Dream-fish, Coelacanths and Super-Predators: The Sarcopterygians

For the subject of today's post, I drew the Sarcopterygii, the 'lobe-finned fishes'. Though something of a poor relation to their considerably more diverse sister-group, the ray-finned fishes of the Actinopterygii, this is a group most of my readers will have probably encountered in some capacity. As their names both formal and vernacular indicate, the Sarcopterygii were originally characterised by the development of the fins as fleshy lobes, with at least some fins possessing an internal skeleton of serial bones. Living sarcopterygians belong to three major groups, the coelacanths, lungfishes and tetrapods (in which, of course, the ancestral fins have been modified into walking limbs). The majority of recent studies have placed the coelacanths as the most divergent of these groups, with lungfishes and tetrapods as sister taxa. As the tetrapods are a particularly tedious group of organisms, with little to interest the casual observer, I'll put them aside for this post (you can go to Tetrapod Zoology if you must). The lungfishes, too, warrant a more detailed look at another time.

The oldest known sarcopterygian (and, indeed, the oldest known crown-group bony fish) is the Guiyu oneiros (shown above in a reconstruction by Brian Choo for Zhu et al. 2009), whose species name suggests the vernacular name of 'dream fish'. The dream-fish is known from the late Silurian of China, with a number of other stem-sarcopterygians such as Psarolepis and Meemannia known from the early Devonian of the same region. These taxa retained a number of ancestral features such as heavy ganoid scales (a type of scale also found in basal actinopterygians), and strong spines in front of the fins. However, crown-group sarcopterygians had also evolved and diverged by the early Devonian, as shown by the presence of the stem-lungfish Youngolepis.

Congregation of West Indian Ocean coelacanths Latimeria chalumnae, photographed by Hans Fricke.

The coelacanths are, of course, best known to most people for the discovery of the living Latimeria chalumnae in 1938 in South Africa, after the lineage had been thought to have become extinct in the Cretaceous. The subsequent media frenzy must have been interesting to fishermen in the area who had long known the coelacanth primarily as an infernal nuisance. Though only captured occasionally as bycatch, a landed coelacanth represents two metres or more of snap-jawed bad temper, while the oily flesh is inedible. More recently, a second species of living coelacanth, Latimeria menadoensis has been described from near Sulawesi in Indonesia.

Because of the circumstances of its discovery, Latimeria became a textbook example of a 'living fossil'. However, all fossil coelacanths were not mere duplicates of Latimeria. To begin with, Latimeria is quite a bit larger than the majority of its fossil relatives (Casane & Laurenti 2013). These included such distinctive forms as the fork-tailed speedster Rebellatrix and the eel-like Holopterygius. And then there was Allenypterus montanus, a Carboniferous taxon that... well, just look at the thing (photo from here):

Though Latimeria may lord it over its immediate relatives, it is far from the largest sarcopterygian (even excluding the tetrapods). The tetrapod stem-group also included a number of large predators, including the famous Eusthenopteron (how many other fossil fish have been referred to by name in an episode of Doraemon?). Particularly dramatic were the Rhizodontida, freshwater ambush predators of the Devonian and Carboniferous. Though probably very low on the tetrapod stem (and hence not directly related to limbed tetrapods), rhizodontids developed enlarged pectoral fins that articulated with the body in a not dissimilar manner to tetrapod forelegs. Like tetrapods, rhizodontids probably used their pectoral fins to push against the substrate and provide explosive propulsion (Davis et al. 2004). The jaw of rhizodontids contained enlarged tusks interspersed among smaller teeth that would have hooked into struggling prey. The largest rhizodontids have been estimated to be about seven metres in length, and were the sort of predator that the term 'apex' was invented for.

Reconstruction of Rhizodus by Mike Coates.

REFERENCES

Casane, D., & P. Laurenti. 2013. Why coelacanths are not 'living fossils'. BioEssays 35: 332-338.

Davis, M. C., N. Shubin & E. B. Daeschler. 2004. A new specimen of Sauripterus taylori (Sarcopterygii, Osteichthyes) from the Famennian Catskill Formation of North America. Journal of Vertebrate Paleontology 24 (1): 26-40.

Zhu, M., W. Zhao, L. Jia, J. Lu, T. Qiao & Q. Qu. 2009. The oldest articulated osteichthyan reveals mosaic gnathostome characters. Nature 458: 469-474.