The 2004 discovery (reported in 2006) of a well-preserved fossil creature dubbed Tiktaalik was hailed as the “missing link” that finally solidified the “fish to tetrapod” transition — a “snapshot” of “a fossil fish in the act of adapting toward a life on land”. Dated to 375-383 Mya, Tiktaalik was concluded to be an intermediary between the sarcopterygian fishes (i.e., Eusthenopteron and Panderichthys) and early tetrapods (i.e., Acanthostega and Ichthyostega). Or, as some call it, a “fishapod”. Some paleontologists have made highly controversial claims of evidence for a “wrist” and fingers, as well as muscular forefins. It had a spiracle, and evidence suggests lungs & gills and eventual development of a more robust ribcage. Along with certain intercranial structures, all these developments were believed to allow Tiktaalik to breathe and support itself on solid ground. Tiktaalik is also the first fish (or, something like it) known to have a “neck”, because it lacked the bony plates in the gill area which restrict head movement.
Some have argued that it is more appropriate to classify Tiktaalik as simply a fish, rather than part-fish/part-amphibian. Notably, its fin is completely finlike, showing no real evidence of transforming into a foot, which is precisely the key feature needed to justify calling it a “transitional form” from fish to tetrapod. It is also an “inconvenience” that Tiktaalik had no precursors to fingers like Panderichthys did, despite the fact that Tiktaalik is supposed to be further along the evolutionary ladder. Although the Tiktaalik specimens are incredibly well-preserved, they shed little light on the evolution of the soft anatomy requisite for living on land. Furthermore, a true intermediate form would have needed to simultaneously be evolving many features & abilities just to survive its transition from water-dweller to land-dweller — e.g., new ways to keep from drying out on land, specialized structures for breathing oxygen, new methods of obtaining food and water, etc.
In 2008, Jennifer Clack, Per Ahlberg, et al. published their findings on Ventastega, another fossil discovery dating to 365 Mya. This basil tetrapod, which some factors suggest lived in a tidal sea, was “probably more aquatic than terrestrial,” yet “it was more tetrapod than fish….” They classified it as intermediate between the ‘elpistostegids’ (e.g., Panderichthys & Tiktaalik) and Devonian tetrapods (e.g., Acanthostega & Ichthyostega). As far as I can tell, though, they have no actual Ventastega fossils dating earlier than Acanthostega, so this transitional status is no more than “educated speculation”. Plus, it isn’t as “advanced” as some other tetrapods, so it seems to be out of place or anachronistic.
Now, here is what blew everything out of the water, so to speak. Recently, at an old quarry in southeastern Poland were found multiple sets of tracks of a tetrapod — actually, from multiple individuals of different sizes — in what looks to be an ancient marine shoreline. According to Per Ahlberg of Sweden’s Uppsala University, a member of the team that found the tracks, “[They are] fossil of footprints that give us the earliest record of how our very distant ancestors moved out of the water and moved on to the land and took their first steps.”
397 Mya Tetrapod tracks (borrowed from BBC News)
The footprints, some of which show distinct signs of digits and ankles, date much earlier than they “should”; specifically, they are quite solidly placed at 395-397 million years old. That is 12 million years or more before Tiktaalik, which rules out Tiktaalik as transitional between fish and the first tetrapod. In fact, this now pushes the earliest known tetrapod (though we don’t have an actual skeletal fossil to name, yet) back by at least 18 million years. Furthermore, these trackways “show that the first tetrapods thrived in the sea, trampling the mud of coral-reef lagoons; this is at odds with the long-held view that river deltas and lakes were the necessary environments for the transition from water to land during vertebrate evolution.”
What to make of all this? It is a reminder of a few things that we often need reminding of. First, the oldest known example of a thing is not always the oldest ever. This is just as true in paleontology as it is in other historical sciences, like archaeology. This is often true for the latest known examples, as well. Second, morphologically transitional forms are not always chronologically transitional, and vice versa. That is, Thing X may have features that look to be a mix of Thing A and Thing B, but dating methods sometimes show that Thing X cannot possibly have been a developmental intermediate between A and B. Thus, any theory that assumed such would be false. (Remember the case of the Archaeopteryx?) This is all the more reason to remember that such theories must be held with reserve, always provisional upon things like further physical specimens for evidence, better and more-refined technologies, and mathematical probabilities of events within known parameters. Also, beware of letting assumptions & biases get in the way of facts.
This brings me to my third observation, which is that, once again, the “proof” for neo-Darwinism isn’t nearly as solid as many of its proponents like to proclaim. Indeed, even before this current discovery, there were many competing ideas of how to best classify the various fishes, potential fishapods, and primitive amphibians, precisely because the different mosaics of “transitional” features found in different epochs make it far from clearcut. For example, was Panderichthys a fish or a fishapod? Was Acanthostega a fishapod or a true tetrapod? One’s opinion on matters like these determine where one thinks they should go in the “tree of life”, which, of course, is actually more of a bush. Indeed, many different models have been advanced and then abandoned over the years. The trackways in Poland just throw another wrench into the works.
With the above said, this latest find does NOT necessarily disprove evolutionary theory. It merely means that the EARLIEST tetrapods are older than previously thought AND that they are not quite where they were expected. Paleontologists must now look at Early Devonian (416-397 Mya) formations for even EARLIER transitions. They will also need to find EVEN EARLIER lobe-finned or other fishes as candidates for the tetrapods’ progenitors. (Perhaps Coelacanth will get another chance?) And what of Tiktaalik and the other creatures discussed above? It is still possible that they do represent a separate and unrelated, fish-to-tetrapod transition. But, one has to wonder if the Darwinian process can account for all the genetic & systemic changes needed to go from fully aquatic to land-dwelling, especially in the required timeframe of just a few million years.
Of course, this assumes that the fishes & fishapods actually transitioned into anything new. Perhaps they were stable forms in their own right, well-adapted (or, well-designed, if you prefer) for living in their particular environment. After all, they seem to have been fully-functional, lacking any unfinished, half-formed adaptations. They were not mere works-in-progress.
I’ll finish with this quote from Henry Gee, editor of the journal Nature: “A fairly complete picture of tetrapod evolution, built up over the past twenty years, has been replaced by a blank canvas overnight…. It means that the neatly gift-wrapped correlation between stratigraphy and phylogeny, in which elpistostegids represent a transitional form in the swift evolution of tetrapods in the mid-Frasnian [stage of the Devonian Period], is a cruel illusion. If – as the Polish footprints show – tetrapods already existed in the Eifelian [stage], then an enormous evolutionary void has opened beneath our feet.”
At the very least, the discovery in Poland represents somewhat of a paradigm shift. It should be fascinating to see what evidence future discoveries bring to this debate….