Mark I. Vuletic

Last updated 21 March 2008

Analysis

(i) The crossopterygian fish Eusthenopteron is linked to the early amphibian Icthyostega by a number of characteristics:

• Eusthenopteron has the same pattern of skill bones as Icthyostega.
• Eusthenopteron has internal nostrils, which are found only in land animals and sarcopterygians (a greater taxonomic group encompassing lungfish and crossopterygians).
• Eusthenopteron has teeth like those of amphibians.
• Eusthenopteron has a two-part cranium (icthyostegids are the only other vertebrates to have this characteristic)
• Eusthenopteron has the same vertebral structure as Icthyostega (list adapted from McGowan 1984:152-153)

(ii) 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) 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)

(iv) 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 remians 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)

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.

References

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.

Zimmer C. 1995. Coming onto the land. Discover 16(6):118-127.