Xenophidiidae Wallach & Guenther , 1998, 1842

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

Xenophidiidae Wallach & Guenther, 1998

General information.

Xenophidiidae represents a rather enigmatic group of snakes that were only recently discovered, comprising a single genus, Xenophidion Günther & Manthey, 1995, with two species, confined in the Malay Peninsula, Borneo, and Sumatra ( Günther and Manthey 1995; Wallach and Günther 1998; Quah et al. 2018). Frustratingly, both species are collectively known only by less than 10 specimens ( Günther and Manthey 1995; Wallach and Günther 1998; Quah et al. 2018; Fukuyama et al. 2020). Xenophidion was originally regarded to be a caenophidian ( Günther and Manthey 1995) and subsequently placed in its own family, Xenophidiidae as the sister group of tropidophiids ( Wallach and Günther 1998). Xenophidiidae possess a unique set of external morphology, soft anatomy, and skeletal features that complicate understanding of their exact interrelationships with other snakes (see Wallach and Günther 1998). Nevertheless, their joint possession with Bolyeriidae of a unique cranial synapomorphy - an intramaxillary joint - suggests a close relationship of xenophidiids with that group ( Gauthier et al. 2012; Georgalis and Smith 2020), a conclusion also coupled with molecular evidence (see entry of Bolyeriidae above). There is as yet no fossil record of xenophidiids.

No xenophidiid skeleton was available for study. In fact, the only so far known information on the vertebrae of Xenophidion is a single X-ray image of the holotype of the type species of the genus, Xenophidion acanthognathus Günther & Manthey, 1995, published by Wallach and Günther (1998: fig. 1). The resolution of that image is low and does not permit many conclusions, but Wallach and Günther (1998) provided a description of some interesting vertebral features, among which they highlighted the presence of "expanded blade-like haemapophyses (larger than the neural spines) in the caudal region", which they considered as unique among snakes. The same authors further mentioned the presence of blade-like hypapophyses in posterior trunk vertebrae and the absence of prezygapophyseal accessory processes ( Wallach and Günther 1998).

Number of vertebrae. Xenophidion acanthognathus (FMNH 235170 [holotype]): 183+4+53 (Agustin Scanferla, unpublished data, personal communication).

This number of vertebrae of Xenophidion acanthognathus is in accordance with the published number of ventrals and subcaudals described in the existing literature for different individuals of the same species: 181-185 ventrals and 51-55 subcaudals ( Günther and Manthey 1995; Fukuyama et al. 2020). Similar data for Xenophidion schaefferi Günther & Manthey, 1995, indicate 176-178 ventrals and 43-45 subcaudals ( Günther and Manthey 1995; Quah et al. 2018). Accordingly, the respective number of trunk and cloacal vertebrae for Xenophidion spp. ranges around 175-185 and that of the caudal vertebrae around 40-55. Therefore, these numbers are slightly lower than in Bolyeria and much lower than in Casarea .