Eusthenopteron (fish) ⇒ Panderichthys (fish) ⇒ Acanthostega (tetrapod)
The transformation of fish into the kind of amphibians we see in the mid Palaeozoic is not inconceivable, but if it did happen, it is difficult to conceive of it happening by any random process. A radical re-organisation of anatomical design and embryonic development was involved, and the fossil record does not document any gradual transition, though it gives us a few clues. By this time, nine-tenths of Earthâ€™s preserved geological time had already passed, and even the origins of fish are obscure.
Modern theory prefers to designate the new animals as â€˜tetrapodsâ€™ â€“ animals with limbs and feet â€“ rather than â€˜amphibiansâ€™. This is because, contrary to the expected story, the first four-footed animals acquired their feet while still living entirely in the water. The term also suggests a closer link with terrestrial animals than the evidence justifies. Modern amphibians did not descend from the first tetrapods, nor did the earliest fully terrestrial animals.
Background: fish living and extinct
Fish take so many wonderful forms that untangling their relationships is a challenge, and schemes for classifying them have changed over the years. 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 that each of these classes stems from a common ancestor â€“ thus that the fish in the seas today arise from no more than five lineages.
Classes subdivide into orders. The hagfish and lamprey classes consist of only 1 order. The cartilaginous fish comprise 14: chimaeras, sharks of various kind, skates, sawfish and rays. The ray-finned fish comprise 44 orders â€“ now encompassing 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:
- Pteraspidomorphs (including the heterostracans)
The general evolutionary scheme, which is probably correct, is that fish were initially both finless and jawless, gradually evolving fins and jaws as competition for resources grew.
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 chronology):
These are not necessarily the times when the class originated. Isolated teeth, scales and bone fragments often occur somewhat earlier â€“ 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 were 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 is very significant. It reflects a world where fish were initially few and far between and 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. Since their food came from algae and microscopic plankton, they had no use for teeth and jaws. Only as near-shore environments became more habitable did such creatures acquire equipment capable of nibbling seaweeds, filtering sediment, cracking open shells and indeed eating other fish. Brackish and freshwater environments were now beginning to host a range of plant and invertebrate life, and as the available niches became more various, the fish moving into the new estuaries and lagoons became more specialised.
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 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. â€¦ Because Eusthenopteron was once cast in the role of the “ancestor” of tetrapods, tetrapodlike behavior was attributed to it. â€¦ However, taking the whole morphology of the fish, with its streamlined torpedo shape, and dorsal, anal, and pelvic fins placed 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)
In the absence of better candidates, Eusthenopteron was regarded as well on the way to becoming a tetrapod, and was portrayed as such in illustrations. Now a more rounded view can be taken and it is no longer seen as a convincing intermediate. â€˜Palaeontologists didn’t previously have a decent fossil representing the intermediate between finned fish and four-footed land animals,â€™ admitted Bob Holmes in the New Scientist. That was before the discovery of Tiktaalik.
Since its discovery was announced in 2006, Tiktaalik has supplanted Panderichthys as the candidate best qualified to bridge the gap, though it is still very much a fish. 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, so the â€˜acquisitionâ€™ of such appendages does not support the story of how â€˜our fishy ancestors began hauling themselves onto dry landâ€™. 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 remains a large morphological gap between Tiktaalik and the first tetrapods.
- Trackways attesting the existence of tetrapods substantially predate Acanthostega and Ichthyostega. They also predate Tiktaalik, so Tiktaalik 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.
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 certain parents in Pennsylvania brought the now famous action Kitzmiller v. Dover against a requirement to teach pupils Intelligent Design alongside the theory of evolution. 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 gives 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 Padianâ€™s comments can be regarded as an objective assessment is very doubtful. As every palaeontologist knows, the occurrence of â€˜the same kinds of structuresâ€™ 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 â€˜convergentâ€™ or â€˜parallelâ€™ 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
Padianâ€™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 â€˜in pressâ€™ 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. â€¦ 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 Padianâ€™s view! The pelvic girdle was small â€“ 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.
The phrase â€˜tetrapod-likeâ€™ 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 ecosystems of the vegetated shallows. The floodplains were transitional environments between sea and land, and it could be that some fish species â€“ amongst the huge range of fish forms arising in the Devonian â€“ acquired tetrapod-like features simply because that is what the exploitation of such environments demanded. The genetic programs which produced the innovations were engineered with the future environments already in mind. Such evolution 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. The still sharp divide between lobe-fins and tetrapods would then mark the limit of the programâ€™s ability to innovate in that direction.
However, the problem with the idea that lobe-fins did not evolve into amphibians is that it leaves the origin of the latter unexplained. Given that the transition did not happen in the late Devonian, it may be better to suppose that amphibians evolved from fish somewhat earlier in the period.
The crucial question is whether tetrapod amphibians evolved into tetrapod non-amphibians (i.e. amniotes). It is not impossible. Since evolution does not happen by magic, such a path could have been directed by a genome programmed to make it happen. However, in the absence of any fossil evidence for such a transition, it seems better to suppose that amphibians and amniotes had separate ancestries â€“ what one might term the Noahâ€™s Ark scenario. (In support of this scenario, there is also no fossil evidence that insects, myriapods and spiders evolved from sea creatures.)
Unfortunately, the National Academy of Sciences is intent on blurring the differences. Tiktaalik, it says, had both the â€˜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.â€™ This is not an accurate statement. Lungs, simple or otherwise, were not mentioned in the scientific report. A mobile neck (â€˜flexibleâ€™ 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 waterâ€™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 â€˜parallel evolutionâ€™), as does the modern eel catfish. And Tiktaalikâ€™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. Acanthostegaâ€™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 â€˜predictâ€™ such a series.