Cryptic diversity of miniaturized vertebrates: systematics of the Mexican salamander genus Thorius

The plethodontid salamander genus Thorius comprises a clade of miniaturized salamanders endemic to southern Mexico. Though the genus was erected (by E. D. Cope) more than 130 years ago, for more than 70 years following its initial description Thorius was thought to include just a single species. Nine named forms were added by 1970, and subsequent morphological and molecular studies have continued to reveal additional species. At present, 23 valid species are recognized, yet at least 11 undescribed species await formal description. These species coexist with and replace one another in complex geographic and elevational patterns, which include numerous instances of sympatry involving two, three, and possibly four species. Phylogenetic analyses suggest surprising trends in morphological evolution associated with miniaturization, including frequent homoplasy involving character loss and gain. While such cryptic diversity was not anticipated by early students of Thorius, it is proving to be a common theme in contemporary studies of amphibian systematics. The total number of valid, named species has been increasing at a rate of nearly 2% or more per year for the last two decades.