The relationship of limbed vertebrates (tetrapods) to lobe-finned fish (sarcopterygians) is well established, but the origin of major tetrapod features has remained obscure for lack of fossils that document the sequence of evolutionary changes. Here we report the discovery of a well-preserved species of fossil sarcopterygian fish from the Late Devonian of Arctic Canada that represents an intermediate between fish with fins and tetrapods with limbs, and provides unique insights into how and in what order important tetrapod characters arose. Although the body scales, fin rays, lower jaw and palate are comparable to those in more primitive sarcopterygians, the new species also has a shortened skull roof, a modified ear region, a mobile neck, a functional wrist joint, and other features that presage tetrapod conditions. The morphological features and geological setting of this new animal are suggestive of life in shallow-water, marginal and subaerial habitats.
Tiktaalik is a lobe-finned fish, in the same broad group as coelacanths and lungfish. In the Devonian period lobe-finned fish were more diverse than they are today and Tiktaalik adds to that diversity. The term ‘tetrapod’ conveys the belief (based on philosophy rather than science, though science plays a role) that all animals with four limbs and a certain number of digits have a common ancestor, and hence it is common ancestry that makes the term meaningful. Tiktaalik then helps to corroborate that story.
Although still a fish, Tiktaalik shares some features with early amphibians, and one could (depending on one’s point of view) interpret the whole animal as transitional to tetrapods. These features include: a lengthened snout (measured from the eyes to the tip of the skull), a mobile neck, overlapping (‘imbricate’) ribs and a pectoral girdle that may have given it an ability to lift the front part of its body by its fins. On these grounds the animal is analysed as being intermediate between the lobe-finned Panderichthys and the first four-limbed animals, Acanthostega and Ichthyostega. Appearing in the fossil record soon afterwards, these two tetrapods are nonetheless very different from each other, so presumably their Tiktaalik-like ancestor evolved in two separate directions, with Acanthostega on one branch and Ichthyostega on the other.
Some caution is appropriate, even with these tetrapod-like features. Among sarcopterygians the mobile neck is not unique to Tiktaalik, being also found in Mandageria, a fish closely related to Eusthenopteron and not thought to have been ancestral to any tetrapod. In evolutionist terms the feature must be interpreted as ‘convergent’, i.e. it evolved in Mandageria, disappeared in Panderichthys, then re-appeared on another branch in Tiktaalik.
Imbricate ribs do not occur in any other fish, nor in Acanthostega, but do occur in Ichthyostega. Shubin et al. state that ‘expansion and imbrication of the ribs is a feature seen in some early tetrapods such as Ichthyostega‘. Apart from Tiktaalik, however, they seem to be unique to Ichthyostega, and since even Ichthyostega was primarily aquatic, it seems unjustified to suggest that they played a role in supporting the weight of the animal. The stiffening of the spine produced by this arrangement would have inhibited horizontal flexion (the wiggling motion of most fishes) just as it would have done in Ichthyostega (Clack 2006), and this fact alone is sufficient to explain the mobile neck and fin joints: without some compensating mobility the animal would not have been viable as a predator. In overall morphology, the ribcages of Tiktaalik and Ichthyostega are unlike each other.
A Devonian rhizodontid, Sauripterus, is known to possess digit-like radials, but phylogenetic analyses indicate that this group is not the closest relative of tetrapods.
The gap between the pectoral fin of Tiktaalik and the front foot of Acanthostega is large, as Ahlberg and Clack note. How far the pelvic fins differed from the hindlimbs of Acanthostega and Ichthyostega is unknown, because the fins are mostly not preserved. In the drawing they are illustrated as fleshy lobes.
Overall, this is not an animal that falls plumb in the middle between Panderichthys and Acanthostega. It is a fish, albeit an unusual one, and while there is only a relatively small gap between Panderichthys and Tiktaalik, there remains a big gap between Tiktaalik and Acanthostega. In a significant number of features, body scales, fin rays, lower jaw, palate, the fossil resembles sarcopterygians that are considered to be evolutionarily less advanced. As Ahlberg and Clack say, ‘we have almost no information about the step between Tiktaalik and the earliest tetrapods, when the anatomy underwent the most drastic changes.’ It may be reasonable enough to link the first pair into a group that had a common ancestor (the ‘elpistostegalian fish’), but it requires a lot more extrapolation to link the second pair together.
There is, moreover, a large gap after Ichthyostega. It often fails to be noticed that, so far as the fossil record is concerned, both Acanthostega and Ichthyostega are dead-ends. They shed no light on the origin of modern tetrapods.
According to Nature‘s announcement of the fossil, Tiktaalik was a fish that crawled out of the water, but if so, the lineage must have crawled back again, for Acanthostega ‘rarely, if ever, made forays onto dry land and its legs would have almost certainly been incapable of supporting its body had it done so’ (Clack 2002, p 128f). That’s less than is being claimed for this animal. We are told in one breath that Tiktaalik had a long snout that would have been suited to catching prey on land, in the next that it hauled itself onto land only to escape predators. It has, one feels, been impressed into the ‘how life conquered the land’ story a little too enthusiastically.
One of the monkfish was observed to move along the seabed for several metres at a time by walking. Several ‘walks’ were observed. The gait involved both the pelvic and pectoral fins and the body and tail were lifted clear of the seabed. The pelvic fins appeared to be the main weight bearing fins lifting the body up from the seabed. They also seemed to be responsible for a considerable proportion of the forwards propulsion.
Frogfish, handfish and monkfish are members of the order Lophiiformes or anglerfish, within the subclass of teleost fish, within the class of ray-finned fish. The evolutionary history of these walkers is obscure but we may presume that their particular forms are the result of immense diversification over time within the order. Certainly their appendages are quite unrelated to those of lobe-finned fish. Anglerfish are an example of how wonderfully unconventional life can be, as if whoever designed them was a non-conformist who delighted in subverting stereotypes.
Thinking that was less ideologically driven might not go amiss in the present case. While locomotion may have been a function of Tiktaalik’s front fins, its probable habitat was shallow water, which it needed for support. Out of water, it could only have dragged its almost 3-metre-long body. Monkfish, which like Tiktaalik have a wide mouth, flattened skull and eyes on top of the skull, again alert us to other possibilities, for in addition they use their fins to scoop out sediment from under their bodies and make a hollow in which, having camouflaged themselves to match their surroundings, they lie in wait. At the right moment, they strike the prey by suddenly pushing with their fins upwards and forwards. The feeding strategy of Tiktaalik won’t have been the same as the monkfish’s, but it may have been similar. Several lineages of fish have ‘evolved’ an ability to crawl out of water, including mudskippers, rockskippers and blennies; but there is no reason to think they are on the way to leaving the water altogether.
Much has been made of the fossil’s being just what palaeontologists would have expected to find, even what they did expect to find. The scientists who discovered Tiktaalik went to Ellesmere Island purposely looking for intermediates between Panderichthys and the first tetrapods, and they were not disappointed. In many respects, nonetheless, the fossil was surprising.
The pelvic girdle is even less tetrapod-like than that of … Eusthenopteron, but the pelvic fin … shares derived characteristics with basal tetrapods despite being more primitive than the pectoral fin of Panderichthys. The evolution of tetrapod locomotion appears to have passed through a stage of body-flexion propulsion, in which the pelvic fins played a relatively minor anchoring part, before the emergence of hindlimb-powered propulsion in the interval between Panderichthys and Acanthostega.
What Boisvert is saying here is that Panderichthys had ‘front-wheel drive’: its front fins were bigger and more powerful than its rear fins. However, the early tetrapods were ‘rear-wheel drive’. Consequently, evolution theory predicted that the emergence of hindlimb-powered propulsion would be seen in the interval between Panderichthys and Acanthostega. Tiktaalik failed that prediction. Indeed, it was more of a ‘front-wheel drive’ animal than Panderichthys was.
- How does one account for tetrapod trackways in beach sediments that predate even Tiktaalik by 10 million years?
- How does one fill the gap between Tiktaalik, which was a fish with no legs, and an aïstopod such as Lethiscus, which within 20 million years had supposedly acquired legs and limb girdles and then lost them again, and changed from a fish to something more like a snake than any tetrapod?
That is the burden of proof that needs to be discharged. As Clack remarked in an academic paper a few months before Tiktaalik, Lethiscus suggests that ‘a great deal happened in the course of tetrapod evolution that we know very little about’. This is one of palaeontology’s trade secrets to which Darwinians do not draw public attention. Until the problem is solved, it seems reasonable to conclude that we know very little about tetrapod evolution at all and to regard Tiktaalik in much the same light as one now regards the lung-fishes, which, for all their superficial appeal as intermediates, are no longer seen as ancestral to tetrapods. The fossil record continues to surprise us because, in certain respects, animal life was once more disparate and more diverse than it is today. It will go on surprising us, whatever our point of view.
From amphibian to reptile, a discussion of the evidence for the crucial transition from aquatic to terrestrial tetrapods.