are several extinct classes, notably:

• Placoderms
• Anaspida
• Osteostracomorphs
• Acanthodians
• Pteraspidomorphs

The earliest fossil occurrences of these classes (excluding isolated teeth, scales and bone fragments) are as follows (dates in millions of years, radioisotope dating):

The periods shown are not necessarily when the class originated. Usually there exist fragments of a somewhat earlier date, insufficient to build up a reliable picture. The pattern of rare fragments followed by more common, better preserved remains reflects a world where there were initially few fish and the potential habitats of continental margins were highly unstable, tending to erode and destroy fragile carcasses. Fauna from the open sea (of which there is only a sparse record, plate tectonics having destroyed pre-Jurassic seafloors) was colonising near-shore environments with but limited success. By the time of the Devonian, brackish and freshwater environments were more hospitable, and as fish moved into these lagoons, estuaries and floodplains they underwent wide diversification, much of it adaptive. The evolution was horizontal rather than vertical.

The theory of vertical evolution has to find evidence that one subgroup underwent a radical transformation that took them from sea to land. Some classes can be immediately excluded. Hagfish and lampreys are invertebrates, and the Anaspids, Osteostracomorphs and Pteraspidomorphs are jawless fish – the first tetrapods, by contrast, were jawed. The point here is not that the gulf between jawless and jawed is considered too big to cross, just that it had already been crossed within the fishes. The class that seems to present the smallest gap between fish and tetrapod, at the appropriate time, is the lobe fins. They had fleshy pectoral and pelvic appendages that could suggest, if one is so minded, an analogy with tetrapod limbs, and in the Devonian they were the most diverse fish group, offering a range of forms from which to choose the most tetrapod-like. Eusthenopteron and Panderichthys are two such fish.

One of the foremost authorities in early tetrapod evolution is Jennifer Clack. In her book Gaining Ground (2002) she writes:

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.

When there were no better candidates, Eusthenopteron was regarded as well on the way to becoming a tetrapod, and in illustrations was duly portrayed as such. 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; scientists just knew that ‘tetrapods evolved from lobe-finned fishes’.

The merits of Panderichthys are discussed in more detail in the article Tiktaalik roseae – a missing link? 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 main points in the article on Tiktaalik include:

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

• The first fossilised tetrapods with limbs and digits were animals that 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 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.

But one does not need a palaeontology degree to judge the strength of the evidence. Put yourself in the shoes of the jury which in October 2005 heard the now famous action of eleven Pennsylvanian parents against a requirement to teach pupils Intelligent Design alongside Darwin’s theory – the Kitzmiller v. Dover case. Kevin Padian, professor at the University of California, supported the parents as expert witness, and one of his key evidences was the transition from Eusthenopteron to Ichthyostega, illustrated with the following diagram:

(There is a transcript 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.”

This was without the benefit of Tiktaalik, which is alleged to have only strengthened the case. Whether Padian’s comments can be regarded as an objective assessment is doubtful, however.

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 (the phenomenon being called ‘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, their presumed descendants. So strong was this expectation that for many years the palaeontologist who discovered Ichthyostega, Erik Jarvik, made out that Ichthyostega actually had only 5 digits.

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

If ‘tetrapod-like’ implies that we are seeing evidence of something evolving towards the tetrapod state, the phrase is tendentious, for it could be that the significance of ‘tetrapod-like’ features is not evolutionary (in the Darwinian sense) so much as ecological: they were the features an animal needed in order to exploit the new shallow-water ecosystems of the Late Devonian. The floodplains were, obviously, themselves transitional between sea and land, and it would be surprising if fish exploiting them did not share features with the tetrapods that exploited them, even if they acquired those features in the course of sarcopterygian evolution. They acquired them by means of a genetic program that had the capability to adapt to shallower water. This would have been a kind of evolution 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 sarcopterygian fish and tetrapods would mark 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 flexible neck, while it quite possibly helped the animal to raise its head above water and target prey by the water’s edge, was not a uniquely tetrapod feature. The 2-metre long tristichopterid lobe fin Mandageria also had a mobile neck (a good example of ‘parallel evolution’), as does the modern eel catfish. And Tiktaalik’s fins were not attached to its backbone; their ability to support its weight was strictly limited. Its rear fins, to judge from Panderichthys and Eusthenopteron, were probably incapable of uplift. Nor should we forget that 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. By the end of the period all the major lineages had appeared. The 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 in our mythology. Given 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.