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Section 12.3Amphibians
Evolution of Amphibians• About 350 million years ago a lineage
of lobe-finned fish were the first vertebrates to make the transition from water to land– This was about 100 million years after
the first plants made the same movement
• These fish had specialized characteristics that allowed for life on land– Limb like fins for crawling– Primitive lungs for breathing air for
short periods• From these ancient fish there would
be descendents who could stay out of water for longer and longer and eventually we got the first amphibians
Amniotes
Amphibians
GreerpetonHynerpeton
lchthyostega
Acanthostega
Metaxygnathus
Elginerpeton
Eusthenopteron
Panderichthys
Lungfishes
Coelacanths
Ray-finned fishes
Paleozoic
Silurian Devonian Carboniferous Permian To present
420 415 400 385 370 355 340 325 310 295 280 265
Millions of years ago
Class Amphibia• Is represented by about 4800
species which are divided into 3 orders:– Legless amphibians, order Apoda
• Include the wormlike caecilians– Tailless amphibians, order Anura
• Include frogs and toads– Tailed amphibians, order Urodela
• Include salamanders and newts• Amphibians have moist skin
that helps with gas exchange along with their lungs
• Most adults are carnivorous
Order Apoda
• As mentioned are legless
• Most are nearly blind
• Most species burrow in moist forest soil in the tropics
Order Anura• Out of all the orders are the
most specialized for life on land– Adults with powerful legs to
hop on the land– Use of sticky tongues to catch
insects• Also have many more
adaptations to reduce predation– Camouflage– Secrete distasteful or even
poisonous mucus from their skin (some advertise this via bright colouring as a warning, e.g. poison dart frogs)
Order Urodela
• Some species are entirely aquatic
• Others are on land as adults or throughout their life
• Most are small organisms, but also includes the largest amphibian the giant salamanders found in Japan and China
Amphibian = “two lives”• Amphibian means two lives
and this is the reference to the common metamorphosis of aquatic larvae to terrestrial adults– For this reason it is as if the
organism has 2 lives– Most common in toads and
frogs (tadpole and adult)• This does not mean all
amphibians have a dualistic life as many do not– Some larvae of some look like
adults– Some species are strictly
aquatic or terrestrial
Amphibian Reproduction• Most have external
fertilization and lay their eggs in fresh water– Oviparous = lay eggs
• Some species though are ovoviparous (eggs hatch within parent) or viviparous (give birth to live young)
• In any case for reproduction for most water is key as the eggs lack shells and dehydrate quickly in dry air
• Most are familiar with reproduction of the the order Anura (frogs and toads)
Case of the Tadpole• Egg hatch as tadpole
– As tadpole most are herbivores
– In some cases though tadpoles are omnivores
• Gas exchange via gills• And have tail for
movement• Overtime tadpole
undergoes metamorphosis developing limbs and loses its tail, and gills
Amphibians Tie to Water• As mentioned is key for most
eggs to survive– Also most lay their eggs in
water• For this reason amphibians are
usually near water or found in damp habitats– This is also due to the fact most
rely heavily on their moist skin to carry out gas exchange over their lungs
• Those adapted to drier habitats spend much of their time in burrows or under moist leaves where humidity is higher
Amphibian Circulation• Adults have a 3 chambered heart,
unlike fish• Blood from the body enters the
right atrium– Then flows into the single ventricle
• Blood from the lungs enter the left atrium– Then flows into the single ventricle
• Since the heart only has one ventricle both oxygenated and deoxygenated blood leave the heart together as they have mixed in the ventricle
• This makes amphibians circulation system to not be very efficient
Amphibian Respiration
• For most species the young that live in water use gills
• Meanwhile for most adults they use the following for gas exchange: – their lungs
• Their lungs have a simple structure with some internal folding to help increase their surface area for gas exchange
– their moist skin• The use of their lungs and skin help make up for
their less efficient circulation
Amphibian Decline• For the past 25 years, zoologists have been documenting a rapid
and alarming decline in amphibian populations throughout the world.
• Several causes that have been proposed include environmental degradation (especially acid rain) and the spread of a pathogen, a chytrid fungus.
• This decline is also part due to the fact amphibians have been found, especially frogs, to be very sensitive to environmental contaminants– This is part to the fact that frogs are found on land and water and can
absorb gas via their skin– Also they eat a variety of food including plant and animal material
• Thus from this many scientists can at times evaluate a habitats contamination and condition based on the amphibian population
Homework
• Read pages 455-456
• Answer Questions– 1-2, 5 page 460