Digging adaptations evolved independently in two lineages of Psammophiid snake: evidence from cranial morphology

ABSTRACT

The Beaked Snakes (Rhamphiophis) are semi-fossorial snakes with reinforced snouts adapted for digging. The Skaapstekers (Psammophylax) are generalist terrestrial snakes. Both belong to the family, Psammophiidae. The Striped Beaked Snake (Kladirostratus acutus) was originally assigned to the genus Rhamphiophis, because of its beak-like snout. However, molecular evidence indicates that K. acutus is more closely related to Psammophylax. Thus, the “beak” of K. acutus must have evolved independently of that seen in Rhamphiophis. We studied the cranial morphology of K. acutus and representatives of Psammophylax and Rhamphiophis to identify possible traits that may reflect independent derivation of a beak-shaped snout relative to Rhamphiophis. We found that K. acutus retains four main cranial features in common with the Psammophylax: (1) an open fenestra between the premaxilla, nasals and septomaxilla, (2) a lack of fusion between the septomaxilla and the vertical lamina of the nasal, (3) the conchal processes of the septomaxilla extend beyond the dorsal margin of the septomaxilla and (4) a lack of contact between the prefrontals and the dorsal laminae of the nasals. Simultaneously, K. acutus displays traits associated with adaptation to semi-fossorial life in snakes such as a shortening of the maxilla, modification of the premaxilla, elongation of the nasal region, and a narrowing of the braincase.

KEYWORDS:

• convergence
• evolution
• computed tomography
• morphology
• phylogeny
• Serpentes
• fossoriality
• skull

Acknowledgements

We would like to acknowledge the Field Museum of Natural History (Chicago), the Museum of Comparative Zoology (Harvard University) and the National Museum of Natural History (Smithsonian Institution) for loaning us specimens as well as Morphosource.org. Access to scan of Malpolon monspessulanus fuscus (LSUMZ 36788) was provided by Louisiana State University & Agricultural and Mechanical College and was funded by oVert TCN; NSF DBI-1701714; NSF DBI-1701402. In addition, we are grateful to Adam Summers and the Karel F Liem Imaging Facility staff at the University of Washington Laboratories at Friday Harbor for their guidance and providing us access to the CT scanner. We would also like to thank Austun Ables and Larry North for technical support and assistance as well as the many reviewers that lent their time and thoughtful suggestions towards the betterment of this manuscript. This research was funded by Whitman College through a Louis B Perry grant for faculty-student research.

Data accessibility

The models and raw data used in this study are available on MorphoSource, Project: Digging adaptations in psammophiid snakes (https://www.morphosource.org/projects/0000C1043).