A couple of years ago I posted on the discovery of Tiktaalik roseae. At the time the fossil was found there were fish-like fossils dating from ca 380 mya (million years ago) and the earliest tetrapod fossil at ca. 365 mya. Thus a likely place for a transitional form between the fish-like and tetrapods was thought to be ca. 375 mya. The search that resulted in the discovery of Tiktaalik roseae is an excellent example of the way scientists seek to test evolutionary theories.
Neil Shubin and coworkers proposed to search for fossils of fresh water aquatic life forms in rock formations dating to the late Devonian era as this is the time and place where transition from lobe-finned fish to tetrapods should be found. In such a deposit they found Tiktaalik roseae. This fish had a number of features consistent with transition from lobe-finned fish to tetrapod. The find was published in Nature in back-to-back articles in 2006 (Nature, v. 440, pp. 757-763, pp. 764-771).
The story doesn’t end there though. In 2008 a second group published a more detailed study of Panderichthys and showed that this fish is even more tetrapod-like than Tiktaalik roseae (Nature v. 456, pp. 636-638). Thus Tiktaalik is not the best example of a transitional species and the transition leading from lobe-finned fish to tetrapods must have begun at least 5 or 10 million years earlier than Tiktaalik. The Tiktaalik fossil was found when and where Shubin had predicted a transitional fossil should be found, but they could have searched in deposits representing a broader time range for new transitional forms.
Often Christians will suggest that this kind of correction and uncertainty is a basis for doubting evolution and preferring some other kind of direct creation hypothesis.
Is this kind of give and take an evidence for weakness in the evolutionary hypothesis?
Is it unexpected waffling or the expected progress of scientific discovery and the refinement of understanding?
There is even more to the story of tetrapod evolution and transitional forms. Scientists 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 fos