Erycidae Bonaparte, 1831 (sensu Pyron et al. 2014), 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

Erycidae Bonaparte, 1831 (sensu Pyron et al. 2014)

General information.

Fitzinger (1843) called them Gongylophes. Commonly known as the Sand Boas, the Old World genus Eryx (including also Gongylophis Wagler, 1830, as a junior synonym in current taxonomies) along with the New World genera Charina Gray, 1849, and Lichanura Cope, 1861, were until recently conceived to represent a distinct subfamily of boids, termed Erycinae (e.g., Hoffstetter 1955; Romer 1956; Hoffstetter and Rage 1972; Underwood 1976; Dowling and Duellman 1978; Rage 1984, 1987; McDowell 1987; Szyndlar 1991a, 1994; Kluge 1993b; Mead and Schubert 2013). However, an array of phylogenies, based primarily on molecular data or combined molecular+morphological evidence, have challenged this topology, and the traditional concept of erycines has been recently considered to be paraphyletic, i.e., with the Old World erycines pertaining to a different group ( Erycidae ) than the New World ones ( Charinaidae ) ( Wilcox et al. 2002; Lawson et al. 2004; Pyron et al. 2013; Reynolds et al. 2014; Hsiang et al. 2015; Burbrink et al. 2020; Scanferla and Smith 2020b). Nevertheless, certain morphology-based or molecular+morphology combined phylogenies contrastingly still recover Erycidae and Charinaidae together as a monophyletic clade (e.g., Kluge 1993b; Gauthier et al. 2012; Scanferla et al. 2016; Smith and Scanferla 2021). It is of course, beyond the scope of this paper to assess the relationships of Erycidae with Charinaidae , but due to the lack of consensus, we treat them independently. However, we should note that vertebral morphology (i.e., the peculiar highly complex osteology of the caudal vertebrae, which is shared among Erycidae and Charinaidae and is totally absent in Ungaliophiidae ) supports affinities between erycids and charinaids.

The current concept of Erycidae is confined to the genus Eryx , which comprises 13 species distributed in southeastern Europe, southwestern Asia, and Africa ( Reynolds and Henderson 2018; Boundy 2021). Erycidae have a moderately rich fossil record, with remains of the extant genus Eryx already identified since the Miocene to the Quaternary of Europe, Asia, and northern Africa (e.g., Hoffstetter and Rage 1972; Rage 1976; Bailon 1989, 1991; Szyndlar 1991a; Szyndlar and Zerova 1992; Szyndlar and Schleich 1994; Ivanov 2000; Malakhov 2005; Szyndlar and Alférez 2005; Nadachowski et al. 2006; Delfino et al. 2011; Maul et al. 2015; Blain 2016; Smith et al. 2016; Villa et al. 2021; Serdyuk et al. 2023; Shi et al. 2023a). Apart from fossils of Eryx , as it is now known that the European Paleogene hosted also charinaids (i.e., Rageryx Smith & Scanferla, 2021), several other forms from the Paleogene and early Neogene of Europe that possess a complex caudal vertebral morphology (e.g., genus Cadurceryx Hoffstetter & Rage, 1972), cannot be confidently identified as Erycidae or Charinaidae , and are tentatively referred as “erycines” (e.g., Georgalis and Scheyer 2021; Smith and Georgalis 2022).

Trunk vertebrae of erycids generally resemble those of other booids (except for candoiids and ungaliophiids) although certain features, such as the (usual) absence of paracotylar foramina distinguish them from others (booids and [to a lesser degree] sanziniids and the charinaid Lichanura ). Nevertheless, it is the highly complex morphology of the caudal vertebrae that characterizes erycids, a feature shared only with charinaids (for more details, see Description and figures of Eryx below). Notably also, erycids are the only snakes, in which (in certain species) osteoderms are present ( Frýdlová et al. 2023). These osteoderms occur around the cloaca and the tail and their unique presence in erycids, together with their highly complex caudal vertebral morphology, have been suggested to be associated with their fossorial locomotion ( Frýdlová et al. 2023).

Previous figures of vertebrae of extant Erycidae were so far presented by Owen (1850), Sood (1941, 1948), Bogert (1968a), Hoffstetter and Gasc (1969), Hoffstetter and Rage (1972), Gasc (1974), Tokar (1989), Szyndlar (1994), Szyndlar and Schleich (1994), Venczel (2000), Georgalis and Scheyer (2019), Frýdlová et al. (2023), and Shi et al. (2023a). Among these, vertebrae from the cloacal and/or caudal series were presented by Sood (1941), Bogert (1968a), Hoffstetter and Gasc (1969), Hoffstetter and Rage (1972), Tokar (1989), Szyndlar (1994), Szyndlar and Schleich (1994), Venczel (2000), and Frýdlová et al. (2023). Figures of the microanatomy and histology / transverse sections of erycid vertebrae were presented by Sood (1948) and Houssaye et al. (2013). Quantitative studies on the intracolumnar variability of erycid vertebrae were conducted by Gasc (1974) and Head (2021). Besides, an extensive analysis of the mode of articulation between neighbouring caudal vertebrae of erycids was provided by Szyndlar (1994).