Abstract

a b s t r a c t The longtailed rattlesnakes of western Mexico represent an enigmatic group of poorly known venomous snake species: Crotalus ericsmithi, C. lannomi, and C. stejnegeri. In the 120 years since their discovery, fewer than twenty individuals have been deposited in natural history collections worldwide. These three species share similar morphological traits, including a particularly long tail that has been interpreted as either an ancestral condition among rattlesnakes or as derived within the longtailed group. An understanding of the phylogenetic distinctiveness and relationships among the longtailed rattlesnakes, and their relationships to other rattlesnake groups, has previously been hampered by a dearth of comparative material and tissues for collection of DNA sequence data. Facilitated by the recent availability of tissue samples from multiple individuals of each species, we estimate the phylogenetic relationships among the longtailed rattlesnakes and their placement among other rattlesnake groups, using DNA sequence data from three mitochondrial and three nuclear gene fragments. We explore phylogenetic signal in our data using Bayesian and maximum likelihood methods, species tree analyses and hypothesis testing. Our results strongly support the monophyly of longtailed rattlesnakes and suggest the three species diverged from each other during the mid to late Pliocene or early Pleistocene ($1.5-5.6 mya). Contrary to prevailing hypotheses, we find no evidence for an early or basal divergence of the longtailed clade within the rattlesnake tree, and instead estimate that it diverged relatively recently ($6.8 mya) from its sister lineage, composed of the diamondback rattlesnakes (C. atrox group) and the prairie rattlesnakes (C. viridis group). With our added sampling of lineages and identification of previously used problematic sequences, we provide a revised hypothesis for relationships among Crotalus species, yet underscore the need for future studies and new data to deliver a well-supported robust estimate of rattlesnake relationships.