Serpent Research: Two new montane rattlesnakes from Mexico

Members of the Mexican dusky rattlesnake species group (Crotalus triseriatus Group) are widely distributed across the highlands of Mexico and the southwestern USA. Currently the group contains five species. The nominate species, C. triseriatus, contains the subspecies C. t. triseriatus and C. t. armstrongi, which inhabit mixed pine-oak forests across the Trans-Mexican Volcanic Belt. Crotalus pusillus ranges across the highlands of the Sierra de Coalcomán and the western portion of the Trans-Mexican Volcanic Belt. Crotalus aquilus, previously considered a subspecies of C. triseriatus, occurs north of the Trans-Mexican Volcanic Belt along the Central Mexican Plateau in mixed pine-oak and rocky mesquite grasslands. Crotalus lepidus is the widest ranging species in the group. It contains four subspecies distributed across a variety of habitats in northern Mexico and southwestern USA. Crotalus l. lepidus occurs in rocky regions of the Chihuahuan Desert and adjacent uplands, C. l. klauberi inhabits the Sierra Madre Occidental and sky islands of the southwestern USA and northern Mexico, C. l. morulus occurs in the northern Sierra Madre Oriental, and C. l. maculosus occupies the Pacific slopes of the southern Sierra Madre Occidental. Crotalus ravus was recently added to the C. triseriatus group and it includes three subspecies, C. r. ravus, C. r. brunneus, and C. r. exiguus, found along the eastern slopes of the Trans-Mexican Volcanic Belt and Sierra Madre del Sur. Species composition of the C. triseriatus group has changed several times over the past 70 years. The most recent molecular studies of the group found strong support for a monophyletic assemblage that includes C. triseriatus, C. pusillus, C. aquilus, C. lepidus, and C. ravus. One of these studies also found evidence that C. triseriatus and C. lepidus are paraphyletic and that at least one cryptic species was present within the C. triseriatus group. Although this study extensively sampled the geographic range of the C. triseriatus group, analyses reconstructed matrilineal relationships only because of a reliance on mitochondrial DNA.

Despite seven decades of systematic study, no study has tested species limits in the C. triseriatus group. Species within the group were recognized and classified long ago based on morphology alone. Recent research has focused on reconstructing phylogenies or on using phylogenies to address evolutionary and biogeographic questions.

In a new study Bryson and colleagues (2014) use data from seven nuclear loci to test competing models of species delimitation in the C. triseriatus group. They tested different models of species delimitation using the recently developed Bayes factor delimitation (BFD) method, and compare models that reflect historical taxonomy against models that reflect phylogeographic structure and contain cryptic species. They also examined museum specimens for morphological congruence to cryptic species along the Trans-Mexican Volcanic Belt hypothesized in a previous study.

They find strong support for a nine-species model and genetic and morphological evidence for recognizing two new species within the group, which are formally describe. The results suggest that the current taxonomy of the C. triseriatus species group does not reflect evolutionary history.

Crotalus tlaloci sp. nova is described based on 11 specimens. It was named after the Aztec god of rain and inhabits open areas in cloud forest and humid oak-pine forest along the lower slopes of the Trans-Mexican Volcanic Belt. Although one record in the Sierra de Taxco (“Arroyo las Damas”) is at 1850 m most records are between 2000–2400 m asl. This species is known from the states of Guerrero, Estado de México, Michoacán, and Morelos, and may range into western Puebla. The Tlaloci rattlesnake uses broad-leaf oak forest with dense undergrowth, a habitat that is distinctly different than the drier pine-oak forest inhabited by C. triseriatus. The distribution of C. tlaloci overlaps the ranges of two alligator lizards, Barisia herrerae and B. rudicollis. Interestingly, both of these alligator lizards occur in similar humid forest habitat at elevations of 2000–2500 m asl, and appear ecologically isolated from B. imbricata, which inhabits the surrounding drier pine-oak forest. Specimens of C. tlaloci are generally found in rocky open forest breaks and edges of cloud or humid oak-pine forest.

Crotalus campbelli sp. Nova was named in honor of Johnathan Campbell and is based upon six specimens. It occurs in rocky, open breaks within montane forest along the far western regions of the Trans-Mexican Volcanic Belt. Much of this forest is covered with remnant patches of cloud forest. This species is known from western Jalisco and the Sierra de Manantlán in southern Jalisco/northwestern Colima. A narrow low-elevation valley appears to separate the range of C. campbelli from C. armstrongi to the east.