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Thamnophis exsul ROSSMAN, 1969

IUCN Red List - Thamnophis exsul - Least Concern, LC

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Higher TaxaColubridae (Natricinae), Colubroidea, Caenophidia, Alethinophidia, Serpentes, Squamata (snakes)
Common NamesE: Montane Garter Snake
G: Zwerg-Strumpfbandnatter
S: Jarretera Mexicana Exilada 
SynonymThamnophis exsul ROSSMAN 1969
Thamnophis exsul — ROSSMAN et al. 1989
Thamnophis exsul — LINER 1992
Thamnophis exsul — LINER 1994
Thamnophis exsul — WALLACH et al. 2014: 722 
DistributionMexico (Tamaulipas, Nuevo León, Coahuila)

Type locality: relatively dry pine forest, 11 mi. E, 3.5 mi S San Antonio de las Alazanas, Coahuila, ca. 9,100 feet elevation.  
TypesHolotype: USNM 166423, adult female 
DiagnosisDefinition and Diagnosis: Thamnophis exsul is the smallest known species in the genus, having a maximum recorded length of 360 mm; 17 dorsal scale rows in 90% of known specimens, usually (80%) with no posterior reduction to 15 rows; 7 supralabials (90%) on at least one side; 8 infralabials (90%) on at least one side; females with 142-150 ventrals (x̅ = 145), males with 152-156 (x̅ = 154); females with 52-56 subcaudals (x̅ = 53), males with 63-65 (x̅ = 64); tail length 18.9-20.0% (x̅ = 19.4) of TL in females, 21.8-22.2% (x̅ = 22.0) in males; 19-21 (x̅ = 20.3) maxillary teeth, the last 2 or 3 slightly enlarged; dorsum almost uniform putty brown with 3 indistinct light stripes, or having 3 or 4 alternating rows of spots or blotches with the light stripes partially or completely suppressed. Specimens from Nuevo León show a marked ontogenetic pattern variation. Juveniles are prominently spotted in 3 or 4 rows. A vertebral stripe is absent or faint and sometimes interrupted at intervals by the dorsal spotting. The nuchal blotch may be divided by the faint vertebral stripe, color may be continuous with the dark dorsal head coloration or separated by a very thin lighter zone immediately posterior to the parietals. The subadult spotted condition is not as prominent as in juveniles and is barely evident, if at all, in larger adults. The single everted hemipenis available is single with the distal half greatly expanded. The sulcus spermaticus is simple, terminating apically between raised lips. The apical surface of the expanded portion is nude, the remainder spinose; spines are very small distally, larger proximally. An enlarged basal hook is slightly distant from the other spines. (Liner 1992)

Detailed description: Rossman 1996: 181 
CommentSmallest member of the genus with a maximal total length of 46 cm.

Not listed by Liner 2007. 
EtymologyNamed after the Latin “exsul” meaning "banished person", in reference to its geographic isolation from its closest relatives. 
  • Bourguignon, T. 2002. Strumpfbandnattern: Herkunft - Pflege - Arten. E. Ulmer Verlag, 125 pp.
  • HALLAS, JOSHUA M.; THOMAS L. PARCHMAN & CHRIS R. FELDMAN. 2021. Phylogenomic analyses resolve relationships among garter snakes (Thamnophis: Natricinae: Colubridae) and elucidate biogeographic history and morphological evolution. Molecular Phylogenetics and Evolution 107374. [2022 in print] - get paper here
  • Hallmen, M. & Chlebowy, J. 2001. Strumpfbandnattern. Natur und Tier Verlag (Münster), 192 pp. - get paper here
  • Heimes, P. 2016. Snakes of Mexico. Chimaira, Frankfurt, 572 pp
  • Lazcano D, Nevárez-de los Reyes M, García-Padilla E, Johnson JD, Mata-Silva V, DeSantis DL, Wilson LD. 2019. The herpetofauna of Coahuila, Mexico: composition, distribution, and conservation status. Amphibian & Reptile Conservation 13(2) [General Section]: 31–94 (e189) - get paper here
  • Lemos-Espinal JA, Smith GR, Cruz A 2016. Amphibians and Reptiles of the state of Nuevo León, Mexico. ZooKeys 594: 123-141, doi: 10.3897/zookeys.594.8289 - get paper here
  • Liner E A 1992. Thamnophis exsul. Catalogue of American Amphibians and Reptiles 549: 1-2 - get paper here
  • Nevárez-de-los-Reyes, Manuel, David Lazcano, Elí García-Padilla, Vicente Mata-Silva, Jerry D. Johnson and Larry David Wilson. 2016. The Herpetofauna of Nuevo León, Mexico: Composition, Distribution, and Conservation. Mesoamerican Herpetology 3 (3): 558–638 - get paper here
  • Queiroz, Alan de; Robin Lawson and Julio A. Lemos-Espinal 2002. Phylogenetic Relationships of North American Garter Snakes (Thamnophis) Based on Four Mitochondrial Genes: How Much DNA Sequence Is Enough? Molecular Phylogenetics and Evolution 22: 315-329 - get paper here
  • Rossman, D. A.; Liner, E. A.;Trevino, C. H.; Chaney, A. H. 1989. Redescription of the garter snake Thamnophis exsul Rossman, 1969 (Serpentes: Colubridae). Proc. Biol. Soc. Washington 102 (2): 507-514 - get paper here
  • Rossman, Douglas A. 1969. A new Natricine snake of the genus Thamnophis from Northern Mexico. Occasional papers of the Museum of Zoology, Louisiana State University (39): 1-4 - get paper here
  • Terán-Juárez, Sergio A., Elí García Padilla, Vicente Mata-Silva, Jerry D. Johnson and Larry David Wilson. 2016. The herpetofauna of Tamaulipas, Mexico: composition, distribution, and conservation status. Mesoamerican Herpetology 3 (1): 43–113 - get paper here
  • Wallach, Van; Kenneth L. Williams , Jeff Boundy 2014. Snakes of the World: A Catalogue of Living and Extinct Species. [type catalogue] Taylor and Francis, CRC Press, 1237 pp.
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