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An Online Guide for the Identification of Amphibians in North America north of Mexico

Produced by the U.S. Department of the Interior || U.S. Geological Survey
Northern Prairie Wildlife Research Center, 8711 37th St. SE, Jamestown, ND 58401 USA
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The Order Anura includes frogs and toads.

Toads are a type of frog. While most frogs have smooth moist skin, toads possess very granulous skin which is comparatively dry. Frogs live in or near water and manage to be where they can keep their skins dry. Toads inhabit drier areas. Frogs have longer hind legs for jumping. Toads have shorter hind limbs for "walking" instead of leaping, although they can and do jump some when they want to move faster than a walk.

The term "anura" essentially means "without a tail". This refers to the absence of a tail in adult frogs and toads. The term "salientia" is a basal term that is applied to amphibians that are more closely related to the order anura, than to the orders caudata or gymnophiona (Ford & Cannatella, 1993).

Frogs are perhaps the most recognizable amphibian species, with their long hind legs and ability to leap, and their great vocalization abilities. There are exceptions to the "typical" frog, as some have developed adaptations for fossorial, aquatic, and arboreal lifestyles. Some may be magnificently colored in bright reds, oranges, blues, pinks, and just about every other color, while others may be subtle browns or greens. Many species can change their colors to better blend into their environment, or by chemical cues.
What truly defines a frog are the morphological features present in all, including a maximum of 9 vertebrae in the front sacrum, such that the posterior three or four vertebrae are fused into a urostyle (Duellman & Trueb, 1986). Furthermore, frogs do not possess tails into adulthood, and possess a radioulna, which is a fused radius and ulna, and a fused tibiofibula, which is a fused tibia and fibula (Larson, 2004). The hind legs of most species are far longer than the front legs, by means of elongated tibiale and fibulare, an adaptation for leaping (Duellman & Trueb, 1986). This, of course, is a reduced characteristic in those species that have adapted to lifestyles that do not require great leaping, such as the fossorial species.

Synapomorphies that define all taxa in Salientia include 14 presacral vertebrae, elongate and anteriorly directed ilium, the presence of a frotoparietal, and a the lack of tail and teeth (Milner, 1988).

The Order Caudata includes Salamanders.

The order Caudata, in the Class Amphibia, is comprised of salamanders, newts, sirens, amphiuma, waterdogs, and mudpuppies. The term caudata originates from the Latin word for tail, cauda, and roughly translates to tailed-amphibian. The more recent term Urodela is often used in place of Caudata to label the salamander order.
There are ten living amphibian families, grouped into three suborders. Hynobiidae and Cryptobranchidae comprise the primitive or ancient suborder, Cryptobranchoidea. Modern caudates, including Salamandridae, Plethodontidae, Rhyacotritonidae, Amphiumidae, Ambystomatidae, Proteidae, and Dicamptodontidae, form the advanced salamander suborder, Salamandroidea. Sirenidae are placed in their own suborder, Sirenoidea. The fossil family Batrachosauroididae is included in Salamandroidea, while the fossil family Karauridae is placed in its own suborder, Karauroidea.

As of today, there are more than 500 recognized caudate species; a number that changes often to reflect new and redefined species. The largest caudate group is the lungless salamanders, family Plethodontidae, which comprises more than half of all known caudate species. Plethodontids are found almost exclusively in North America, with a large radiation into the tropical Central and South Americas. The smallest groups are the giant salamanders (Cryptobranchidae), and Amphiuma (Amphiumidae), for which there are only three known species in each family. For more information about each caudate family, see the Caudate Families and the Taxonomic Model section.

All caudates, or urodeles, possess tails, a general characteristic that separates this order from Anura. Some living caecilians, order Gymnophiona, do possess tails, but differ from salamanders in that caecilian tails are generally indistinguishable from the body, and are highly reduced compared to caudates. The tails of most caudates are obvious, being approximately of equal length to the body, however, some may possess extraordinarily long tails, while the tails of others may be hard to distinguish from an elongate, snake-like body. Caudates also possess four limbs of relatively equal size, with the exceptions of the Sirens, which lack hind limbs. Unlike the anurans, caudates do not leap to move from one place to the next, but rather walk, sometimes run. Aquatic species may walk along the floor of their watery homes, and are rather capable swimmers. Some aquatic species walk well on land, while others, such as the amphiuma, possess rudimentary limbs that serve little purpose for locomotion.

Although highly variable on the surface, caudates all share a few fundamental characteristics. These characteristics define the caudate group, and include the following: (1) absence of an otic notch and middle ear, (2) presence of a large footplate and short stylus on the columella in most taxa, (3) absence of postorbital, postparietal, tabular, supratemporal, jugal, quadratojugal (present in Karauridae), supraoccipital, basioccipital and ectopterygoid bones, (4) presence of ribs, (5) presence of true teeth on both jaws, (6) gill slits and external gills in aquatic larvae (when present), (7) origin of the adductor mandibulae internus superficialis muscle on the top and back of the skull (except Karauridae) and small size of the levator mandibulae posterior (Duellman & Trueb, 1986; Tree of Life Web).

Many caudates are biphasic, emerging from eggs as aquatic larvae, and later metamorphosing into terrestrial, aquatic, or semi-aquatic adults. Biphasic species typically reproduce in water, however there are some exceptions, such as Ambystoma opacum, a species that deposits eggs on land, and waits for seasonal rains to raise the water levels and submerse them.

Some species can be considered only partially biphasic in that they do not metamorphose completely, but retain larval characteristics into adulthood, and reproduce in this semi-larval state. Such species are referred to as neotenic (also sometimes called paedomorphic). The families Amphiumidae, Sirenidae, Cryptobranchidae, and Proteidae retain larval characteristics into adulthood, to varying degrees. There are also neotenic species, and neotenic tendencies in the families Salamandridae, Ambystomatidae, Plethodontidae, Dicamptodontidae, and Hynobiidae. Perhaps the most famous neotene is the Mexican Axololt, Ambystoma mexicanum. This species is commonly used in laboratory research, and is often kept and bred in captivity. For more information about neoteny, see the Biology section, and Caudate Families and the Taxonomic Model.

A few species, including some Salamandrids, are viviparous, producing fully formed miniature adults, as opposed to eggs. In these cases, the larval stage is passed within the mother. Such species produce considerably less offspring, usually around 1-4, compared to the hundreds of eggs that may be deposited each season by biphasic species.

Many Plethodontid salamanders deposit eggs on land, which pass the aquatic larval stage within the egg casing, before emerging as fully formed miniature adults. Such species do not require bodies of water to reproduce, but must still maintain a level of moisture to sustain life.

Adult Plethodontids are also unique in that they lack lungs, and rely mainly on cutaneous respiration and buccopharyngeal respiration. Adult of most other species utilize a combination of buccopharyngeal, cutaneous, and pulmonary respiration, and sometimes branchial respiration. Adult amphiuma lack gills, although they are nearly completely aquatic, and rely mainly on pulmonary respiration. Many neotenes retain external gills into adulthood, and like aquatic larvae, rely heavily on branchial respiration.

Caudates are found mainly in the cooler Northern Hemisphere, with the exception of some genera of the family Plethodontidae that inhabit tropical zones of Central and South America, and a few other unique species. North America is home to the greatest phylogenetic diversity of caudates, which includes species from 9 of the 10 living amphibian families. The only family without representatives in North America is the Asiatic family Hynobiidae. The distribution map at right shows the approximate global range of caudates.

The Order Gymnophiona (, or Apoda) includes Caecilians

The order Gymnophiona is comprised of caecilians. Caecilians are unusual amphibians possessing reduced tails, with the exception of the family Ichthyophiidae, reduced eyes, segmented skin with tiny scales, powerful heads for burrowing, and acute olfactory systems. Caecilians actually resemble giant earthworms, rather than typical amphibians. Caecilians are found throughout most of northeast South America, a few patches in southeast Africa, and much of southern Asia.

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