Skull morphology of some highly aquatic South American xenodontines

In all recently published molecular phylogenies, the South American Xenodontinae form a clade that comprises
several monophyletic units (tribes), one of which consists of the three genera of the tribe Hydropsini (Helicops,
Hydrops and Pseudoeryx. Complete information on the bony skull is available for nearly a third (29 genera) of the Dipsadidae genera, whereas fragmentary descriptions were published for an additional 29 genera. Information on the bony skull of Hydropsini is more complete, since descriptions are available for Helicops carinicaudus, H. infrataeniatus, Hydrops marti, H. triangularis, and Pseudoeryx plicatilis.
A comparison of the cranial osteology among the three Hydropsini genera may prove useful characters to discuss the systematic value of previously studied features and to identify new ones that could unravel relationships among species within the tribe.  Di Pietro and colleagues (2014) describe the bony skull variation of Helicops
infrataeniatus, H. leopardinus, Hydrops caesurus, and Pseudoeryx plicatilis and compare it to that of Dipsadidae
species. This comprehensive comparison includes terrestrial, semi-fossorial, fossorial, semi-aquatic,
fully-aquatic, and arboreal forms of Dipsadidae.
The authors compare them with several genera of Dipsadidae and found that the unpaired foramen on the parabasisphenoid with anterior position is the only skull feature shared by all Hydropsini genera. This feature also occurs in the semi-aquatic (Erythrolamphrus semiaureus) and fully-aquatic (Farancia abacura) dipsadids. All species of Hydrops with available skull descriptions and Pseudoeryx plicatilis share four features: (1) The anterior border of the angular is higher than the posterior border of the splenial, (2) the vomerine processes of the premaxilla are long, (3) the ascending process of the premaxilla overlaps the horizontal lamina of the nasals, and (4) an anterior projection of the prefrontal is present.
All species of Helicops with available skull descriptions and Pseudoeryx plicatilis share three features: (1) A vertical
lamina of the nasal with a notch, (2) a single foramen rotundum, and (3) the presence of a ventral projection of the
transverse crista of the basioccipital. Finally, they also found small, paired parietal foramina in most of the dipsadids studied here, which are filled with a Sudan-Black-positive tissue of possible nervous origin.
The species studied here and those for which are available skull descriptions, Helicops and Hydrops share no unique features. These authors found the two genera share a long and triangular-shaped maxillary process of the palatine,
a character not found in the species of Helicops and Hydrops reported on by previous authors.

Di Pietro, D. O., Alcalde, L., & Williams, J. D. (2014). New cranial characters in the tribe Hydropsini (Serpentes: Dipsadidae: Xenodontinae). Acta Herpetologica, 9(1), 1-14.

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