Ancient Tracks Question Ideas About Tetrapod Origins

Tiktaalik is practically a household name. Since its description in 2006 the flat-headed "fishapod" has appeared in books, on t-shirts, and has even starred in its own music video. Hailed as a "missing link", Tiktaalik has become a poster child fossil for evolution, but it is hardly the first such creature to be given this honor.

Way back in the 1840's, well over a decade before Charles Darwin's On the Origin of Species was published, the Victorian anatomist Richard Owen was mulling over the concept of transitional forms. He was not so much thinking about actual fossils as the way anatomical frameworks could be modified by natural laws, but even so the anatomy of several creatures Owen had examined appeared to throw credence to the idea that one form could be derived from another. The lungfish Lepidosiren and Protopterus, for example, were fish that had lungs and wispy fins supported by stacks of bone. These traits made the fish seem very similar to some amphibians, and a fossil creature approached the "fish/amphibian boundary" from the other side. The extinct, crocodile-like amphibian Archegosaurus showed some close resemblances to Lepidosiren, and together Owen took the two forms to represent a divergent juncture in vertebrate forms. The anatomy of Lepidosiren, on the one hand, appeared capable of forming the basis for salamanders with gills such as the axolotl, while the Archegosaurus, on the other, could have been derived from a gar-like fish. Together they were "transitional types" that seemed to represent gradations along anatomical chains, but, frustratingly, Owen was vague about just what he meant by all this.

It was Darwin's 1859 work, of course, that spurred a greater scientific interest in evolution, but he did not co-opt Lepidosiren and Archegosaurus as transitional forms. Perhaps this was a wise move. Clearly the first land-dwelling vertebrates (called "tetrapods" for their possession of four limbs) could not have evolved from a living fish, and as Archegosaurus became better understood it was moved further and further away from the origin of the first land-dwelling vertebrates. (Today we know Archegosaurus as a temnospondyl that lived tens of millions of years after the first tetrapods.)

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With so few fossils mapping out the transition debates went on for decades about the details of how vertebrates became adapted to life on land. This was not helped by some delays in the description of important specimens. Many fossils of early tetrapods, such as Acanthostega and Ichthyostega, had been found in the over 365 million year old rock of Greenland during the early part of the 20th century. Unfortunately, however, the description of these fossils was held up due to the death of one scientist, the almost glacial work pace of another, and the relegation of some specimens to museum basements where they gathered dust for years.

By 1990, though, our picture of tetrapod origins seemed a little more complete. The earliest tetrapods had evolved from "bony-finned" fish akin to Eusthenopteron (a long-time representative of the fish side of the transition) and had evolved through graded states into forms such as Acanthostega and Ichthyostega. Yet there was still a wide anatomical gap between the "fish side" and the "tetrapod side" of the transition, a gap that Tiktaalik and its lesser-known relative Panderichthys would come to fit in.

Nor were these the only fossils relevant to questions about this transition. They became the most famous because they are the most complete, but there are plenty of other critters known from fragments that illustrate that the origin of tetrapods was not some straight-line march as is commonly seen in cartoons. What this all means is that even though Tiktaalik is a celebrity today there is still a lot out there to be discovered, and in a few years we very well may be celebrating some other tetrapodomorph with an anatomy that fits snugly between Tiktaalik and Acanthostega.

Indeed, since the 1980's scientific investigations into the origin of tetrapods has exploded, and new discoveries are being made all the time. One new finding, just published in Nature, may even cause us to revise what we thought we knew about the tempo and mode of tetrapod evolution. It is a collection of approximately 395 million year old tracks from Poland, tracks that predate Tiktaalik and its kind by several million years.

As described by Grzegorz Niedzwiedzki, Piotr Szrek, Katarzyna Narkiewicz, Marek Narkiewicz, and Per Ahlberg, the tracks were found in quarry in Poland. Rather than representing a shallow freshwater swamp or stream, however, these deposits were marine. This is significant as the evolution of the first tetrapods has generally been thought that have occurred in brackish-to-freshwater environments. Instead the tracks appear to have been made in an area alternately covered and exposed by saltwater, such as a lagoon or shallow tidal area. Whatever the environment was like, though, the creatures walked all over it. While devoid of body fossils (as is often, and frustratingly the case with such sites) the deposit contains numerous tracks made by the animals.

Among the most impressive of the specimens is a short trackway left by one of the animals. It preserves the hand and footprints of an animal moving in a straight line, and there are no body drag marks. Think about that for a second. Tiktaalik, which lived about 10 million years or so after the tracks were made, had short, stubby arms and even less-well-developed legs that would not have allowed it to do this. No doubt that it could have raised its body to move, but it could not have moved it all the way off the bottom and hence would have left a drag mark between the footprints. The creatures described by Niedzwiedzki and colleagues, however, appear to have raised their bodies higher off the bottom, although they may have also floated their bodies in the water and moving themselves about with their limbs (thus removing some of the weight-bearing stress from their arms and legs).

What is especially interesting, though, is that this trackway appears to show that this animal, which was larger than Ichthyostega, moved in a side-to-side manner similar to that of living salamanders. This might not have been possible for some of the earliest tetrapods known from complete skeletons such as Ichthyostega. In fact, just a few years ago Ahlberg and colleagues published a reexamination of Ichthyostega in Nature in which they proposed that its overlapping ribs would have hindered its ability to move its body from side-to-side. Instead the restrictions of its skeleton made it seem more likely that it would have moved in a way similar to a seal or an inchworm by flexing its body up-and-down. Clearly the animals whose tracks were preserved in the quarry in Poland were moving more like living amphibians, making them unlike Tiktaalik and (presumably) Ichthyostega.

Some of the individual tracks are also of great interest. If the scientists are correct, they represent the earliest animals yet known with differentiated toes. Toes are a key tetrapod trait, Tiktaalik did not have them, and one of the best-preserved representations of a foot with toes is specimen Muz. PGI 1728.II.1. Altogether it seems to be an impression of almost the entire lower left hindlimb exhibiting at least five toes (there may be more, though, given the track is smudged). It looks akin to what you would expect the foot of something like Ichthyostega to make, but it is not an exact match.

But are those really toes? It looks like the outlines of toes, but could the same impressions have been made by a modified fin? How can we tell whether or not those notches really represent the ends of fingers or something else entirely? As it stands now, we can't. The tracks appear to be consistent with what an early tetrapod could make but the trouble with tracks is that the animals that much such prints almost never die in their tracks. We need bones to be sure, and in lieu of bones we need to try to reconstruct how those kind of tracks could have been made.

These tracks very well might be the earliest traces of tetrapods on record, but that is a hypothesis, not a fact. The fact is that some marine vertebrate with limb-like appendages made these tracks about 395 million years ago, but just what that vertebrate was and what it looked like will require further evidence to determine. I am comfortable saying that the tracks were made by a tetrapod in the vernacular sense (i.e. a four-footed vertebrate), but what is truly a creature related to the common ancestor of all land-dwelling vertebrates?

Here is another, hypothetical, example that might help explain some of my reservations about these tracks. Bipedalism has long been treasured as the defining trait of humans (=hominins). Find something "ape-like" between 6 and 4 million years ago that exhibits evidence of bipedalism and you have yourself a hominin, right? But lets say you find what appears to be a track made by a bipedal ape in sediments 10 million years old. Does this mean that "Ardi" is suddenly irrelevant? Of course not! It is entirely possible, for example, that another group of apes, as yet unknown, independently evolved bipedalism before going extinct. Then again, such a track could mean that our previous hypotheses were wrong and require revision according to new evidence. Without body fossils, bones to compare to what has already been collected, it is impossible to know which scenario is correct.

We are faced with a similar situation here. The hypothesis that the tracks were made by tetrapods seems pretty reasonable, but it is going to take more evidence to support. I am in no way trying to downplay this study. Instead I think it is wonderful because it brings up so many new questions! If the scientists behind this new research are correct then tetrapods evolved much earlier than we previously supposed, and what we have taken up till now as the general evolutionary sequence of forms in early tetrapod evolution are actually disparate forms which are part of a more complex radiation of early tetrapods. In this case, as the authors note, creatures like Tiktaalik did not quickly give way to early tetrapods but lived alongside them for 10 million years or more. This does not mean that Tiktaalik, Acanthostega, and the rest are irrelevant to tetrapods origins, but rather that we need to revise our hypotheses about how they relate to one another.

Some people might consider my uncertain admissions here to be something of a downer, but I cannot agree. In science uncertainty is exciting. The authors of this new paper have proposed an interesting hypothesis that could rearrange what we thought we know about the origins of tetrapods and a lot of work, both in the lab and the field, will need to be done to sort this all out. We should not feel compelled to throw all our weight behind one hypothesis or another without more evidence. We have been presented with some really intriguing questions, and I look forward to reading the future reports of how scientists went about trying to find some answers.

For more, see these posts by Ed Yong, Adam Rutherford, and Henry Gee, as well as the supplementary material on the Nature website.

Niedźwiedzki, G., Szrek, P., Narkiewicz, K., Narkiewicz, M., & Ahlberg, P. (2010). Tetrapod trackways from the early Middle Devonian period of Poland Nature, 463 (7277), 43-48 DOI: 10.1038/nature08623