Leptotyphlopidae Stejneger, 1891

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

Leptotyphlopidae Stejneger, 1891

General information.

Leptotyphlopidae comprise the smallest known snakes, with the record holding a species of the genus Tetracheilostoma Jan, 1861, which achieves a maximum snout-vent length of only 105 mm (i.e., Tetracheilostoma carlae [Hedges, 2008]; see Hedges 2008). At the same time, as their etymology also suggests (from the Greek “λεπτός”, meaning “thin”), this group also comprises the snakes with the minimum body width (diameter), i.e., a species of the genus Mitophis Hedges, Adalsteinsson & Branch in Adalsteinsson et al., 2009 (around only 2.5 mm; see Adalsteinsson et al. 2009). They consist of more than 140 species, distributed in Africa, the Middle East and parts of southern Asia, and the Americas ( Adalsteinsson et al. 2009; Boundy 2021). Most leptotyphlopid species were once placed into the genus Leptotyphlops Fitzinger, 1843, however, recent molecular studies coupled with evidence from external morphology have suggested their partitioning into several different genera ( Adalsteinsson et al. 2009; see also Wallach et al. 2014 and Boundy 2021). Divergence date estimates suggest that leptotyphlopids split from other scolecophidians already during the Early Cretaceous ( Zheng and Wiens 2016; Miralles et al. 2018; Sidharthan and Karanth 2021). Leptotyphlopidae were variously known, especially during the 19th and early 20th centuries, under the names Catodontia (or Catodonta or Catodontiens) (e.g., Duméril et al. 1854; Cope 1864, 1887, 1894, 1895, 1898; Carus 1868), a term apparently evoking to the fact that they possess teeth (Greek “οδόντες”) solely on their lower (Greek “κάτω”) jaws, as well as Stenosomata ( Ritgen 1828), evoking to their narrow bodies (Greek for narrow [ “στενός”] and body [ “σώμα”]), Stenostomidae (or Stenostomatidae or Stenostomata or Stenostomina or Stenostomi or Sténostomiens) (e.g., Bonaparte 1845, 1852; Boulenger 1882; Peters 1881, 1882; Boettger 1884; Cope 1887), evoking to their narrow mouth (Greek for narrow [ “στενός”] and mouth [ “στόμα”]), or more commonly as Glauconiidae (e.g., Boulenger 1890b, 1893; Bocage 1895; Cope 1898; Gadow 1901; Janensch 1906; Lydekker 1912; Werner 1916; Abel 1919; Mahendra 1936).

The leptotyphlopid vertebral morphology is generally reminiscent of other scolecophidians (see Description and figures below).

Previous figures of vertebrae of extant Leptotyphlopidae have been so far presented by List (1966), Dowling and Duellman (1978), Fabrezi et al. (1985), Pinto et al. (2015), Koch et al. (2019, 2021), Martins et al. (2019, 2021a, 2021b), Herrel et al. (2021), Alfonso-Rojas et al. (2023), and Peralta and Ferrero (2023). Among these, vertebrae from the cloacal and/or caudal series were presented by List (1966), Fabrezi et al. (1985), Pinto et al. (2015), Koch et al. (2019), and Martins et al. (2021a, 2021b). Quantitative studies on the intracolumnar variability of leptotyphlopid vertebrae have been conducted by Head (2021).

Material examined.

Epacrophis boulengeri (Boettger, 1913) ( SMF 16700 [holotype]); Epictia albifrons (Wagler, 1824) (MCZ Herp R-17393 [Morphosource.org: Media 000077182, ark:/87602/m4/ M77182 ]); Epictia ater (Taylor, 1940) (USNM 580323 [Morphosource.org: Media 000165850, ark:/87602/m4/M165850]); Epictia borapeliotes (Vanzolini, 1996) (MCZ Herp R-182170 [Morphosource.org: Media 000059305, ark:/87602/m4/ M59305 ]); Epictia columbi (Klauber, 1939) (MCZ Herp R-48773 [Morphosource.org: Media 000077341, ark:/87602/m4/ M77341 ]); Epictia guayaquilensis (Orejas-Miranda & Peters, 1970) (X-ray of ZMB 4508 [holotype]); Leptotyphlops nigricans (Schlegel, 1839 in Schlegel 1837 -1844) (CAS Herp 173933); Leptotyphlops scutifrons (Peters, 1854) (UF Herp 187225 [Morphosource. org: Media 000099311, ark:/87602/m4/ M99311 ]); Mitophis pyrites (Thomas, 1965) (MCZ Herp R-77239 [Morphosource. org: Media 000060820, ark:/87602/m4/ M60820 ]); Myriopholis longicauda (Peters, 1854) (MCZ Herp R-184447 [Morphosource. org: Media 000063946, ark:/87602/m4/ M63946 ]); Rena humilis Baird & Girard, 1853 (AMNH R 73716 ); Rena myopica (Garman, 1884) (UCM Herp 64598 [Morphosource. org: Media 000439463, ark:/87602/m4/439463]); Rena segrega (Klauber, 1939) (UCM Herp 16011 [Morphosource. org: Media 000444494, ark:/87602/m4/444494]); Siagonodon borrichianus ( Degerbøl, 1923) (MCZ Herp R-15899 [Morphosource. org: Media 000407561, ark:/87602/m4/407561); Siagonodon cupinensis (Bailey & Carvalho, 1946) (MCZ Herp R-142653 [Morphosource. org: Media 000397266, ark:/87602/m4/397266]); Siagonodon septemstriatus (Schneider, 1801) (MNHN L20.3177b); Tetracheilostoma bilineatum (Schlegel, 1839 in Schlegel 1837 -1844) (MCZ Herp R-10693 [Morphosource. org: Media 000397689, ark:/87602/m4/397689]; USNM 222954 [Morphosource. org: Media 000165848, ark:/87602/m4/M165848]); Tricheilostoma bicolor (Jan, 1860 in Jan and Sordelli 1860-1866) (MCZ Herp R-48934 [Morphosource. org: Media 000408221, ark:/87602/m4/408221]) .

Description (Figs 2-4).

Trunk vertebrae. The morphology of all vertebrae is very simple. Centrum elongate and cylindrical; cotyle and condyle strongly depressed; neural arch depressed; posterior median notch of the neural arch absent or very shallow; neural spine (except for a few anteriormost vertebrae) vestigial and restricted to the posteriormost part of neural arch or absent; prezygapophyseal accessory processes very long; paradiapophyses situated at a high position, higher than the ventral margin of the cotyle; the presence of hypapophyses restricted to a few anteriormost vertebrae (up to V 5); haemal keel absent in more posterior vertebrae, where the centrum is flattened and smooth, with no subcentral structures - but Hoffstetter (1968) highlighted that only in Siagonodon septemstriatus the haemal keel exists up to V 37; paracotylar foramina absent. Subcentral foramina were absent in all studied leptotyphlopids but they have been well documented as present throughout the trunk vertebrae of Trilepida ( Pinto et al. 2015).

Trunk / caudal transition. No subcentral structures occur in cloacal and caudal vertebrae. In some caudal vertebrae zygosphenes and zygantra may be missing.

Following the published literature, a range of 2-6 cloacal vertebrae are known in leptotyphlopids, and Mitophis is observed to show the maximum ( Martins et al. 2021a), while this number is also variable in Epictia Gray, 1845 (3-5 in Epictia rioignis Koch, Martins & Schweiger, 2019; Koch et al. 2019). In a few small species ( Tetracheilostoma spp.), pleurapophyses of the caudal vertebrae are extremely reduced. Moreover, in a single species (i.e., Mitophis leptepileptus ), pleurapophyses seem to be totally absent, a feature that is unique among all snakes, extant or extinct (see Martins et al. 2021a). The posteriormost two or three caudal vertebrae can be fused in several species, with this fused unit being conical with a bifurcated posterior tip ( Martins et al. 2021b).

Number of vertebrae. Epacrophis boulengeri (SMF 16700 [holotype]): 185 (160+3+22, including a final fusion); Epictia ater (USNM 580323): 242 (220+4+18, including a final fusion); Epictia albifrons (MCZ Herp R-17393): 251 (231+4+16, including a final fusion); Epictia borapeliotes (MCZ Herp R-182170): 263 (244+4+15, including a final fusion); Epictia columbi (MCZ Herp R-48773): 254 (225+4+25, including a final fusion); Epictia guayaquilensis (ZMB 4508 [holotype]): 246 (222 trunk vertebrae plus 24 cloacal and caudal vertebrae, including a final fusion); Leptotyphlops scutifrons (UF Herp 187225): 269 (244+3+22, including a final fusion); Mitophis pyrites (MCZ Herp R-77239): 269 (252+4+13, including a final fusion); Myriopholis longicauda (MCZ Herp R-184447): 307+ (273+3+31+ [posteriormost caudal vertebrae lacking]); Rena myopica (UCM Herp 64598): 210 (193+3+14, including a final fusion); Rena segrega (UCM Herp 16011): 289 (271+4+14, including a final fusion); Siagonodon borrichianus (MCZ Herp R-15899): 289 (273+3+13, including a final fusion); Siagonodon cupinensis (MCZ Herp R-142653): 268 (252+4+12, including a final fusion); Siagonodon septemstriatus (MNHN L20.3177b): 185+ (184+1+0+ [most cloacal and all caudal vertebrae missing]); Tetracheilostoma bilineatum (MCZ Herp R-10693): 179 (161+3+15, including a final fusion); Tetracheilostoma bilineatum (USNM 222954): 178 (159+4+15, including a final fusion); Tricheilostoma bicolor (MCZ Herp R-48934): 266 (251+3+12, including a final fusion).

Data from literature and unpublished data from personal communications: Epictia albipuncta (Burmeister, 1861): 205-228 trunk vertebrae plus 2-5 cloacal vertebrae plus 22-23 caudal vertebrae ( Fabrezi et al. 1985); Epictia munoai (Orejas-Miranda, 1961): 207 trunk vertebrae plus unknown number of cloacal and caudal vertebrae ( Koch et al. 2019); Epictia rioignis : 231-248 trunk vertebrae plus 3-5 cloacal vertebrae plus 14-21 caudal vertebrae ( Koch et al. 2019); Epictia magnamaculata (Taylor, 1940): 227 trunk vertebrae plus 4 cloacal vertebrae plus 19 caudal vertebrae (posteriormost 3 caudal vertebrae are fused) ( List 1966); Epictia magnamaculata : 199 trunk vertebrae plus unknown number of cloacal and caudal vertebrae ( Koch et al. 2019); Epictia phenops (Cope, 1875): 213-246 trunk vertebrae plus unknown number of cloacal and caudal vertebrae ( Koch et al. 2019); Epictia tenella (Klauber, 1939): 190-204 trunk vertebrae plus unknown number of cloacal and caudal vertebrae ( Koch et al. 2019); Epictia tricolor (Orejas-Miranda & Zug, 1974): 282 trunk vertebrae plus unknown number of cloacal and caudal vertebrae ( Koch et al. 2019); Habrophallos collaris (Hoogmoed, 1977): 132-144 trunk vertebrae plus 3-4 cloacal vertebrae plus 14-17 caudal vertebrae (posteriormost 2 caudal vertebrae are fused) ( Martins et al. 2019); Leptotyphlops conjunctus (Jan, 1861): 191-223 trunk and cloacal vertebrae plus?21-26 caudal vertebrae ( Alexander and Gans 1966); Leptotyphlops emini (Boulenger, 1890): 220 trunk vertebrae plus 4 cloacal vertebrae plus 25 caudal vertebrae (posteriormost 3 caudal vertebrae are fused) ( List 1966); Leptotyphlops nigricans : 195 trunk vertebrae plus 3 cloacal vertebrae plus 29 caudal vertebrae (posteriormost 3 caudal vertebrae are fused) ( List 1966); Leptotyphlops nigricans : 130 trunk vertebrae plus 3 cloacal vertebrae plus 9 caudal vertebrae ( Rochebrune 1881; potentially erroneous counts); Mitophis asbolepis (Thomas, McDiarmid & Thompson, 1985): 279 trunk vertebrae plus unknown number of cloacal and caudal vertebrae ( Martins et al. 2021a); Mitophis calypso (Thomas, McDiarmid & Thompson, 1985): 350-352 trunk vertebrae plus unknown number of cloacal and caudal vertebrae ( Martins et al. 2021a); Mitophis leptepileptus : 354-391 trunk vertebrae plus 6 cloacal vertebrae plus 19-21 caudal vertebrae ( Martins et al. 2021a); Mitophis pyrites : 259-260 trunk vertebrae plus unknown number of cloacal and caudal vertebrae ( Martins et al. 2021a); Myriopholis longicauda : 208-215 trunk and cloacal vertebrae plus?25-?32 caudal vertebrae ( Alexander and Gans 1966); Myriopholis phillipsi (Barbour, 1914): 335-343 trunk and cloacal vertebrae plus?41-49 caudal vertebrae ( Alexander and Gans 1966); Namibiana occidentalis (FitzSimons, 1962): 268-302 trunk and cloacal vertebrae plus 24-29 caudal vertebrae (Claudia Koch, unpublished data, personal communication to GLG); Rena dissecta (Cope, 1896): 212-214 trunk vertebrae plus 4 cloacal vertebrae plus 16 caudal vertebrae (posteriormost 3 caudal vertebrae are fused) ( List 1966); Rena dulcis Baird & Girard, 1853: 209 trunk vertebrae plus unknown number of cloacal and caudal vertebrae ( Tsuihiji et al. 2012); Rena humilis : 254-265 trunk vertebrae plus 4-5 cloacal vertebrae plus 20-21 caudal vertebrae (posteriormost 2 caudal vertebrae are fused) ( List 1966); Rena maxima (Loveridge, 1932): 198 trunk vertebrae plus 4 cloacal vertebrae plus 17 caudal vertebrae (posteriormost 2 caudal vertebrae are fused) ( List 1966); Rhinoleptus koniagui (Villiers, 1956): 546 vertebrae in total ( Guibé et al. 1967); Siagonodon exiguum Martins et al., 2023: 239-263 trunk vertebrae plus 3-4 cloacal vertebrae plus 18-20 caudal vertebrae in males and 248-260 trunk vertebrae plus 4-5 cloacal vertebrae plus 18 caudal vertebrae in females ( Martins et al. 2023); Tetracheilostoma bilineatum : 152-160 trunk vertebrae plus 3 cloacal vertebrae plus 16 caudal vertebrae ( Martins et al. 2021a); Tetracheilostoma breuili (Hedges, 2008): 155-162 trunk vertebrae plus 3 cloacal vertebrae plus 16 caudal vertebrae ( Martins et al. 2021a); Tetracheilostoma carlae : 165-167 trunk vertebrae plus 3-4 cloacal vertebrae plus 15-16 caudal vertebrae ( Martins et al. 2021a); Tricheilostoma bicolor : 240-253 trunk and cloacal vertebrae plus 13-16 caudal vertebrae (Claudia Koch, unpublished data, personal communication to GLG); Trilepida affinis (Boulenger, 1884): 192 trunk vertebrae plus 4 cloacal vertebrae plus 21 caudal vertebrae (posteriormost 3 caudal vertebrae are fused) ( Martins et al. 2021b); Trilepida brasiliensis (Laurent, 1949): 172-198 trunk vertebrae plus 4 cloacal vertebrae plus 18-20 caudal vertebrae (posteriormost 2-3 caudal vertebrae are fused) ( Martins et al. 2021b); Trilepida dimidiata (Jan, 1861): 180 trunk vertebrae plus 4 cloacal vertebrae plus 17-18 caudal vertebrae (posteriormost 3 caudal vertebrae are fused) ( Martins et al. 2021b); Trilepida fuliginosa (Passos, Caramaschi & Pinto, 2006): 179-190 trunk vertebrae plus 4 cloacal vertebrae plus 22 caudal vertebrae (posteriormost 2 caudal vertebrae are fused) ( Martins et al. 2021b); Trilepida jani (Pinto & Fernandes, 2012): 154-165 trunk vertebrae plus 3-4 cloacal vertebrae plus 22 caudal vertebrae (posteriormost 2 caudal vertebrae are fused) ( Martins et al. 2021b); Trilepida joshuai (Dunn, 1944): 158-162 trunk vertebrae plus 3-5 cloacal vertebrae plus 15-19 caudal vertebrae (posteriormost 3 caudal vertebrae are fused) ( Martins et al. 2021b); Trilepida macrolepis (Peters, 1857): 196-219 trunk vertebrae plus 3-4 cloacal vertebrae plus 19 caudal vertebrae (posteriormost 3 caudal vertebrae are fused) ( Martins et al. 2021b); Trilepida macrolepis : 206 trunk vertebrae plus 21 cloacal and caudal vertebrae ( Nopcsa 1923); Trilepida nicefori (Dunn, 1946): 135 trunk vertebrae plus 4 cloacal vertebrae plus 16 caudal vertebrae (posteriormost 3 caudal vertebrae are fused) ( Martins et al. 2021b); Trilepida pastusa Salazar-Valenzuela et al., 2015: 176-184 trunk (?and cloacal) vertebrae plus 23-24 caudal (?and cloacal) vertebrae (posteriormost 3 caudal vertebrae are fused) ( Salazar-Valenzuela et al. 2015); Trilepida salgueiroi (Amaral, 1955): 203-228 trunk vertebrae plus 3-5 cloacal vertebrae plus 21-28 caudal vertebrae in males, and 211-231 trunk vertebrae plus 3-5 cloacal vertebrae plus 17-27 caudal vertebrae in females ( Pinto et al. 2015).

In general, the vertebral counts are highly variable with a single species, even subjected to sexual variation. With 546 vertebrae in total (see Guibé et al. 1967), the African monotypic genus Rhinoleptus possesses by far the highest count of vertebrae among leptotyphlopids, and also the largest recorded among all extant snakes, actually being surpassed only be a few vertebrae from the record holder snake of all time, the Eocene Archaeophis (565 vs. 546); it is unclear how many of these vertebrae of Rhinoleptus pertain to the trunk, cloacal, and caudal region. Mitophis leptepileptus possesses the second highest number of trunk vertebrae known among leptotyphlopids, reaching 391, with the same number in all other species of Mitophis being also high (above 252). The next highest number of trunk vertebrae is recorded in Myriopholis Hedges, Adalsteinsson & Branch in Adalsteinsson et al., 2009, where it can surpass 330, in Namibiana Hedges, Adalsteinsson & Branch in Adalsteinsson et al., 2009, where it ranges between around 270 to 300, in a few species of Epictia , in which it can surpass 240 ( E. borapeliotes , E. rioignis , and E. phenops ) and reach up to 282 ( E. tricolor ), in two species of Siagonodon Peters, 1881, where it can reach up to 263 ( Siagonodon exiguum ) or even 273 ( S. borrichianus ), in Tricheilostoma Jan, 1860 in Jan & Sordelli 1860-1866, ranging between around 240 and 250, in one species of Leptotyphlops where it can reach up to 244 ( L. scutifrons ), and in Rena Baird & Girard, 1853, where it is almost always above 200 and can reach up to 271. In contrast, the number of trunk vertebrae in the remaining taxa is around 200, with some genera even possessing as few as around 135 trunk vertebrae or less (i.e., Habrophallos Martins et al., 2019, and some species of Trilepida ). The highest caudal vertebral counts in leptotyphlopids are observed in Myriopholis , where it ranges between 25 and 49, followed by Namibiana (24 to 29), and Epacrophis Hedges, Adalsteinsson & Branch in Adalsteinsson et al., 2009 (22), while instead that number in the remaining genera ranges between 12 and 26.