A year ago, Nature published an educational booklet with the title 15 Evolutionary gems (as a resource for the Darwin Bicentennial). Number 2 gem is Tiktaalik a well-preserved fish that has been widely acclaimed as documenting the transition from fish to tetrapod. Tiktaalik was an elpistostegalian fish: a large, shallow-water dwelling carnivore with tetrapod affinities yet possessing fins. Unfortunately, until Tiktaalik, most elpistostegids remains were poorly preserved fragments.
“In 2006, Edward Daeschler and his colleagues described spectacularly well preserved fossils of an elpistostegid known as Tiktaalik that allow us to build up a good picture of an aquatic predator with distinct similarities to tetrapods – from its flexible neck, to its very limb-like fin structure. The discovery and painstaking analysis of Tiktaalik illuminates the stage before tetrapods evolved, and shows how the fossil record throws up surprises, albeit ones that are entirely compatible with evolutionary thinking.”
Just when everyone thought that a consensus had emerged, a new fossil find is reported – throwing everything into the melting pot (again!). Trackways of an unknown tetrapod have been recovered from rocks dated 10 million years earlier than Tiktaalik. The authors say that the trackways occur in rocks that: “can be securely assigned to the lower-middle Eifelian, corresponding to an age of approximately 395 million years”. At a stroke, this rules out not only Tiktaalik as a tetrapod ancestor, but also all known representatives of the elpistostegids. The arrival of tetrapods is now considered to be 20 million years earlier than previously thought and these tetrapods must now be regarded as coexisting with the elpistostegids. Once again, the fossil record has thrown up a big surprise, but this one is not “entirely compatible with evolutionary thinking”. It is a find that was not predicted and it does not fit at all into the emerging consensus.
“Now, however, Niedzwiedzki et al. lob a grenade into that picture. They report the stunning discovery of tetrapod trackways with distinct digit imprints from Zachemie, Poland, that are unambiguously dated to the lowermost Eifelian (397 Myr ago). This site (an old quarry) has yielded a dozen trackways made by several individuals that ranged from about 0.5 to 2.5 metres in total length, and numerous isolated footprints found on fragments of scree. The tracks predate the oldest tetrapod skeletal remains by 18 Myr and, more surprisingly, the earliest elpistostegalian fishes by about 10 Myr.” (Janvier & Clement, 2010)
The Nature Editor’s summary explained: “The finds suggests that the elpistostegids that we know were late-surviving relics rather than direct transitional forms, and they highlight just how little we know of the earliest history of land vertebrates.” Henry Gee, one of the Nature editors, wrote in a blog:
“What does it all mean?
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, is a cruel illusion. If – as the Polish footprints show – tetrapods already existed in the Eifelian, then an enormous evolutionary void has opened beneath our feet.”
For more, go here:
Lobbing a grenade into the Tetrapod Evolution picture
Additional note: The Henry Gee quote is interesting for the words “elpistostegids represent a transitional form”. In some circles, transitional forms are ‘out’ because Darwinism presupposes gradualism and every form is no more and no less transitional than any other form. Gee reminds us that in the editorial office of Nature, it is still legitimate to refer to old-fashioned transitional forms!