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 gillsLungsBothNeither

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 breathingMouth – buccal breathingLungs

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.