Chapter 19

A New Era of Amphibian Taxonomy

Jennifer B. Pramuk and Joseph R. Mendelson, III

The more than 7000 species of amphibians represent a monophyletic group (i.e., sharing a common ancestor) that is both highly successful and geographically widespread. The earliest amphibians, of which the fossils of Ichthyostega and Elginerpeton are representatives, date from the Late Devonian, more than 360 million years ago. These fossil forms represent the earliest known ancestors to all extant tetrapods, which include amphibians, birds, reptiles, and mammals. Amphibians were the first vertebrates to hunt, vocalize, and breed on land, and their basic body plan has changed little since the Jurassic period. Because of their highly permeable skin, amphibians are usually intolerant of saltwater; therefore, their wide present-day distribution is largely the result of deeper lineages predating the split of continental landmasses—today, they are present on all continents except Antarctica.

Because closely related organisms are expected to function similarly, understanding taxonomy and phylogeny might yield clues to their physiology and natural history. In most cases, all but the most commonly kept amphibian species have little to nothing about them in the literature regarding their medical treatment or husbandry. A fundamental understanding of the evolutionary relationships or phylogeny of the amphibians is therefore integral to an understanding of their biology, captivity, and medical care. For example, if one is treating a member of a poorly known taxon, such as a leaf frog in the genus Phasmahyla, clues to its captive management (e.g., sensitivities to a particular medication or susceptibility to diseases such as Batrachochytrium dendrobatidis [amphibian chytrid fungus]) might be revealed by researching literature on its most widely studied sister taxon, in this case, the Red-eyed Tree frog (Agalychnis callidryas).

It is ironic that the notion of stability is so entrenched in the concept of taxonomy that the very system of names designed to allow communication across cultural and language barriers cannot itself be changed without reactionary outbursts from the greater community of biologists, land managers, veterinarians, and even the taxonomists themselves. It is as if some feel that taxonomic stability (tradition) is more important than the development of knowledge of biodiversity (progress). Is one not supposed to learn and move forward in understanding of biodiversity? To witness the reaction some have to proposed taxonomic changes, one would infer not! Taxonomic stability is a good thing to have but only if the taxonomy in use adequately communicates both the diversity and evolutionary history of the group of organisms in question.

In the case of amphibians, a stable taxonomy is elusive in part because knowledge of amphibian diversity is still growing at an astonishing rate. Compared with the largely stable taxonomies for mammals or birds, new major lineages (e.g., genera or even families) of amphibians are still being discovered regularly. To put this into perspective, approximately one quarter of all known amphibian diversity has been described in the past 20 years, and the rate of species discovery has not yet plateaued. To confound matters, the last century had been spent tolerating a flawed traditional taxonomy that used useless names for artificial and large groups of animals, such as “Leptodactylidae,” a catchall group of neotropical frogs that was far from phylogenetically accurate. The new, molecular data–based phylogenies have revealed that the vast age of amphibians and the phenomenon in many groups in which larval characteristics are retained in adults (i.e., neoteny) have led to repeated appearances and reversals of morphologic traits that were formerly considered to be reliable indicators of shared ancestry. These results have confounded or completely revised earlier morphologic-based phylogenies and classifications. Presented herein are resources for understanding not only current amphibian taxonomy but also the challenges of relying on a stable taxonomy that is undergoing constant flux.

From activities ranging from teaching to veterinary practice, a taxonomy should accurately communicate the state of the art in knowledge related to both actual biodiversity and the phylogenetic relationships among species and groups (e.g., genera). A recent pulse of activity in amphibian systematics, spurred largely by technologic improvements in collecting and analyzing massive molecular data sets, has greatly increased the understanding of the relationships among and within the major groups; this new information has appropriately spurred efforts to revise the antiquated, misleading taxonomy of the last century or so. Given that novel taxonomic arrangements for long-familiar groups can be confusing and frustrating, this chapter endeavors to summarize in a user-friendly fashion the most recent taxonomic changes, especially as they relate to groups likely to be encountered by readers of this chapter. Ideally a taxonomy should reflect the phylogeny—the actual evolutionary history—of the creatures under consideration. Thus, this summary will hopefully be useful for keepers, curators, and veterinarians working with species that are poorly known or with which they are simply unfamiliar. Importantly, knowledge of the phylogenetic position of a species may allow one to extrapolate information relevant to other closely related species, genera, or from another family.

Because taxonomic monographs and treatments include exhaustive citations of all relevant literature and complete reviews of taxonomic changes and synonymies, those efforts will not be duplicated here and readers are referred to the original monographs. The current renaissance in amphibian taxonomy was spurred by a series of three monographs published by the American Museum of Natural History.1–3 The Amphibian Tree of Life³ was especially influential because it represented the first modern attempt to consider amphibian phylogeny in its entirety. Never claiming to be the last word on knowledge of amphibian phylogeny or taxonomy, it did proceed to frame and encourage a flood of subsequent articles that, considered together, have greatly increased knowledge of the history of Amphibia and represent major steps toward producing a taxonomy that is consistent with their history. Some important recent articles include treatments of the phyllomedusine frogs,⁴ hemiphractid frogs (formerly part of Hylidae),⁵ microhylid frogs,⁶ glass frogs,⁷ ranid frogs,⁸ bufonid frogs,^9–12 and the terraranan frogs (the so-called eleutherodactylines, formerly included in Leptodactylidae).¹³ Additional recent efforts have informed the understanding of the broader groups such as caecilians^14–16 and salamanders.^17–19 Recently, Pyron and Wiens²⁰ assembled virtually all of the molecular data produced by the mentioned series of articles (plus others not here listed) into a single massive analysis. Their study largely confirmed these previous more focused efforts and added some refinements to the ever-problematic assemblage of South American frogs formerly referred to “Leptodactylidae.” However, relationships within the large superfamily Hyloidea (including many families widely kept in captivity, such as the hylid tree frogs) were mostly poorly supported. Moreover, their phylogeny inexplicably retained an untenable nonmonophyletic taxonomy for Bufonidae and Ranidae. Table 19-1 lists a taxonomic summary of generally recognized families of amphibians, including approximate information on species-level diversity within each and geographic distribution.

TABLE 19-1

A Taxonomic Summary of Generally Recognized Families of Amphibians ^∗

^∗Based on Frost DR, Grant T, Faivovich J, et al. The amphibian tree of life. Bull Am Museum Nat Hist 2006;297:1-370; Pyron RA, Wiens JJ. A large-scale phylogeny of Amphibia including over 2800 species, and a revised classification of extant frogs, salamanders, and caecilians. Mol Phylogenet Evol 2011;61:543-583; and Frost DR. Amphibian species of the world: an online reference. Version 5.5. Electronic database available at http://research.amnh.org/vz/herpetology/amphibia/. Accessed January 31, 2011. Approximate numbers of species in each group and a very general description of geographic distribution are also presented (species counts are from Amphibiaweb.org).

Current generic-level taxonomy has not been emphasized nor summarized in this chapter because generic limits and species allocations are in constant flux, and, in the case of amphibians, considerable changes at this level can be anticipated in the upcoming years. For current information regarding generic allocations, the reader is referred to these frequently updated online references: The American Museum of Natural History’s Amphibian Species of the World at http://research.amnh.org/herpetology/amphibia/index.php and The University of California’s Amphibiaweb at http://www.amphibiaweb.org/.

For species occurring in North America, the standard English and scientific names list cosponsored by all four of the major herpetological societies based in the United States is recommended.²¹ The most recent edition is available for free download by the Society for the Study of Amphibians and Reptiles at http://www.ssarherps.org/pages/comm_names/Index.php.

Readers working in institutions accredited by the Association of Zoos and Aquariums (AZA) should note that AZA follows the taxonomy of Amphibian Species of the World Web site ²²; however, this Web site is updated frequently, and it has not been standard for AZA institutions to update their records accordingly. The International Species Information System (ISIS) record-keeping system officially lists Frost²³ plus the addendum by Duellman²⁴ as their taxonomic standard. However, because both of those sources are quite outdated, it can be assumed that they now refer to the continuation of that same project, which now exists online in the form of Amphibian Species of the World.²² An additional useful Web site is the International Union for Conservation of Nature (IUCN) Red List (http://www.iucnredlist.org/initiatives/amphibians), which provides detailed information for every known species of amphibian regarding distribution, conservation status, and more, although this particular database is not intended to be a source of the most current taxonomic status of any species or group.