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."
How one of the Devonian animals might have made the tracks (Source BBC News)
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."
In another blog, Ed Yong discussed the significance of the find and is obviously impressed by the endorsement of one seasoned researcher directly involved in trying to understand the evolution of tetrapods:
"Jenny Clack, the Cambridge scientist who discovered Acanthostega, has seen the Polish tracks for herself and finds them more convincing. Her only reservation is that the detailed prints don't have any trackways to show how their maker moved, while the trackways themselves consist of blobs. "But so do lots of previously known tracks," she says. "If you'd found those in other deposits in the last part of the Devonian, you wouldn't have any qualms about them." She'd like to see trackways of the detailed prints but she's nonetheless excited. "It's going to change all our ideas about why tetrapods emerged from the water, as well as when and where.""
Rethinking the why and where is another aspect of this explosive discovery. The first tetrapods have been recognised as animals that lived in water. People have wondered whether fins evolved into legs as the animals negotiated plant material in shallow waters, perhaps brackish or even freshwater. These ideas may still be applicable to Acanthostega and Ichthyostega, but they are not realistic for the new tetrapod trackways - which are found in marine tidal flat sediments.
"Niedzwiedzki and colleagues' apparently anachronistic Eifelian tetrapod trackways will thus shake up thinking about tetrapod origins. They 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."
The ID interest in this story is for at least two reasons. First, the case documents an example of a failed evolutionary prediction - although, for a while, evolutionists have claimed it as a triumph (see the blog by Casey Luskin on this). Second, the evolution of tetrapods is an important test case for the relevance of design thinking - we ask the question whether tetrapods are here by Design or whether Law+Chance processes are sufficient explanation. Research is proceeding assuming the latter option, but the new discovery suggests that pursuing multiple working hypotheses (including design-based options) might be more prudent.
Tetrapod trackways from the early Middle Devonian period of Poland
Grzegorz Niedzwiedzki, Piotr Szrek, Katarzyna Narkiewicz, Marek Narkiewicz & Per E. Ahlberg
Nature, 463, 43-48 (7 January 2010) | doi:10.1038/nature08623
Abstract: The fossil record of the earliest tetrapods (vertebrates with limbs rather than paired fins) consists of body fossils and trackways. The earliest body fossils of tetrapods date to the Late Devonian period (late Frasnian stage) and are preceded by transitional elpistostegids such as Panderichthys and Tiktaalik that still have paired fins. Claims of tetrapod trackways predating these body fossils have remained controversial with regard to both age and the identity of the track makers. Here we present well preserved and securely dated tetrapod tracks from Polish marine tidal flat sediments of early Middle Devonian (Eifelian stage) age that are approximately 18 million years older than the earliest tetrapod body fossils and 10 million years earlier than the oldest elpistostegids. They force a radical reassessment of the timing, ecology and environmental setting of the fish tetrapod transition, as well as the completeness of the body fossil record.
Janvier, P. & ClÃ©ment, G. Muddy tetrapod origins, Nature 463, 40-41 (7 January 2010) | doi:10.1038/463040a
Dalton, R. Discovery pushes back date of first four-legged animal, Nature News, 6 January 2010 | doi:10.1038/news.2010.1
Yong, E. Fossil tracks push back the invasion of land by 18 million years, Not exactly rocket science, January 6, 2010
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