Fish with fingers, whales with legs – the sub-title of Carl Zimmer’s 1998 book on the evolution of amphibians & whales – seems even more apt with the announcement of a new fossil find: a fish whose pectoral fins contained bones homologous to tetrapod fingers (Boisvert et al. 2008).
Panderichthys is a lobe-finned fish that’s generally seen as belonging to the lineage leading to amphibians. Inside the fleshy lobes at the base of its pectoral & pelvic fins lie a series of bones in a 1-2-many arrangement that is homologous to the humerus-ulna & radius-carpals (or femur-tibia & fibia-tarsals) of a tetrapod limb. But palaeontologists have tended to view digits (fingers & toes) as an evolutionary novelty, unique to the tetrapods.
Boisvert & her colleagues studied a fairly complete Panderichthys that dates back to the Devonian, around 385 million years ago. Fossils this old are often squashed fairly flat by the weight of sediments atop them, but this one has more of a 3-D structure (plus a pelvis & pelvic fin skeleton – the first found for this genus of fish). The researchers used a CT scan – a technique that’s used quite often now – to visualise details of the fish’s right pectoral fin & shoulder girdle. They must have been rather excited by what they found:
The fin endoskeleton is complete and is composed of the humerus, radius, ulna, ulnare, bipartite intermedium and four small distal radials arranged in a transverse terminal array.
In other words, the bony skeleton supporting the fin was complete, & contained not only the bones of the forearm & wrist – but also bones homologous to the fingers of tetrapods. THe authors go on to say that
Our reinterpretation of the distal fin endoskeleton of Panderichthys removes the final piece of evidence supporting the formerly popular hypothesis that tetrapod digits are wholly new structures without homologues in [lobe-finned] fish fins. This hypothesis … has already been called into question by the discovery of digit-like radials in Tiktaalik and the fact that Hox gene expression patterns closely resembling those associated with digit formation in tetrapods occur in the distal fin skeletons of paddlefish & Australian lungfish.
So Zimmer was even closer than he knew to the mark. Fish fingers, anyone?
C.A. Boisvert, E. Mark-Kurik & P.E. Ahlberg (2008) The pectoral fin of Panderichthys and the origin of digits. Nature doi:10.1038/nature07339