Old Six Lobes

_Latimeria chalumnae_ from the Harvard Museum of Natural History. The famous _Kronosaurus_ lurks in the reflection.

What is going on with coelacanths? The first dorsal fin is a typically fishy rayed structure yet the second is fleshy and lobed. Considering that tetrapod limbs are derived lobed fins and the anal fin is also lobate this makes coelacanths hexapedes of a sort. It is baffling how the nickname “Old Fourlegs” got attached to a creature with such bizarrely arrayed and numerous appendages.

A phylogeny of lobed fins from Friedman et al. (2007).

Coelacanths can’t really be said to have “legs” or even proper limbs. Tetrapod limbs are defined as having mobile wrists, ankles and digits (Shubin et al. 2006) whereas coelacanth lobed fins are made out of a series of metapterygia with radials on the anterior and posterior margins (see above). The Devonian coelacanth Shoshonia had pectoral fins with some traits more typical of early tetrapodomorphs than extant coelacanths, most notably by having pronounced asymmetry (Friedman et al. 2007). This indicates that coelacanth lobe fins became less limb-like during their evolution and Friedman et al. (2007) argue that the extant fishes most likely to give insight into early tetrapod limbs are basal actinopterygians such as bichir and sturgeon. Thus extant coelacanths should not be viewed as evolutionarily stagnant and the nickname “Old Fourlegs” somehow manages to be even more wrong.

From Wikipedia Commons.

The medial second dorsal and anal fins of Latimeria are “practically identical” and mirror one another (Forey 1998). These lobed medial fins exhibit skeletal, muscular and innervational similarities with the paired lobed fins (Ahlberg 1992 – citing Millot and Anthony 1958) and all the lobed fins appear capable of considerable rotation (Forey 1998). Judging by videos it appears that the medial lobed fins beat in sync whereas the pectoral and pelvic pair does not. The medial lobed fins particularly resemble the pelvic fins and the basal plate supporting the former appears very similar to the pelvis of the latter rotated 90 degrees (Ahlberg 1992). Ahlberg (1992) suggested that the lobed median fins came into being through a switch in gene expression although noted more fossils and evidence from development and genetics would be needed. Unfortunately so far as I can tell this topic has yet to be explored further.

From Wikipedia Commons

The caudal fin of coelacanths is normally interpreted as a three-part structure (Forey 1998) however in the description of the Jurassic coelacanth Parnaibaia maranhaoensis Yabumoto (2008) interprets it as being composed of a third dorsal, second anal fin and a proper caudal. Third dorsals and second anals are not without precedent (e.g. cod) however the degree of disparity between fins that coelacanths display certainly is. Perhaps the medial lobed fins of coelacanths aren’t true dorsal or anal fins at all but totally novel replications of the pelvic fins. Alternately it could be possible that coelacanths had ancestors with a superfluous number of medial fins or that the caudal fin split into three parts for some reason; either way it seems likely that something very very strange is happening to coelacanths genetically and developmentally.

References:

Ahlberg, P. E. (1992). Coelacanth Fins and Evolution. Nature 358, 459. Available.

Forey, P. L. (1998). History of the Coelacanth Fishes. Natural History Museum: London.

Friedman, M. (2007). First discovery of a primitive coelacanth fin fills a major gap in the evolution of lobed fins and limbs. Evolution & Development 9(4), 329-337. Available.

Millot, J. & Anthony, J. (1958-1965) Anatomy de Latimeria chalumnae. Vol. I-II. Paris.

Shubin, N. H. et al. (2006). The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb. Nature 440 (6), 764-771. Available.

Yabumoto, Y. (2008). A new Mesozoic coelacanth from Brazil (Sarcopterygii, Actinistia). Paleontological Research 12(4), 329-343. Available.

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