Crotalus molossus nigrescens
The two species of blacktail rattlesnakes Crotalus molossus molossus and Crotalus ornatus are medium-sized rattlesnakes found in the southwestern United States and throughout much of Mexico. It is an ecological generalist being encountered in a variety of habitats from desert scrub to pine-oak forest. The blacktail rattlesnake was previously divided into four subspecies:

Crotalus molossus molossus (northern blacktail rattlesnake)
Crotalus molossus estebanensis (San Esteban Island blacktail rattlesnake)
Crotalus molossus nigrescens (Mexican blacktail rattlesnake)
Crotalus molossus oaxacus (Oaxacan blacktail rattlesnake)

Crotalus molossus has been shown to be polyphyletic1. Mexican populations of Crotalus m. molossus show an east-west divide with western individuals falling into a clade with the Mexican west coast rattlesnake (Crotalus basiliscus) and eastern individuals falling into a clade with the Totonacan rattlesnake (Crotalus totonacus). The Mexican blacktail rattlesnake (Crotalus m. nigrescens) has the largest distribution of all the blacktail rattlesnakes and is the most basal taxon of the group. The Oaxacan blacktail rattlesnake (Crotalus m. oaxacus) which is found at the southernmost portion of the distribution of Crotalus molossus falls into a clade with western Mexican Crotalus m. molossus and Crotalus basiliscus.

The San Esteban Island blacktail rattlesnake (Crotalus estebanensis) - endemic to the island of San Esteban in the Gulf of California - has been elevated to full species status.

Current research shows Crotalus molossus molossus containing five clades in the United States leading to the resurrection of the name Crotalus ornatus for populations in Texas and eastern New Mexico2.
Map adapted from Anderson, Christopher G. and Eli Greenbaum. 2012.


The current taxonomy of blacktail rattlesnakes is as follows:

Crotalus estebanensis (Klauber 1949)
COMMON NAME: San Esteban Island blacktail rattlesnake, San Esteban Island rattlesnake
MEXICO: the island of San Esteban in the Gulf of California.

Crotalus molossus molossus (Baird & Girard 1853)
COMMON NAME: northern blacktail rattlesnake, western blacktail rattlesnake
MEXICO: occurs from western Sonora to northwestern Chihuahua.
UNITED STATES: occurs from western Arizona to southwestern New Mexico.

Crotalus molossus nigrescens (Gloyd 1936)
COMMON NAME: Mexican blacktail rattlesnake
MEXICO: occurs from southern Sonora to Nuevo Leon and south into northern Puebla.

Crotalus molossus oaxacus (Gloyd 1948)
COMMON NAME: Oaxacan blacktail rattlesnake
MEXICO: occurs from southern Puebla to central Oaxaca.

Crotalus ornatus (Hallowell 1854)
COMMON NAME: eastern blacktail rattlesnake
MEXICO: occurs from eastern Chihuahua to central Coahuila.
UNITED STATES: occurs from western New Mexico to central Texas.

DISTRIBUTION of the BLACKTAIL RATTLESNAKES

This website contains information mostly dealing with Crotalus molossus and Crotalus ornatus in the United States as that is unfortunately what I am most familiar with right now. I would love the opportunity to spend more time learning about the blacktail rattlesnakes in the field in Mexico.




1 Wüster, W., J.E. Ferguson, J.A. Quijada-Mascareñas, C.E. Pook, M. da Graça Salomão, and R.S. Thorpe. 2005. Tracing an invasion: landbridges, refugia, and the phylogeography of the Neotropical rattlesnake (Serpentes: Viperidae: Crotalus durissus). Molecular Ecology 14:1095-1108.

2 Anderson, Christopher G. and Eli Greenbaum. 2012. Phylogeography of Northern Populations of the blacktail Rattlesnake (Crotalus molossus Baird And Girard, 1853), With the Revalidation of C. ornatus Hallowell, 1854. Herpetological Monographs 26 (1): 19-57.