The Class Amphibia

The class amphibia is represented today by three orders: Anura (frogs and toads), Caudata (salamanders, newts, amphiumas, and sirens), and Gymnophiona (caecilians). The caecilians appear to have diverged first from frogs and salamanders which together form a clade called Batrachia. The three orders of living amphibians form the subclass Lissamphibia.

The three orders of modern amphibians form a monophyletic group, but the origin of the group remains unclear. A wild menagerie of tetrapod-like amphibians flourished in the Carboniferous, Permian, and Triassic periods. Most of these creatures either belonged to the Temnopondyls or the Lepospondyls.  These two clades are differentiated based on the shape of their vertebrae.  Which clade gave rise to class lissamphibia is still debated. The most widely accepted hypothesis is a single origin within the temnospondyls (tree c in the figure below). Some analyses however support a single origin within the Lepospondyls (tree b in the figure below) and a few analyses favour a mixed origin in which frogs and salamanders are derived from Temnospondyls and caecilians from Lepospondyls (tree a in the figure below).

The most notable differences between amphibians and amniotes (reptiles, birds, and mammals) are related to reproduction. Amphibians, like most animals,  have a larval stage and thus have to undergo a metamorphosis. A larval stage with metamorphosis is the ancestral condition in animals. Amniotes, which include reptiles, birds, and mammals, have lost the larval stage and their young develop into adults without going through a metamorphosis (direct development).

All amniotes have internal fertilization, a necessity for life on land but most amphibians retained the ancestral external fertilization. Most anurans have external fertilization in which a male fertilizes the eggs as they are laid by the female. Fertilization is however internal in most urodeles but it is accomplished without an intromission organ. Male salamanders and newts deposit sperm packets called spermatophores on the ground or at the bottom of water bodies. Receptive females then pick up the spermatophores with their cloaca and fertilization ensues. Caecilians have an intromission organ called a phallodeum which is not homologous to the penis of reptiles but serves the same purpose. The phallodeum is formed by a portion of the cloacal wall.

Derived Traits of Modern Amphibians

Modern amphibians share many derived traits which include but are not limited to the following:

1. Pedicellate and bicuspid teeth. The crown of amphibian teeth is separated from the root by a pedestal of fibrous tissue. These teeth contrast with the teeth of reptiles which rest in a socket in the jaw bone. Moreover, the crown of amphibian teeth has two cusps, hence the name bicuspid.

2. Opercular system.  Frogs and salamanders have a unique sound conducting apparatus called the opercular system. It is easier to make sense of the opercular system by comparing it to the more familiar hearing system of mammals. In mammals, airborn vibrations are transmitted to the inner ear via a chain of tiny bones located in the middle ear called the malleus, the incus and the stapes. The innermost of these bones, the stapes, connects to the inner ear via the oval window. When the oval window vibrates through the mechanical action of the stapes, the fluid in the inner ear is compressed which stimulates hair cells and neurons fire up. Amphibians (and reptiles), the stapes is the only bone found in the inner ear. The malleus and the incus remain in their ancestral position as part of the jaw bone. The incus is homologous to the quadrate (part of the upper jaw) and the inclus is homologous to the articular (part of the lower jaw). In amphibians and reptiles, the stapes alone transmits vibrations from the typanum (a.k.a. eardrum) to the inner ear. The middle ear of amphibians also has a unique feature: a small bony or cartilaginous disc called the operculum. The operculum rests against the oval window and is connected to the shoulder blade (suprascapular) via the opercular muscle. This muscle transmits the vibrations from the forelegs to the oval window allowing amphibians to pick up minute vibrations from the ground using their forelegs.  Although, share by all modern amphibians, the opercular system is more developed in frogs than in salamanders, and it has been lost in caecilians.

3. Integument with granular and mucus glands. The skin of amphibians is smooth, scaleless, permeable, and glandular. Interestingly, early tetrapods were scaly indicating that the absence of scales in modern amphibians is a derived characteristic rather than an ancestral one. The loss of scales was likely associated with the evolution of cutaneous respiration (see notes on gas exchange). Two main types of glands are present in the skin of amphibians: mucus glands and granular glands (also called poison glands). Both types are in the innermost layer of skin, the dermis. The mucus glands secrete mucoproteins that help the skin maintain the necessary moisture to allow gas exchange. Poison glands secrete various noxious compounds rendering amphibians unpalatable. These glands tend to be concentrated into macro glands like the large parotid glands found on the head of toads.

4. Cutaneous respiration. Most adult amphibians have lungs, but gas exchange is not restricted to this organ. The skin and part of the buccal cavity are important gas exchange surfaces.  The skin is a particularly important respiratory organ for amphibians, and some species such as the lungless salamanders (Family Plethodontidae) rely nearly exclusively on their skin to breath even on land.

5. Short and straight ribs that do not encircle the body.  This trait is shared by all amphibians. In other tetrapods, including humans, ribs encircle the viscera and meet ventrally to form the sternum. The rib cage plays an important role in lung ventilation and as we will see later, amphibians have a unique way of ventilating their lungs.

6. Retractor bulbi muscle. Frogs and salamanders have a pair of large ventral openings (interpterygoid vacuities) in their skulls to accommodate their large eyes. They can retract their eyeball inside those openings by contracting a pair of unique muscles called the retractor bulbi muscle.  Amphibians can thus blink even though they lack eyelids! Interestingly, swallowing and eye retractions are coordinated in amphibians. Amphibians ingest relatively large prey whole. The eyes appear to help push prey down the oesophagus during feeding.

The Amphibian Body Plan

Among amphibians, salamanders retain most features of the ancestral tetrapod body plan while anurans and caecilians have undergone major modifications. Caecilians are elongated, and limbless creatures that are well adapted to their fossorial life. While the skull of salamanders and frogs is lightly built, the skull of caecilians is compact and robust. Caecilians have a unique chemosensory organ called the tentacle that protrude through the skull via its own aperture (tentacular foramen).  Some species of caecilians sport scale-like structures that are embedded in their dermis, between the body segments called annuli. Little is known about the function of these dermal scales.

The anuran skeleton is very different from the basic tetrapod blueprint (think of a salamander). Over evolutionary times anurans have lost many bones while others fused with one another. The skull of anurans is open. Their axial skeleton is composed of only nine presacral vertebrae while salamanders have ten to sixty. The tail is absent, and the caudal vertebrae are fused into a long rod called the urostyle. The urostyle transmits the thrust from the hind limbs to the vertebral column during jumping. The pelvic girdle of anurans is also highly modified. The ilium is elongated while the two other hip bones (the ischium and the pubis) are reduced. The pelvis is mainly cartilaginous and may play a role in shock absorption during landing. The long ilium serves as attachment for the massive gluteal and quadriceps muscles of the legs.

The tibia and fibula of the hind limbs are fused to form a fibulotibia. The radius and ulna of the forelimbs are also fused forming a radioulna. Fusing bones together adds stiffness which help dealing with mechanical stress of jumping and landing. The hindlimbs of frogs are famously elongated especially in jumping species. The ankle bones (astragalus and calcaneum) in particular have essentially become an additional leg segment. Although many frogs can jump substantial distances, many groups only hop or walk. These modes of locomotion appear to be the ancestral condition in anurans.

References

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