Xenopeltidae Bonaparte, 1845, 1880

Szyndlar, Zbigniew & Georgalis, Georgios L., 2023, An illustrated atlas of the vertebral morphology of extant non-caenophidian snakes, with special emphasis on the cloacal and caudal portions of the column, Vertebrate Zoology 73, pp. 717-886 : 717

Xenopeltidae Bonaparte, 1845

General information.

Xenopeltidae represents a monotypic family, with a single genus, Xenopeltis , with only three species distributed in southeastern Asia ( Orlov et al. 2022). Unlike other constrictors that were ubiquitously lumped into an expanded " Boidae ", Xenopeltis was instead already recognized as its own distinct family (or subfamily) for a long time (e.g., Bonaparte 1845, 1852; Cope 1864, 1887, 1895, 1898; Nopcsa 1923; Smith 1943; Romer 1956; Dowling 1959; Guibé 1970). Indeed, Bonaparte (1845) established Xenopeltina as a member of the broader family Herpetidae , which included also uropeltids (his Uropeltina), calamariids (his Calamarina), and the homalopsid Erpeton Lacépède, 1801 (his Herpetina). In any case, the exact affinities of Xenopeltis with other snakes were unresolved for a long time, and the genus was occasionally considered to pertain to (the traditional concept of) “aniliids” (e.g., Jan 1857, 1865; Romer 1956; Kuhn 1961; Smith et al. 1977; McDowell 1987) or even within caenophidians ( Colubroidea in Cope 1898). Nevertheless, although a single analysis recovered Xenopeltis as close to Cylindrophis ( Noonan and Chippindale 2006), the majority of recent phylogenetic analyses have instead recovered Xenopeltis as closely related to pythonids (e.g., Slowinski and Lawson 2002; Wiens et al. 2008; Pyron and Burbrink 2012; Pyron et al. 2013 Reynolds et al. 2014; Figueroa et al. 2016; Streicher and Wiens 2016; Zheng and Wiens 2016; Harrington and Reeder 2017; Burbrink et al. 2020; Onary et al. 2022; Zaher et al. 2023). Accordingly, Xenopeltidae has been placed into Pythonoidea in current taxonomic schemes ( Wallach et al. 2014; Georgalis and Smith 2020). Affinities of Xenopeltis within Constrictores is also supported by its dorsal scale microstructure morphology ( Pauwels et al. 2000). Despite the fact that recent divergence dates estimate that xenopeltids split from other snakes already by the Late Cretaceous (e.g., Pyron and Burbrink 2012), there is no existing fossil record for the group.

Vertebral morphology of Xenopeltidae is characterized by being heavily built with centra distinctly longer than wide, the anterior ventral projection of the axis fused to the bone (this is sutured in most other snakes except for uropeltids), the presence of longitudinal bilateral ridges on the zygantral mounds, and the unique shape of the neural spine. We further highlight here the distinct notch in the ventral edge (visible in lateral view) of the hypapophyses of the anterior (but not anteriormost) trunk vertebrae, as unique among known snakes. Another, almost unique feature among non-caenophidian snakes seems to be the first appearance of the haemapophyses already on the cloacal vertebrae, but this is intraspecifically variable (for more details see Description and figures of Xenopeltis below) - a similar case with haemapophyses already appearing in the cloacal vertebrae is observed in the boid Epicrates and the pythonid Morelia Gray, 1842 - also in those two taxa it is intraspecifically variable (see the respective parts above and below).

Previous figures of vertebrae of extant Xenopeltidae have been so far presented by Hoffstetter and Gasc (1969), Gasc (1974), Lee and Scanlon (2002), Ikeda (2007), Garberoglio et al. (2019), Orlov et al. (2022), and Frýdlová et al. (2023). Among these, vertebrae from the cloacal and/or caudal series have been presented by Gasc (1974), Garberoglio et al. (2019), Orlov et al. (2022), and Frýdlová et al. (2023). Quantitative studies on the intracolumnar variability of xenopeltid vertebrae has been also conducted by Gasc (1974) and Head (2021). Beyond these, important descriptions of xenopeltid vertebral features were made by Szyndlar and Böhme (1996), Rage (2001), and Smith (2013).