Early Tetrapods and Modern Amphibians Chapter 25

Early Tetrapods Tetrapods are gnathostomes that have limbs and feet. Tetrapods are a monophyletic group. One of the most significant events in vertebrate history was when the fins of some lobe-fins evolved into the limbs and feet of tetrapods.

Early Tetrapods The movement from water to land required more than the change from fins to feet.

Early Tetrapods The terrestrial environment is different than the aquatic environment. A higher oxygen content means oxygen can diffuse more rapidly. Air is less dense – less buoyant. Stronger skeletal structure, sturdier limbs. Temperature fluctuates more. Terrestrial environment offers a whole new array of habitats.

Early Tetrapods Tetrapods have many adaptations including: Lungs for efficiently breathing air. Four limbs and feet with digits. Ears for detecting airborne sounds.

The Origin of Tetrapods The Devonian (beginning 400 mya) had mild temperatures and alternating droughts & floods. Freshwater sources unstable. Reduced oxygen content as pools & streams dried up.

The Origin of Tetrapods Freshwater fishes alive at this time had a kind of lung that developed as an outgrowth of the pharynx. Efficiency of this air-filled cavity was enhanced by improving the vascularity with a capillary network. Oxygenated blood was then sent to the heart & pumped to the rest of the body.

The Origin of Tetrapods This was the origin of the double circulation that we find in all tetrapods. A systemic circulation serving the body. A pulmonary circulation supplying the lungs.

The Origin of Tetrapods Vertebrate limbs also arose during the Devonian period. Fins of lobe-finned fishes have bony elements that are homologous to tetrapod limbs. Some Devonian lobe-finned fishes have identifiable humerus, radius, & ulna, and could sort of walk on the bottom of pools.

The Origin of Tetrapods In one lineage of lobe-fins, the fins became progressively more limb-like while the rest of the body retained adaptations for aquatic life.

The Origin of Tetrapods Extraordinary fossil discoveries over the past 20 years have allowed paleontologists to reconstruct the origin of tetrapods.

The Origin of Tetrapods One theory about how early tetrapods became adapted for life on land suggests that they had to be able to move from pool to pool to find water. Those with stronger fin/limbs would survive & reproduce. Alfred Romer

The Origin of Tetrapods Acanthostega had well developed limbs with digits, but was fully aquatic in other ways. The limbs were not adequate for proper walking on land. Only later did tetrapods move onto land. We do not currently know what caused this move.

The Origin of Tetrapods Lobe-finned fishes are probably the closest relatives of tetrapods. Sister group Share several characteristics with early tetrapods like Acanthostega & Ichthyostega. Characteristics of the skull, teeth, & pectoral girdle.

The Origin of Tetrapods Ichthyostega had characteristics of a terrestrial vertebrate: Jointed limbs Stronger vertebrae & associated muscles to support the body in air. Muscles to elevate the head. Stronger shoulder & hip girdles. Protective rib cage. Modified ear structures for detecting airborne sounds. Modifications for detecting airborne odors.

The Origin of Tetrapods Ichthyostega also retained some aquatic characteristics: The tail has fin rays. Opercular (gill cover) bones are present. Ichthyostega represents an early offshoot of tetrapod phylogeny, not an immediate ancestor of amphibians.

The Origin of Tetrapods Several extinct lineages plus the Lissamphibia with modern amphibians formed the temnospondyls. Lepospondyls and anthracosaurs are, based on skull structure, closer to amniotes than temnospondyls.

The Origin of Tetrapods The warm and wet Carboniferous period followed the Devonian. Tetrapods radiated quickly.

Class Amphibia Class Amphibia is represented by about 5,000 species. Most amphibians have moist skin that complements the lungs in gas exchange. Amphibians require moist environments. Amphibians are ectothermic with a low metabolic rate.

Metamorphosis Amphibian means “two lives”, a reference to the metamorphosis of an aquatic larva into a terrestrial adult.

Terrestrial Amphibians Some amphibians have adapted to a more terrestrial life. Some caecilians and salamanders have direct development. Some frogs have unusual adaptations for caring for their young.

Order Gymnophiona Order Gymnophiona includes caecilians, which are legless and resemble worms. Burrowing & aquatic forms.

Order Gymnophiona – Reproduction Fertilization is internal in this group. Larvae are aquatic. Some complete larval development inside the egg. Some species are viviparous with the young obtaining nourishment by eating the wall of the oviduct.

Order Urodela Order Urodela includes salamanders, which have tails.

Order Urodela – Metamorphosis The ancestral condition in salamanders is to have aquatic larvae and terrestrial adults that live in moist places. Some species are fully aquatic, others are fully terrestrial.

Order Urodela – Reproduction Fertilization is internal. The male deposits a spermatophore (package of sperm) on the substrate, the female then picks it up.

Order Urodela – Reproduction Aquatic larvae have external gills and a finlike tail. Blue-spotted Salamander larva (Photo from the Massachussetts Vernal Pool Association )

Order Urodela – Reproduction Terrestrial species have direct development – no larval form. Eggs are laid in moist areas and are sometimes guarded by the parents.

Order Urodela – Life Cycle Some American newts have a complex life cycle: Aquatic larvae metamorphose into a terrestrial red eft stage that later metamorphose again into aquatic adults. Some skip the terrestrial stage and remain aquatic.

Order Urodela – Respiration Salamanders have a diverse array of respiratory mechanisms. Respiration through the skin is important. At some point in the life cycle they may have: External gills Lungs Both Neither

Order Urodela – Paedomorphosis Paedomorphosis is found in some salamanders. Larval characteristics are retained in mature adults. Some species do not metamorphose, retaining their external gills and finlike tail and remaining aquatic while mature. Mudpuppies

Order Urodela Some species only metamorphose under certain conditions. Gilled forms called axolotls. Metamorphose when the pond evaporates.

Order Urodela Another example of paedomorphosis occurs in the feet of salamanders in the genus Bolitoglossa. Growth of the feet stops early leading to pad-like feet that are useful for climbing.

Order Anura Order Anura includes frogs and toads, which lack tails.

Order Anura – Habitats Adult anurans may be found near water throughout their lives, or in more terrestrial habitats. Some return to water only to breed.

Order Anura – Habitats Some species are fully terrestrial with tadpoles living in small water accumulations in plants, or in the mouth or vocal sac of the parent.

Order Anura – Defense Adult frogs show a variety of defense mechanisms. Leaping away is a commonly seen defense. Most anurans can inflate their lungs so they look to big to swallow. Some have poison glands. Dendrobatids (poison dart frogs) Toads Some will bite at a potential predator.

Order Anura – Coloration Skin color in amphibians is produced by pigment cells called chromatophores. Xanthophores contain yellow, orange or red pigment. Iridophores contain a silvery, light reflecting pigment. Melanophores contain black or brown melanin.

Order Anura – Coloration Green hue is an interaction of xanthophores containing yellow pigment and underlying iridophores. Many can adjust color to match with background and camouflage themselves.

Order Anura – Respiration Amphibians can use three respiratory surfaces for breathing: Skin – cutaneous breathing Mouth – buccal breathing Lungs Anurans are more reliant on lungs than salamanders. Carbon dioxide still lost through skin.

Order Anura – Respiration Air must be forced into lungs – positive pressure breathing.

Order Anura – Vocalization Males have better developed vocal chords in their larynx (voice box) than females. Males use their voices to attract females. Calls are species specific.

Order Anura – Circulation Tetrapods have separated pulmonary and systemic circulation. This is made more efficient by partitioning the heart. This partitioning is not complete in amphibians. Frog hearts have two atria, but a single ventricle. Blood still remains mostly separated.

Order Anura – Feeding Frogs, like most adult amphibians, are carnivorous. Most will feed on just about anything that moves and is small enough to swallow whole. Most have a sticky, protrusible tongue. Tadpoles are usually herbivorous.

Order Anura – Senses Lateral line system present in larval forms only. Ears are used for detecting airborne sounds. Vision is important in most amphibians. Frogs have color vision provided by rods and cones on the retina.

Order Anura – Senses Frogs also have tactile & chemical receptors in their skin, taste buds on the tongue and palate, and a well-developed olfactory epithelium in the nasal cavity.

Order Anura – Reproduction Spring weather signals breeding season and is marked by calling frogs.

Order Anura – Reproduction Frogs pair up with the male riding on the females back – amplexus. Fertilization is external. Eggs usually deposited in water, anchored to vegetation.

Order Anura – Reproduction Tadpoles, the aquatic larval stage, hatch after 2-21 days depending on temperature. Usually herbivorous with keratinized jaws for scraping algae. Three pairs of external gills which become internal gills.

Order Anura – Reproduction Hind limbs appear first, followed by front limbs. The tail is gradually resorbed. The tadpole stage may last a few weeks or a couple of years, depending on species.