Anura

Family: New Zealand Frogs
Family: Tailed Frogs
Family: Fire-Bellied Toads and Barbourulas
Family: Midwife Toads and Painted Frogs
Family: Mesoamerican Burrowing Toads
Family: Clawed Frogs and Surinam Toads
Family: Asian Toadfrogs
Family: Spadefoot Toads
Family: Parsley Frogs
Family: Ghost Frogs
Family: Seychelles Frogs
Family: Australian Ground Frogs
Family: Australian Toadlets and Water Frogs
Family: Leptodactylid Frogs
Family: Vocal Sac-Brooding Frogs
Family: Three-Toed Toadlets
Family: True Toads, Harlequin Frogs, and Relatives
Family: Poison Frogs
Family: Ruthven's Frogs
Family: Glass Frogs
Family: Amero-Australian Treefrogs
Family: True Frogs
Family: Squeakers and Cricket Frogs
Family: Shovel-Nosed Frogs
Family: African Treefrogs
Family: Asian Treefrogs
Family: Narrow-Mouthed Frogs
Family: Madagascaran Toadlets

(Frogs and toads)

Class: Amphibia

Order: Anura

Number of families: 28

Number of genera, species: 352 genera; 4,837 species

Evolution and systematics

Anurans (frogs and toads) usually are divided into two informal groups. Of these groups, the archaeobatrachians include the basal living families Ascaphidae, Leiopelmatidae, Bombinatoridae, Discoglossidae, Pipidae, Rhinophrynidae, Megophryidae, Pelobatidae, and Pelodytidae as well as the fossil family Paleobatrachidae. Some authors consider Megophryidae, Pelobatidae, Pelodytidae, Pipidae, and Rhinophrynidae to constitute an "intermediate" group, the mesobatrachians; all other families are placed among the neobatrachians.

Even the most experienced herpetologists can have difficulty ascertaining the family to which a frog belongs by examining only the external features, because many species closely resemble other species in unrelated families. The most important morphological characters are features of the internal anatomy, especially the skeleton. In the nineteenth century, biologists discovered that some frogs lacked tongues and so divided the Anura into two suborders—tongued frogs (Phaneroglossa) and tongueless frogs (Aglossa). Subsequently, two basic types of pectoral girdles were recognized—two halves overlapped ventrally (arciferal condition) and two halves fused midventrally (firmisternal condition).

Early in the twentieth century many additional suites of characters were discovered, including different kinds of vertebral articulations, presence of free ribs, dentition, and thigh musculature. By the middle of the twentieth century, classification of anurans commonly consisted of five suborders (Amphicoela, Anomocoela, Diplasiocoela, Opisthocoela, and Procoela) based on the nature of the articulating surfaces of the vertebrae and the intervertebral elements, but contemporary herpetologists no longer accept this arrangement. Later in the century, larval characters, developmental patterns, nature of the mating embrace (amplexus), and pupil shape were added to the growing number of characters used in classification. By the end of the twentieth century, molecular data sets provided support for some but not all arrangements based on morphological features; also, by this time, rigorous analyses were used to propose testable hypotheses of phylogenetic relationships of both living and extinct anurans.

The resulting phylogenies and classifications place the Triassic Triadobatrachus as the sister taxon to anurans and establishes the monophyly (descendents of a single ancestor) of Anura, within which the basal familes Ascaphidae, Leiopelmatidae, Bombinatoridae, and Discoglossidae form a grade. The assumed sister relationships of Pipoidea (Pipidae and Rhinophrynidae) and Pelobatoidea (Megophryidae, Pelobatidae, and Pelodytidae) are the subject of controversy. Many unresolved problems exist within the neobatrachians, but most evidence supports a clade (all descended from one ancestor) usually referred to as the ranoids (Arthroleptidae, Hemisotidae, Hyperoliidae, Microhylidae, Ranidae, Rhacophoridae, and Scaphiophrynidae). A group of Madagascar frogs recognized by some workers as Mantellidae has been placed in the Rhacophoridae or Ranidae by various researchers (covered here in Rhacophoridae).

The remaining neobatrachians, sometimes referred to as bufonoids, may be viewed as a grade between archaeobatrachians and ranoids. Among the bufonoids, no evidence supports the monophyly of Leptodactylidae; morphological and molecular data support the monophyly of one group of families—Allophrynidae, Centrolenidae, and Hylidae (including Pseudidae). Morphological data associate Sooglossidae, endemic to the Seychelles Islands group, with the Australo-Papuan Limnodynastidae and Myobatrachidae, but molecular evidence places Sooglossidae as the sister taxon to ranoids. Also, Dendrobatidae has been placed in the ranoids by some authors, but molecular evidence does not support that arrangement. Relationships of Bufonidae, Brachycephalidae, Heleophrynidae, Limnodynastidae, Myobatrachidae, and Rhinodermatidae have yet to be determined with any degree of certainty, but some molecular evidence supports the relationships of Bufonidae and Rhinodermatidae and of Heleophrynidae and Myobatrachidae.

Physical characteristics

Anurans are unique among amphibians and all other vertebrates in having a broad head, large mouth, large protuberant eyes, short body, and no tail. The hind limbs are long and modified for jumping by having an extra segment composed of elongated "ankle" bones—fibulare and tibiale (astragalus and calcaneum, respectively). The vertebral column is short and consists of no more than nine (usually eight, but 10 in the Jurassic Notobatrachus) presacral vertebrae; the presacral vertebrae are articulated firmly so as to allow only slight lateral and dorsoventral flexure, and the postsacral vertebrae are fused into a bony rod, the urostyle (coccyx).

Although most anurans have snout-vent lengths of about 1.5–3.0 in (35–75 mm), many are much smaller, and a few are much larger. The smallest frogs are the Brazilian two-toed toadlet (Psyllophryne didactyla) and the Cuban Iberian rain frog (Eleutherodactylus iberia), which have lengths of 0.42 in (10.2 mm) and 0.43 in (10.5 mm), respectively. By far the largest anuran is the West African ranid, the Goliath frog (Conraua goliath), which reaches a length of 13 in (32 cm).

Larval anurans (tadpoles) are unlike the aquatic larvae of other amphibians. Tadpoles have short, globular bodies and long tails. The mouth is a unique structure usually containing keratinized rows of labial teeth and jaw sheaths supported by cartilaginous elements. During metamorphosis the tail is absorbed, and the mouthparts and their support structures change dramatically to produce the adult condition.

Distribution and habitat

Frogs and toads are nearly worldwide in distribution, except for Antarctica, Greenland, Arctic regions of North America and Eurasia, and some oceanic islands. In desert regions, such as the Sahara, they are restricted to oases. Few species live at high latitudes. The ranges of only three species extend north of the Arctic Circle; these are the brown frog (Rana temporaria) and the moor frog (Rana arvalis) in Eurasia and the wood frog (Rana sylvatica) in North America. The southernmost frog is the gray four-eyed frog (Pleurodema bufonina), which reaches the Straits of Magellan. Most frogs and toads live at low to moderate elevations, but a few are found at high elevations. The highest known record is for the Pakistani toad (Bufo siachinensis) at an elevation of 16,971 ft (5,238 m) in the Himalaya Mountains of Pakistan. In South America, the range of the puna frog (Pleurodema marmorata) extends to 16,200 ft (5,000 m) in the Andes of Peru.

Far more anurans live in the tropical parts of the world than in the northern temperate climates. Only 90 species live in North America and 116 species in temperate Eurasia. The greatest diversity of anurans is in the neotropical region (Central America, South America, and the West Indies), which is home to about 2,200 species. This number is about three times those in tropical Asia or tropical Africa and about five times that in the Australo-Papuan region.

Different historical patterns are evident among anurans. The basal living families Ascaphidae (northwestern North America) and Leiopelmatidae (New Zealand) apparently are relics of former widespread Pangaean distributions and presumably are related to Notobatrachus from the Middle to Upper Jurassic boundary in Argentina. With the breakup of Pangaea in the Triassic, the ancestors of the archaeobatrachian families Bombinatoridae, Discoglossidae, Megophryidae, Pelobatidae, and Pelodytidae were in Laurasia, whereas the ancestors of the other families of archaeobatrachians and the neobatrachians were in Gondwana.

Behavior

Because of their thin skin, through which water is lost, most frogs live in humid regions or are active only during rainy seasons of the year. During dry times of the year, anurans estivate, usually below ground. Likewise, in temperate regions, anurans hibernate below the frost line. Despite these physiological limitations, anurans display a wide variety of activity, mostly at night, when they feed and breed. Most respiration is cutaneous (through the skin) and is facilitated by dermal mucous glands that secrete a moist coating.

The long hind limbs facilitate a saltatorial (jumping) locomotion; most frogs can leap two to 10 times their body length, and a few can approach 30 times their body length. A few anurans (e.g., species in the Andean bufonid genus Osornophryne) have relatively short hind limbs and slowly walk instead of jumping. Because of their saltatorial locomotion, anurans do not leave a scent trail, and females do not locate males by chemosensory means. Instead, male frogs vocalize; air is forced from the lungs over the vocal cords and is resonated by a single or paired vocal sacs. Acute hearing allows females (and other males) not only to recognize the unique vocalization of their species but also to locate the calling males.

Resources

Duellman, William E., ed. Patterns of Distribution of Amphibians: A Global Perspective. Baltimore: Johns Hopkins University Press, 1999.

Duellman, William E., and Linda Trueb. Biology of Amphibians. Baltimore: Johns Hopkins University Press, 1994.

Lynch, John D. "The Transition from Archaic to Advanced Frogs." In Evolutionary Biology of Anurans, edited by James L. Vial. Columbia: University of Missouri Press, 1973.

Savage, Jay M. "The Geographic Distribution of Frogs: Patterns and Predictions." In Evolutionary Biology of Anurans, edited by James L. Vial. Columbia: University of Missouri Press, 1973.

Tyler, Michael J. Frogs. Sydney, Australia: Collins, 1976.

Bossuyt, Franky, and Michel C. Milinkovitch. "Convergent Adaptive Radiations in Madagascan and Asian Ranid Frogs Reveal Co-variation Between Larval and Adult Traits."

Proceedings of the National Academy of Sciences of the United States of America 97 (2000): 6585–6590. ——. "Amphibians as Indicators of Early Tertiary 'Out-of-India' Dispersal of Vertebrates." Science 292 (2001): 93–95.

Ford, Linda S., and David C. Cannatella. "The Major Clades of Frogs." Herpetological Monographs 7 (1993): 94–117.

Hay, Jennifer M., et al. "Phylogenetic Relationships of Amphibian Families Inferred from DNA Sequences of Mitochondial 12S and 16S ribosomal RNA genes." Molecular Biology and Evolution 12 (1995): 928–937.

Maglia, Anne S., L. Analía Pugener, and Linda Trueb. "Comparative Development of Anurans: Using Phylogeny to Understand Ontogeny." American Zoologist 41 (2001): 538–551.

Orton, Grace L. "The Systematics of Vertebrate Larvae." Systematic Zoology 2 (1953): 63–75.

[Article by: William E. Duellman, PhD]