Once again, I find myself interested in a paper about the analysis of a fossil! This time the point of interest is origin of the tetrapod limb digits. The origins of the proximal elements of the tetrapod limb are well understood, and can be seen as homologues of elements of the fins of sarcopterygian (lobe-finned fish). The origins of the digits may have been fin rays, or possibly evolutionary novelties. The latter explanation was supported by developmental genetic studies and analysis of fossils (including the subject of this paper, the transitional fish/tetrapod Panderichthys). In this paper, CT scanning was used to generate 3D images of Panderichthys limbs, demonstrating the presence of distal radials, and correcting a mistaken reconstruction.
The figure below shows the skeletal structure of the limb, coloured to show homologies to the elements of the tetrapod limb.
The authors provide some striking movies of 360 degree rotations of their reconstruction in the supplementary material to this paper. The figure below shows the homologies of the limb elements of Eusthenopteron (a), Panderichthys (b), Tiktaalik (c) and Acanthostega (d). Acanthostega has eight digits, and was the subject of Stephen Jay Gould’s essay Eight Little Piggies.
It seems from the CT scan based reconstruction here that these fossil pectoral fins had structural elements that can be interpreted as homologues for tetrapod limb digits.
Catherine A. Boisvert, Elga Mark-Kurik, Per E. Ahlberg (2008). The pectoral fin of Panderichthys and the origin of digits Nature, 456 (7222), 636-638 DOI: 10.1038/nature07339