Lobbing a grenade into the Tetrapod Evolution picture
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.”