Early Tetrapods and Modern Amphibians Chapter 25.
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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.
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