Catherine A. Boisvert1, Elga Mark-Kurik2 & Per E. Ahlberg1
Correspondence to: Catherine A. Boisvert1 Correspondence and requests for materials should be addressed to C.A.B. (Email: catherine.boisvert@ebc.uu.se).
One of the identifying characteristics of tetrapods (limbed vertebrates) is the presence of fingers and toes. Whereas the proximal part of the tetrapod limb skeleton can easily be homologized with the paired fin skeletons of sarcopterygian (lobe-finned) fish, there has been much debate about the origin of digits. Early hypotheses1 interpreted digits as derivatives of fin radials, but during the 1990s the idea gained acceptance that digits are evolutionary novelties without direct equivalents in fish fin skeletons. This was partly based on developmental genetic data2, but also substantially on the pectoral fin skeleton of the elpistostegid (transitional fish/tetrapod) Panderichthys, which appeared to lack distal digit-like radials3. Here we present a CT scan study of an undisturbed pectoral fin of Panderichthys demonstrating that the plate-like 'ulnare' of previous reconstructions is an artefact and that distal radials are in fact present. This distal portion is more tetrapod-like than that found in Tiktaalik4 and, in combination with new data about fin development in basal actinopterygians5, sharks6 and lungfish7, makes a strong case for fingers not being a novelty of tetrapods but derived from pre-existing distal radials present in all sarcopterygian fish.
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Previous data from another ancient fish called Tiktaalik showed distal radials as well -- although the quality of that specimen was poor. And the orientation of the radials did not seem to match the way modern fingers and toes radiate from a joint, parallel to each other.
"The disposition of distal radials in Panderichthys are much more tetrapod-like than in Tiktaalik," Boisvert wrote. "Combined with fossil evidence from Tiktaalik and genetic evidence from sharks, paddlefish and the Australian lungfish, it is now completely proven that fingers have evolved from distal radials already present in fish that gave rise to the tetrapod."
The Scientist: NewsBlog: Ancient fingers and toes
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Curiously, the radial bones of Panderichthys are more finger-like than those of Tiktaalik, a fish with stubby leg-like limbs that lived about five million years later.
Many scientists regard Tiktaalik as a "missing link": the crucial transitional animal between fish and the first tetrapods.
One possibility, Alhberg said, is that finger development took a step backward with Tiktaalik, and that Tiktaalik's fins represented an evolutionary return to a more primitive form.
Michael Coates, an evolutionary biologist at the University of Chicago, called the new findings "intriguing" but is not convinced that the digit-like structures in Panderichthys's fin are the equivalent of our fingers.
For one thing, they seem unusually flat for radial bones, Coates said.
"Radials are generally cylindrical. When you look at [a] cross-section [of the digit], they're dumbbell-shaped."
The structures are so peculiar, they might just be fragments of damaged bone, he added.
Coates agreed, however, that fingers and toes—or at least their precursors—were probably present in early fish.
"Nothing comes from nothing in evolution," he said.
Ancient Fish Had Primitive Fingers, Toes --------------------------------
Cordially,
I wouldn't go around saying anything in science is ever “completely proven”; but your article authors tend to view Tiktaaik as a brilliant find that sheds much light on the subject of tetrapod evolution.
Do you disagree with this view of the authors of the article you sourced?