A New Era of Amphibian Taxonomy

Jennifer B. Pramuk and Joseph R. Mendelson III

CHAPTER 19

230

The more than 7000 species of amphibians represent a mono-phyletic 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 Elginer-peton 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 Juras-sic period. Because of their highly permeable skin, amphib-ians 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, captiv-ity, 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 fun-gus]) 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, veterinar-ians, 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 stabil-ity 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 grow-ing 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 discov-ered 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 use-less 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 morpho-logic traits that were formerly considered to be reliable indi-cators of shared ancestry. These results have confounded or completely revised earlier morphologic-based phylogenies and classifications. Presented herein are resources for under-standing 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 prac-tice, 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, cura-tors, 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 pub-lished 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 caecilians14-16 and salaman-ders.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-prob-lematic 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 geo-graphic distribution.

Current generic-level taxonomy has not been emphasized nor summarized in this chapter because generic limits and spe-cies allocations are in constant flux, and, in the case of amphib-ians, 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 Califor-nia’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 Asso-ciation of Zoos and Aquariums (AZA) should note that AZA follows the taxonomy of Amphibian Species of the World Web site22; however, this Web site is updated frequently, and it has not been standard for AZA institutions to update their records

231CHAPTER 19 • A New Era of Amphibian Taxonomy

accordingly. The International Species Information System (ISIS) record-keeping system officially lists Frost23 plus the addendum by Duellman24 as their taxonomic standard. How-ever, 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 Amphib-ian 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.

PHYLOGENY AND TAXONOMY OF AMPHIBIANSAmphibians are a distinct group of tetrapod vertebrates, and despite centuries of communications to the opposite, they are in no way “transitional” between fishes and other groups of tetrapods (e.g., reptiles). For the most part, refer-ring a specimen in hand to one of the three major groups of amphibians (i.e., caecilians, frogs, and salamanders) has never been problematic. However, reconciling the relation-ships among these three groups has never been intuitive. This is because frogs and caecilians both have highly derived body forms while salamanders remain with what is frequent-ly considered to be the generic tetrapod body plan. Some debate has occurred regarding the relationships among these three groups,25 but the arrangement shown in Figure 19-1 is now generally accepted.

Taxonomically, or in terms of nomenclature, this arrange-ment can be summarized as follows: Class Amphibia: amphibiansOrder Gymnophiona: caeciliansInfraclass Batrachia: frogs and salamandersOrder Caudata: salamandersOrder Anura: frogs

Currently, 7170 species of amphibians are known, and the majority of those are anurans (note: the homepage for amphibiaweb.org includes a convenient daily update of the number of recognized species). Frost et al3 characterize amphibians as having, among many other unique features, loss of several bones in the typical tetrapod skull (e.g., supratempo-ral, jugal, postorbital bones, Figure 19-2, A), biscuspid, pedi-cellate teeth (Figure 19-2, B), a unique papilla amphibiorum in the ear associated with the and opercular element (Figure 19-2, C), glandular skin (Figure 19-2, D), gonadal fatbodies, and a lack of epidermal scales.

PHYLOGENY AND TAXONOMY OF CAECILIANSThe caecilians, or Gymnophiona, are the most geographically restricted (limited in distribution to tropical South America, Asia, and Africa but absent from Madagascar) and secretive of all amphibian orders. As with all aspects of caecilian biol-ogy, their true species-level diversity is poorly known. None-theless, known taxa are commonly referred to 10 families (Figure 19-3), and approximately 192 species are currently recognized. Future taxonomic changes are to be expected

232 SECTION II I • ADVANCES IN AMPHIBIAN MEDICINE

TABLE 19-1

A Taxonomic Summary of Generally Recognized Families of Amphibians*

Taxon English NameApproximate Number of Species General Distribution

GYMNOPHIONA CAECILIANS 192 PANTROPICALCaeciliidae N/A 42 South and Central AmericaChikilidae N/A 1 IndiaDermophiidae N/A 14 Central and South America and AfricaHerpelidae N/A 9 AfricaIchthyophiidae N/A 53 South and Southeast AsiaIndotyphlidae Including yellow-banded caecilians 20 India, Africa and the SeychellesRhinatrematidae N/A 11 South AmericaScolecomorphidae N/A 6 AfricaSiphonopidae N/A 23 South AmericaTyphlonectidae Aquatic caecilians 13 northern South AmericaCAUDATA SALAMANDERS 648 EURASIA AND AMERICASAmbystomatidae Mole salamanders 32 North AmericaAmphiumidae Amphiumas 3 North AmericaCryptobranchidae Giant salamanders and hellbenders 3 Asia and North AmericaDicamptontidae Pacific giant salamanders 4 North AmericaHynobiidae Asian salamanders 59 AsiaPlethodontidae Lungless salamanders 435 Asia, Europe, and AmericasProteidae Muduppies and waterdogs 6 Europe and North AmericaRhyacotritonidae Torrent salamanders 4 North AmericaSalamandridae “True” salamanders 98 Eurasia and North AmericaSirenidae Sirens 4 North AmericaANURA FROGS 6289 COSMOPOLITAN, EXCEPT ANTARCTICAAllophrynidae N/A, but including Tukeit-hill frog 2 South AmericaAlsodidae N/A 30 South AmericaAlytidae Midwife toads 5 Europe and North AfricaArthroleptidae N/A, but including Hairy frogs 147 Sub-Saharan AfricaAscaphidae Tailed frogs 2 Northwestern United States and CanadaBatrachylidae N/A 15 Argentina and ChileBombinatoridae Fire-bellied toads 10 Eurasia and Southeast AsiaBrachycephalidae N/A 52 South AmericaBrevicipitidae N/A, but including rain frogs 34 Sub-Saharan AfricaBufonidae “True” toads 585 Cosmopolitan, except Australo-Papua,

Oceania and MadagascarCalyptocephalellidae N/A, but including the Helmeted

Water toad4 South America

Centrolenidae Glass frogs 152 Mesoamerica and South AmericaCeratobatrachidae N/A 86 Southeast AsiaCeratophryidae Including Budgett’s and Horned frogs 12 South AmericaCeuthomantidae N/A 4 South AmericaConrauidae N/A, but including the Goliath frog 6 Equatorial AfricaCraugastoridae Stream frogs 115 Mesoamerica and South AmericaCycloramphidae N/A 34 South AmericaDendrobatidae Poison frogs 293 Central and South AmericaDicroglossidae N/A 185 Africa and AsiaDiscoglossidae Painted frogs 6 Europe and northern AfricaEleutherodactylidae Rain frogs 206 Americas, including CaribbeanHeleophrynidae Ghost frogs 6 South AfricaHemiphractidae Marsupial frogs 101 Central and South AmericaHemisotidae Shovel-nosed frogs 9 Sub-Saharan AfricaHylidae Tree frogs 929 Eurasia, Australo-Papua, and AmericasHylodidae N/A 42 South AmericaHyperoliidae Reed frogs 223 Africa and MadagascarLeiopelmatidae New Zealand frogs 4 North America and New ZealandLeptodactylidae Southern frogs 200 Mesoamerica and South AmericaMantellidae Mantellas 205 MadagascarMegophryidae N/A but including Horned frogs and

Toothed toads174 Southeast Asia

Micrixalidae N/A 11 India

Continued

233CHAPTER 19 • A New Era of Amphibian Taxonomy

Taxon English NameApproximate Number of Species General Distribution

Microhylidae Narrow-mouthed toads 525 Americas, Africa, Southeast Asia, and Australo-Papua

Myobatrachidae Australian ground and water frogs 131 Australo-PapuaNasikabatrachidae Purple frog 1 IndiaNycibatrachidae N/A 28 IndiaOdontophrynidae Includes Horned frogs 41 South AmericaPelobatidae Spadefoot toads 4 Eurasia and North AfricaPelodytidae Parsley frogs 3 EuropePetropedetidae N/A 12 Sub-Saharan AfricaPhrynobatrachidae N/A 86 Sub-Saharan AfricaPipidae Clawed frogs and Surinam toads 33 Central and South America and sub-

Saharan AfricaPtychadenidae Rocket frogs 51 Sub-Saharan AfricaPyxicephalidae N/A, including African Bullfrogs 73 Sub-Saharan AfricaRanidae “True” frogs 362 CosmopolitanRanixalidae N/A 10 IndiaRhacophoridae N/A, but including foam-nesting and

flying frogs361 Sub-Saharan Africa and Asia

Rhinodermatidae Darwin’s frogs 3 ChileRhinophrynidae Mexican Burrowing toad 1 MesoamericaScaphiopodidae American Spadefoot toads 7 North America, including MexicoSooglossidae Seychelles frogs 4 SeychellesStrabomantidae Rain frogs 603 Central and South AmericaTelmatobiidae Includes water frogs 61 South America

*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).

TABLE 19-1

A Taxonomic Summary of Generally Recognized Families of Amphibians—cont’d

in this group as additional material for study, discoveries of new species, and studies are forthcoming. For example, Kamei et al26 recently discovered a new major lineage (fam-ily Chikilidae) and suggest that it contains numerous yet unnamed species.

Caecilians, superficially resembling earthworms or snakes in general appearance, share a suite of morphologic features,

Amphibia

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FIGURE 19-1 Relationships among the three major groups of amphibians: Gymnophiona (caecilians), Anura (frogs), and Caudata (salamanders). (Adapted from Frost DR, Grant T, Faivovich J, et al. The amphibian tree of life. Bull Am Museum Nat Hist 2006;297:1-370.)

including a heavily ossified skull that makes them highly adapted to a fossorial or burrowing lifestyle. Frost et al3 char-acterize the gymnophionans as being legless amphibians that lack both limbs and girdles (but one fossil taxon is known to have had four limbs) and having a unique dual jaw-closing mechanism, an eversible male intromittent organ (the phal-lodeum), dermal annuli that encircle the body, and unique paired sensory tentacles on the snout. With the exception of one species (Atretochoana eiselti), all caecilians have lungs, but often the left lung is reduced in size, which is an adaptation for an elongate body shape, much as in snakes. Some species also have dermal calcite scales and are the only living amphibians known to possess them.

PHYLOGENY AND TAXONOMY OF SALAMANDERSThe salamanders represent a moderately speciose group (approximately 648 species) distributed primarily in North America, Central America, northern South America, and Eur-asia. Recent efforts in the systematics of salamanders did not greatly modify the understanding of the relationships among the major groups (and Figure 19-4) nor their family-level taxonomy. Readers should be aware that some continuing controversy exists regarding subfamilial classification of sala-manders and should consult the Web sites of Amphibiaweb

234 SECTION II I • ADVANCES IN AMPHIBIAN MEDICINE

PT

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FIGURE 19-2 Amphibians are characterized as having several synapomorphies (i.e., unique, shared, derived characteristics) some of which are illustrated here. A, Amphibian evolution includes the loss of several bones found in the typical tetrapod skull (e.g., supra-temporal, jugal, postorbital bones) through the fusion of elements such as the frontal and parietal (frontoparietal). Dorsal view of the skull of the North American bullfrog Lithobates catesbeianus. E, Exoccipital; FP, frontoparietal; M, maxilla; N, nasal; PR, prootic; PM, premax-illa, PT, pterygoid; QJ, quadratojugal; SP, sphenethmoid; SQ, squamosal. (Drawing made from the DigiMorph online catalog of CT scans: http://www.digimorph.org/specimens/ Rana_catesbeiana//.) B, Lingual view of the biscuspid, pedicellate teeth of the anuran Calyptocephalella caudiverbera. C, Crown; P, pedicel; RC, replacement crown; TS, trans-verse suture. (Drawing adapted from Parsons TS, Williams EE. The teeth of Amphibia and their relation to amphibian phylogeny. J Morphol 1962;110:375-389.) C, The inner ear of Lithobates catesbeianus illustrating the papilla amphibiorum (PA), as well as the opercular element (O) and the columella (C). MO, Medulla oblongata; OC, otic capsule; PC, perilym-phatic cistern; T, tympanum; U, utriculus. (Drawing adapted from Duellman WE, Trueb L. Biology of Amphibians. Baltimore: The John Hopkins University Press, 1994.) D, Cross section of the skin of the poisonous toad Melanophryniscus montevidensis illustrating granular glands. (Drawing adapted from Mebs D, Pogoda W, Maneyro R, et al. Studies on the poisonous skin secretion of individual red bellied toads, Melanophryniscus montevi-densis [Anura, Bufonidae], from Uruguay. Toxicon 2005;46:641-650.)

and Amphibian Species of the World to consider current perspectives; this controversy does not, however, greatly affect familial or generic level classifications. Salamanders have always been difficult to characterize in terms of unique morphologic aspects because most persons assume that they

have simply retained the primitive ancestral tetrapod body plan. This is an overstatement in many senses, and Frost et al3 characterize the caudatans as being unique among amphibians by having both forelimbs and a tail and an incomplete maxil-lary arcade (upper jaw). Additionally, salamanders are unique

C

in that many species, or entire groups, retain various aspects of the larval morphology into sexual maturity (a phenomenon known as neoteny). Salamanders usually do not vocalize as most anurans do; thus they rely on pheromones and courtship displays to locate and attract a mate.

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dae

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ae

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FIGURE 19-3 Relationships among the major clades (fami-lies) of caecilians. (Adapted from Kamei RG, Mauro DS, Gower DJ, et al. Discovery of a new family of amphibians from northeast India with ancient links to Africa. Proc Biol Sci 2012;279:2396-2401.)

Caudata

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FIGURE 19-4 Consensus of phylogenetic relationships among the major clades (families) of salamanders. (Adapted from Zhang P, Wake DB. Higher-level salamander relationships and divergence dates inferred from complete mitochondrial genomes. Mol Phylogenet Evol 2009;53:492-508 and Pyron RA, Wiens JJ. A large-scale phylogeny of Amphibia including over 2800 species, and a revised classification of extant frogs, sala-manders, and caecilians. Mol Phylogenet Evol 2011;61:543-583.)

235HAPTER 19 • A New Era of Amphibian Taxonomy

PHYLOGENY AND TAXONOMY OF FROGSFrogs, represented by more than 6289 species, have always rep-resented a great challenge to systematists, as their evolutionary history evidently includes explosive radiations, along with high levels of both convergence and extreme morphologic derivation. Because attempts to recover the phylogeny of frogs with morpho-logic data alone have generally failed, molecular data have been crucial in shaping our understanding of this complex phylogeny. Building on the foundation of the efforts by Frost et al,3 numer-ous studies have been produced11,20 and are greatly increasing the knowledge about this vast and taxonomically difficult group. As soon as an agreement on a phylogeny of frogs is reached, numer-ous taxonomic changes will be forthcoming. Some stakeholders have expressed frustration about changes already proposed, such as when long-familiar species such as Rana catesbeiana and Bufo americanus suddenly appeared as Lithobates catesbeianus and Anaxyrus americanus. Simply put, such changes are necessary and important so that a taxonomy that is both informative and internally consistent can be used when communicating about amphibians. Figure 19-5 summarizes the agreement among recent studies on frog phylogeny. Polytomies, or unresolved branches, represent areas of the tree where recent analyses have produced conflicting results or poor statistical support; in this sense, the phylogeny shown here is conservative.

Anurans are perhaps the most familiar amphibians to most people, and indeed, their unique body plan, adapted largely for saltatorial locomotion (e.g., elongate hindlimbs), makes all but the most bizarre species instantly recognizable to layperson as frogs. The many unique features of this group that Frost et al3 listed as characteristic include the following: reduction of ver-tebral number to nine or fewer; fusion of the caudal vertebrae to form the bony urostyle; hindlimbs significantly longer than forelimbs (with a few exceptions), produced, in part, by the elongation of the ankle bones; fusion of the radius and ulna of the forelimb and fusion of the tibia and fibula of the hindlimb; presence of keratinous jaw sheaths and keratodonts on larval mouthparts (with some exceptions); skin with large subcu-taneous lymph spaces; and during mating, a unique clasping behavior (amplexus) common to most species.

CONCLUSIONTremendous growth in knowledge of the phylogeny of amphibians has occurred in the last few years. This knowledge is crucial for informing the study of all aspects of their biol-ogy, as well as their husbandry and veterinary care, and for pressing conservation concerns. However, more work remains to be done, and changes in the trees presented here are antici-pated. Certainly, still more taxonomic changes will accompany increased knowledge of the evolution of amphibians. The regularly updated Amphibian Species of the World22 is recom-mended as a standard reference for amphibian taxonomy, but the primary literature should be the source for the most recent information on amphibian phylogeny.

ACKNOWLEDGMENTSThe authors would like to thank Darrel Frost for providing in-formation on the latest version of the Amphibian Species of the World Taxonomy.

236 SECTION II I • ADVANCES IN AMPHIBIAN MEDICINE

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FIGURE 19-5 Consensus of phylogenetic relationships among the major clades (fami-lies) of anurans or frogs. (Adapted from Pyron RA, Wiens JJ. A large-scale phylogeny of Amphibia including over 2800 species, and a revised classification of extant frogs, sala-manders, and caecilians. Mol Phylogenet Evol 2011;61:543-583.)

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frog family Hylidae, with special reference to Hylinae: phylogenetic anal-ysis and taxonomic revision. Bull Am Museum Nat Hist 2005;294:1-240.

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8. Bossuyt F, Brown RM, Hillis DM, et al. Phylogeny and biogeogra-phy of a cosmopolitan frog radiation: late Cretaceous diversification resulted in continent-scale endemism in the family Ranidae. Syst Biol 2006;55:579-594.

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