Aniliidae Stejneger, 1907, 2007

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

Aniliidae Stejneger, 1907

General information.

The taxonomic content and exact affinities of the group that is commonly known as “pipesnakes”, i.e., the American Anilius Oken, 1816, and the Asian Anomochilus Berg, 1901, and Cylindrophis Wagler, 1828, has been variously altered throughout decades of systematic studies of snakes. They were once known during the 19th century under the names Ilysiidae (or Ilysioidea) (e.g., Fitzinger 1826; Bonaparte 1852; Boulenger 1890b, 1893; Cope 1894, 1898; Gadow 1901; Janensch 1906) or Tortricidae (or Tortricina or Tortriciens) (e.g., Müller 1831; Duméril and Bibron 1844; Jan 1857, 1862, 1863; Peters 1861; Cope 1864, 1887, 1893, 1895, 1898; Günther 1864; Jan 1865; Carus 1868; Müller 1880; Rochebrune 1884; Zittel 1887-1890; Palacký 1898). Cope (1887) also included the uropeltids in anilioids, while Romer (1956), Kuhn (1961), Smith et al. (1977), and McDowell (1975, 1987) further added Xenopeltis Reinwardt in Boié, 1827, and Loxocemus Cope, 1861. Guibé (1970) placed in Aniliidae only Anilius , Cylindrophis , and Anomochilus . In a similar manner, Anilioidea was later confined to include these three genera, all pertaining to their own families or subfamilies (e.g., Cundall et al. 1993). That traditional concept of Anilioidea was considered as a paraphyletic assemblage by Gower et al. (2005), a view that has been subsequently followed by others ( Smith 2013; Head 2021; Smith and Georgalis 2022). Indeed, recent molecular studies treat Anilius as the sister group of Tropidophiidae , united in a group termed Amerophidia , whereas Uropeltis Cuvier, 1829, and Cylindrophis are united in a more distantly related group termed Uropeltoidea (e.g., Miralles et al. 2018; Burbrink et al. 2020; Zaher et al. 2023; see the respective entry below). As such, Aniliidae is currently conceived to include among extant snakes solely Anilius and its single species, Anilius scytale (Linnaeus, 1758), distributed only in northern South America. Fossorial habits for these snakes are already clearly indicated by their etymology, i.e., from the Greek “ἀν-” (privative affix for “no”, “without”) and “ἥλιος” ( “sun”).

Although a number of fossil remains and taxa has been referred to aniliids in the past few decades (e.g., Rage 1974, 1984, 1998; Szyndlar 1994, 2009; Szyndlar and Alférez 2005; Augé and Rage 2006; Syromyatnikova et al. 2019), taking also into consideration the paraphyly of the traditional concept of " Anilioidea ", it is not possible to determine where exactly most of these lie within Alethinophidia (see Head 2021; Smith and Georgalis 2022). Accordingly, the sole few definite fossil occurrences of Aniliidae are known from the Late Cretaceous and Cenozoic of the Americas ( Head 2021; Head et al. 2022; Smith and Georgalis 2022), including also the only known fossil record of the extant Anilius scytale , from the Pliocene of Venezuela ( Carrillo-Briceño et al. 2021).

Vertebral morphology of Aniliidae is characterized by being relatively heavily built, an elongate centrum, depressed cotyle and condyle, depressed neural arch, neural spine with short anterior lamina that is strongly reduced dorsoventrally but crosses most of the anteroposterior length of the neural arch, shallow (but not absent) median notch of the neural arch, elongate prezygapophyses elevated to just shorter than zygosphene and angled at around 20°-25°, prominent (very thick and plate-like in shape) hypapophysis in anterior trunk vertebrae and a distinct haemal keel in succeeding trunk vertebrae, lack of haemapophyses or hypapophyses in caudal vertebrae, and a very low number of caudal vertebrae (for more details see Description and figures of Anilius below).

Previous figures of vertebrae of extant Aniliidae have been so far presented by Rochebrune (1881), Hoffstetter and Gasc (1969), Gasc (1974), Hoffstetter and Rage (1977), Dowling and Duellman (1978), Rieppel (1979), Ikeda (2007), Palci et al. (2013a, 2018), Garberoglio et al. (2019), Fachini et al. (2020), Head (2021), and Alfonso-Rojas et al. (2023). Among these, vertebrae from the cloacal and/or caudal series have only been figured so far by Gasc (1974) and Alfonso-Rojas et al. (2023). Documentation of the embryonic development of aniliid vertebrae was provided by Guerra-Fuentes et al. (2023). Quantitative studies on the intracolumnar variability of aniliid vertebrae have been conducted by Gasc (1974) and Head (2021). Besides, important observations on the vertebrae of Anilius were made by Smith (2013), while Head (2021) recently provided a comprehensive study of the vertebral morphology of Aniliidae coupled with an emended diagnosis for the family, recognizing diagnostic features on the vertebrae that could differentiate them from uropeltoids.