One little ‘fix’ on the piranha premaxilla resolves phylogenetic issues

Revised January 14, 2021
with new bone identities for the premaxilla and maxilla.

Confession time!
Now that I have discovered this error, the piranha (Serrasalmus, Figs. 1, 2) no longer disrupts the ‘flow’ of evolution in that corner of the large reptile tree (LRT, 1709+ taxa then, 1796 taxa now; Fig. 1).

The mistake:
Previously I misidentified some bones in the piranha. These are now corrected… again.

Figure 1.  On the piranha, Serrasalmus, the premaxilla (yellow) does not extend to the tooth row. Instead the maxilla (green) carries all the upper teeth. Serrasalmus skeleton image courtesy of ©Steve Huskey and used with permission.

Figure 1.  On the piranha, Serrasalmus, the premaxilla (yellow) does not extend to the tooth row. Instead the maxilla (green) carries all the upper teeth. Serrasalmus skeleton image courtesy of ©Steve Huskey and used with permission.

Figure 2. Anterior view of the piranha, Serrasalmus, showing the premaxilla extends to the tooth row.

Figure 2. Anterior view of the piranha, Serrasalmus, showing the premaxilla extends to the tooth row.

The correction: 
As the anterior view of Serrasalmus shows (Fig. 2), the premaxilla (yellow) extends to the tooth row, as in all other fish. This uncorrected error vexed the resolution of the fish portion of the LRT for several months.

Serrasalmus rhombeus (Lacepède 1803) is one of the extant piranhas, predatory fish restricted to the Amazon River.  A deep, narrow and short body with a propensity to school distinguish this genus from its sisters. Note the sagittal crest.

Hoplerythrinus
(Fig. 3) is a piranha sister with a skull diagram close to what we see in the piranha.

Figure 3. Araimaia (Hoplerythrinus) skull.

Figure 3. Araimaia (Hoplerythrinus) skull.

Xiphactinus audax (Leidy 1870; Late Cretaceous; up to 6m in length) was a large traditional ray-fin fish.  The teeth are longer and stronger in this clade. Like the piranha a parasagittal crest is present. The torso is much longer.

Figure 1. Xiphactinus skull revised using Portheus as a guide. The 'shadow' area in the reconstruction indicates a lack of cheek bones, exposing the large pterygoids and quadrate.

Figure 1. Xiphactinus skull revised using Portheus as a guide. The ‘shadow’ area in the reconstruction indicates a lack of cheek bones, exposing the large pterygoids and quadrate.

Portheus molossus (Cope 1872; Late Cretaceous; is considered a junior synonym for Xiphactinus. However, note the expanded jugal and postorbital and different shapes for several other facial bones, included the tabular (red).

Figure 4. The skull of Portheus from Gregory 1938. Many workers consider this a junior synonym of Xipactinus.

Figure 4. The skull of Portheus from Gregory 1938. Many workers consider this a junior synonym of Xipactinus.

According to Schwimmer, Stewart and Williams 1997,
“Joseph Leidy and Edward D. Cope independently described the taxon as Xiphactinus audax Leidy 1870 and Portheus molossus Cope, 1871. Although Cope’s type specimen was a finely preserved individual, whereas Leidy’s type was an isolated pectoral fin spine, the rules of priority (Ride, et al. 1985) require that the widely-known name Portheus molossus be suppressed as a junior synonym. In their early descriptions of Xiphactinus and Portheus, Leidy and Cope followed prevailing practice and recognized numerous species, all of which, at least for North America, were subsequently subsumed into the single species X. audax by Bardack (1965). which was the last substantive taxonomic
analysis of the genus.”

Among living taxa,
Bardack (1965) allied Xiphactinus to the overall similar modern wolf-herring, Chirocentrus (Fig. 5) based on eyeballing it. That hypothesis predated MacClade and PAUP. By contrast, the LRT nests Chirocentrus with the lizardfish, Trachinocephalus and the viperfish, Chaulidos apart from Xiphactinus and Serrasalmus. 

Figure 1. The wolf herring (Chirocentrus) enters the LRT.

Figure 5. The wolf herring (Chirocentrus) enters the LRT.

Apparently the connection between
Xiphactinus and the piranha, Serrasalmus, was never made in the academic literature. While Googling I was able to find connections only in the popular press which promoted Xiphactinus as a giant fish with ‘piranha-like jaws’. Let me know if there is an earlier citation in the literature so I can promote it.

References
Bardack D 1965. Anatomy and evolution of Chirocentrid fishes. The University of Kansas Paleontological Contributions 10:1-88.
Cope ED 1871. Account of a journey in the valley of the Smoky Hill River in Kansas. Proceedings of the American Philosophical Society 12:174-176
de Lacepéde BG 1803. Histoire naturelle des poissons. Tome Cinquieme. 5(1-21):1-803 + index.
Leidy J 1870. [Remarks on ichthyodorulites and on certain fossil Mammalia]. Proceedings of the Academy of Natural Sciences, Philadelphia 22:12–13.
Schwimmer DR, Stewart JD and Williams GD 1997. Xiphactinus vetus and the distribution of Xiphactinus species in the Eastern United States. Journal of Vertebrate Paleontology 17(3):610–615.

wiki/Piranha
wiki/Serrasalmus
wiki/Xiphactinus

https://pterosaurheresies.wordpress.com/2020/02/02/the-wolf-herring-chirocentrus-enters-the-lrt/

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