Sunandan Das (1), Jonathan Brecko (2) (3), Olivier S. G. Pauwels (2), Juha Merila (1) (4)
Journal of Morphology. Volume 283, Issue 4, pages 510-538 (30 January 2022). DOI: https://doi.org/10.1002/jmor.21457

Keywords

Atractaspis, cranium, fang evolution, fossoriality, phylogeny, systematics

Abstract

Fossoriality evolved early in snakes, and has left its signature on the cranial morphology of many extinct Mesozoic and early Caenozoic forms. Knowledge of the cranial osteology of extant snakes is indispensable for associating the crania of extinct lineages with a particular mode of life; this applies to fossorial taxa as well. In the present work, we provide a detailed description of the cranium of Hypoptophis wilsonii, a member of the subfamily Aparallactinae, using micro-computed tomography (CT). This is also the first thorough micro-CT-based description of any snake assigned to this African subfamily of predominantly mildly venomous, fossorial, and elusive snakes. The cranium of Hypoptophis is adapted for a fossorial lifestyle, with increased consolidation of skull bones. Aparallactines show a tendency toward reduction of maxillary length by bringing the rear fangs forward. This development attains its pinnacle in the sister subfamily Atractaspidinae, in which the rear fang has become the “front fang” by a loss of the part of the maxilla lying ahead of the fang. These dentitional changes likely reflect adaptation to subdue prey in snug burrows. An endocast of the inner ear of Hypoptophis shows that this genus has the inner ear typical of fossorial snakes, with a large, globular sacculus. A phylogenetic analysis based on morphology recovers Hypoptophis as a sister taxon to Aparallactus. We also discuss the implications of our observations on the burrowing origin hypothesis of snakes.

How Our Software Was Used

Segmentation of the scans was done using Dragonfly. Visualization of scans, including those obtained from databases, and preparation of figures were then done with MeshLab.

Author Affiliation

(1) Ecological Genetics Research Unit, Organismal and Evolutionary Biology Research Programme, Faculty of Biological and Environmental Sciences, FI-00014 University of Helsinki, Helsinki, Finland
(2) Department of Recent Vertebrates, Royal Belgian Institute of Natural Sciences (RBINS), Brussels, Belgium
(3) Royal Museum for Central Africa, Tervuren, Belgium
(4) Division of Ecology and Biodiversity, Faculty of Science, The University of Hong Kong, Pok Fu Lam, Hong Kong SAR