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The pectoral fin of Panderichthys and the origin of digits


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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Figure 1: Pectoral fin, girdle and scale plus lepidotrichia cover of Panderichthys.
Figure 2: Endoskeleton of the pectoral fin of Panderichthys rhombolepis.
Figure 3: Pectoral endoskeleton of tetrapodomorph fish and a tetrapod in ventral view.

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We thank P. Ross and the team at East-Tallinn Central Hospital, Estonia for the CT-scanning, E. Vorobyeva for permission to examine the Panderichthys material in her care, N. Shubin for access to Tiktaalik, and M. I. Coates for valuable discussions. C.A.B. and P.E.A. are supported by Vetenskapsrådet.

Author Contributions E.M.-K wrote one of the Supplementary Information files and arranged the logistics for the CT scan, P.E.A. and E.M.-K. scanned the specimen, and C.A.B and P.E.A. produced the models and descriptions.

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Correspondence to Catherine A. Boisvert.

Supplementary information

Supplementary Information

This file contains Supplementary Information about the localities, range and age dating of Panderichthys rhombolepis, Supplementary References and Supplementary Figure with Legend. (PDF 3737 kb)

Supplementary Movie 1

This file contains 360° rotation of the fin endoskeleton and pectoral girdle with translucent scale and lepidotrichia cover. (MPG 1820 kb)

Supplementary Movie 2

This file contains 360° rotation of the fin endoskeleton and pectoral girdle. Note that the scapulocoracoid is truncated anteriorly where the scan resolution became too poor to model due to mineral infill to confidently model. The supracleithrum and the scale and lepidotrichia cover end where the scan began and ended, respectively. (MPG 1812 kb)

Supplementary Movie 3

This file contains 360° rotation of the fin endoskeleton. (MPG 1814 kb)

Supplementary Movie 4

This file contains 360° rotation of the fin endoskeleton pausing in post-axial, dorsal, pre-axial and ventral views. (MPG 2015 kb)

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Boisvert, C., Mark-Kurik, E. & Ahlberg, P. The pectoral fin of Panderichthys and the origin of digits. Nature 456, 636–638 (2008).

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