sts are constantly testing theories, looking for more data, refining their understanding, posing questions. In as fluid and tentative a field as tetrapod evolution nothing is taken as a given until proven. The presence of a tetrapod fossil at ca. 365 mya means that tetrapods appeared on the scene sometime before this. The presence of transitional forms in Panderichthys and Tiktaalik means that the evolutionary appearance of features leading to tetrapods began sometime before these specimen (ca. 380 mya). The gap between the transitional forms and the first tetrapod fossil was thought to be where a changeover to true tetrapods would be found. Of course the fossils set a limit on the young end, but do not establish the initial appearance of tetrapods or transitional forms between lobbed-fish and tetrapods.
In January 2010 a new paper was published by Niedźwiedzki, Szrek, Narkiewicz, Narkiewicz and Ahlberg, also in Nature (v. 463, pp. 43-48) which rocked the whole scenario again. This paper presented the findings of a tetrapod trackway dated to ca. 397 mya, pushing the date for the appearance of tetrapods back well before the date of either the Panderichthys or the Tiktaalik fossils. Tetrapod trackways are far more common than tetrapod fossils. They are used extensively to study the tetrapods in the later stages, say 330 mya or so, and palaeontologists regularly search for trackways as well as for whole body fossils.
Instead of a branch producing tetrapods at ca. 380 mya (blue-dashed line), this branch is pushed back to ca. 400 mya. Panderichthys and Tiktaalik don’t represent transitional forms between lobe-finned fish and tetrapods. Apparently they represent the continued lineage of forms transitional between lobe-finned fish and tetrapods.
Such a continued lineage is not unusual. Ancient forms can persist for very long periods. The lobe-finned fish coelacanth was believed to have gone extinct millions of years ago until a fisherman caught one 0ff the east coast of South Africa in 1938. This was called a living fossil – as ancient fossils were known long before the fish was found. The modern coelacanth is not identical to the ancient fish, but is sufficiently similar to represent a continuation of the same lineage.
The continuing saga of the dating of the appearance of tetrapods is an excellent example of the way science and scientists work. There is no “party line” that must be maintained. The data determines the consensus with much arguing and discussion along the way. New hypotheses are proposed to account for the data and new investigations undertaken to test the hypotheses. This is not evidence of a theory in trouble, but a picture of scientists and scholars at work.
What do you think?
Does this kind of investigation and revision challenge your acceptance of evolution?
Does it strengthen your confidence in the process?
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