Claeson, Bemis and Hagadorn 2007
use 3D-digital reconstruction to bring us the skull of the extant eel-like polypterid fish, Erpetoichthys calabaricus (Figs. 1–3). The authors considered polypterid fish “the basalmost group of extant actinopterygians and may be a direct link to understanding the systematics and evolution of the first bony fishes.” The LRT indicates this traditional nesting is due to taxon exclusion. Several other bony fish (like catfish) are much more primitive.
FIgure 1. The eel-like polypterid, Erpetoichthys in vivo. At standard 72dpi monitor resolution, this is shown at 3x full size, so this is a very tiny fish.
Several bony fish
are much more primitive in the large reptile tree (LRT, 1566 taxa). The cladograms of Claeson et al. do not indicate an outgroup taxon. That creates a problem.
Figure 2. The skull of Erpetoichthys in three views from Claeson, Bemis and Hagadorn 2007. Colors added. Note the frontals are colored blue here, distinct from the original authors label. The preoperculars are fused to the squamosals (magenta).
Claeson et al. report,
“Erpetoichthys calabaricus differs in external morphology from Polypterus in several ways. Most notable are the elongation of the body, the lack of pelvic fins, and the extremely smaller size of the adult rope fish skull.”
“The Polypteridae possess several apomorphies including elongate, cylindrical bodies, thick rhomboid ganoid scales that interlock with a ‘‘peg and socket’’ articulation, a spiracle, a firm articulation between the preopercle and the maxilla, highly modified dorsal and caudal fins, and the absence of a working spiracular organ.”
Distinct from Claeson et al.
the frontals (blue) are not over the orbits, but between the naris and orbit. The other bones are also colored according to their tetrapod homologs. Claeson et al. documented the presence of extensive lateral line canals.
Figure 3. Erpetoichthys palate from Claeson et al. 2007, colors added.
The palate
Claeson et al. provide an image of the palate of Erpetoichthys (Fig. 2), a rare opportunity. Distinct from most tetrapods, the ectopterygoid is much larger than the palatine. The vomers are tiny. As we knew earlier, the pterygoids are multipart.
Figure 4. Subset of the LRT focusing on basal vertebrates. Colors indicate various naris configurations. Some palates are not known. Epretoichthys would nest with Polypterus here, but this graph has not been updated to reflect that. See the LRT for that update.
According to Claeson et al.
“Palatal teeth are usually small and come in patches. There are several rows of teeth on the ventral surface of the dermopalatine. Although it is present in Polypterus, the quadratojugal is absent in E. calabaricus.” The parasphenoid has tiny teeth.
Figure 5. The Nile bichir (Polypterus), skull, skeleton and bones colorized for ease of comparison. Compare to the placoderm, Entelognathus, (Fig. 2) and the stem tetrapod Tinirau (Fig. 3). The colors on this drawing have been updated based on data from Erpetoichthys.
Like Polypterus (Fig. 5), Erpetoichthys can breathe air and is related to lungfish in the LRT (Fig. 4). Claeson et al. do not identify or even mention an internal naris, but several potential openings are indicated by their diagram (Fig. 3). The origin of the internal naris (chown, Fig. 4) was first covered here.
References
Claeson KM, Bemis WE and Hagadorn JW 2007. New interpretations of the skull of a primitive bony fish Erpetoichthys calabaricus (Actinopterygii: Cladistia). Journal of Morphology 268:1021–1039.
The Pterosaur Heresies 