There are no transitional forms between fish and amphibians.
The crossopterygian fish Eusthenopteron is linked to the early amphibian Icthyostega by a number of characteristics:
The skeletal characteristics of Acanthostega, the most primitive tetrapod known (360 million years), reveal that
tetrapod anatomy evolved while our ancestors lived exclusively underwater — and it evolved for life underwater. The first vertebrate that walked onto land didn't crawl on fins; it had evolved well-tuned legs millions of years beforehand. (Zimmer 1995:120)
Acanthostega's arms could not support it very well on land, but were functional in water, allowing the creature to pull itself along the bottom of plant-rich coastal lagoons, and enabling it to ambush prey better than normal fish, which must move their fins to stay afloat, kicking up detectable waves. Additionally, despite being a tetrapod, Acanthostega breathed like a fish and had a hearing system more similar to that of fish than to that of landgoing creatures.
III. Transitional contemporaries of Acanthostega
Paleontologists have also found
fragments from five more tetrapods, all of which were roughly contemporaries of Acanthostega and some of which were more advanced and thus closer to a terrestrial life. (Zimmer 1995:126)
Recent years have been very kind to scientists looking for additional transitional forms from fish to tetrapod. According to paleontologist Jennifer A. Clack
we now have the remains of nine genera documenting around 20 million years of early tetrapod evolution and an even clearer idea of how the rest of the vertebrate body became adapted for life on land. (Clack 2005:103)
One of the most notable among recent finds was Tiktaalik roseae, a creature intermediate between Eusthenopteron and other transitional forms like Acanthostega. Something worth noting about Tiktaalik is that the paleontologists who found it went looking for it in a specific location based on evolutionary predictions: they reasoned that if tetrapod evolution occurred the way they thought it did, intermediates should have lived in a certain type of location, and (if they were lucky) might have left fossilized remains there. When they looked, they found what they had predicted. So much for the idea that evolutionary theory cannot make predictions or be tested.
Most recently, Ventastega curonica, described as "a transitional intermediate form between the 'elpistostegids' Panderichthys and Tiktaalik and the Devonian tetrapods (limbed vertebrates) Acanthostega and Ichthyostega" (Ahlberg et al., 2008:1119), has been added to the list.
V. Is Tiktaalik invalidated by earlier tetrapod footprints?
The latest wrinkle in the story is the discovery (Niedzwiedzki et. al, 2010) of fossilized footprints of tetrapods dating to 10 million years before Tiktaalik. These footprints show that Tiktaalik is not the most immediate ancestor of the very earliest tetrapods; this is in keeping with the branching structure of evolution, and does not diminish the value of Tiktaalik or the other transitional tetrapods we have found (see PZ Myers's takedown of creationist misunderstandings about this discovery), but also means that paleontologists can expect to find transitional tetrapods earlier than Tiktaalik.
Ahlberg PE, Clack JA, Lukševičs E, Blom H, Zupinš I. 2008. Ventastega curonica and the origin of tetrapod morphology. Nature 453:1119-1204.
Clack JA. 2005. Getting a leg up on land. Scientific American 293(6):100-107.
McGowan C. 1984. In the Beginning...: A Scientist Shows Why the Creationists Are Wrong. Buffalo: Prometheus.
Niedzwiedzki G, Szrek P, Narkiewicz K, Narkiewicz M, and Ahlberg PE. 2010. Tetrapod trackways from the early middle devonian period of poland. Nature, 463(7277):43-48.
Zimmer C. 1995. Coming onto the land. Discover 16(6):118-127.
Last updated: 09 Jan 2010
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