Panderichthys rhombolepis

Panderichthys is a widely recognized transitional form in tetrapod evolution (you know, one of those transitional fossils we're so often told don't exist). A description of a specimen with a well-preserved pelvic girdle has just been described in Nature, and it tells us some more about the history of tetrapod locomotion.

Panderichthys is an interesting animal—it definitely looks more like a fish than a salamander, but its fins are stout and bony, and other characteristics of its skeleton clearly ally it with the tetrapods. In the shift from an aquatic to a fully terrestrial life, the limbs and their supporting pectoral and pelvic girdles had to undergo major changes. In fish, the pectoral girdles are coupled to the skull, while the pelvic girdles are small and 'floating' in the musculature. To bear the animal's weight, the pectoral girdles lost their connection to the skull, and both became thicker, stronger, and more closely bound to the axial skeleton. The fins themselves had to change from a fan of slender fin-rays to more solid load-bearing digits. In Panderichthys, we see a mixture of these changes in process.

Continue reading "Panderichthys rhombolepis" (on Pharyngula)