vertebrate;Amphibia

vertebrate;Amphibia
 Amphibia



There are currently 5,300 known species of Amphibia From Greek amphi = both sides, bios = life.
Amphibians = live in two worlds: land and water (dual life).Include frogs, toads, salamanders
Gave rise to all higher vertebrates First vertebrates on land, ~300 mya
Evolved from lobe-finned fish (similar to the coelacanth)Terrestrial forms had tough skin to resist desiccation.

Classification of Class Amphibia;3 major orders:
Order Apoda (Gymnophiona in other sources): caecilians
Order Anura: frogs and toad
Order Urodela or Caudata: salamanders

Food for Thought“While the Amphibians were the First Terrestrial Vertebrates,
the Reptiles were the First Successful Terrestrial Vertebrates”. Comment
Amphibian Early Evolution
The 1st amphibians probably evolved in the Devonian period about 350mill. years ago
During Carboniferous (350 - 280 mya)- was good environment for amphibian development.
characterized by wet warm environments lots of swampy areas during this time amphibians
had their greatest rate of evolution The climate of the world changed at the end of the Devonian period and beginning of Permian During this period the earth became colder and dryer.
At about this time, some fish started to crawl out of the water They then began walking on land
and breathing air.Amphibians dominated in the Permian period “the age of amphibians”
They begun to decline in the late Permian, until only 3 groups remained.

1st theory: Lungfish as Ancestral Tetrapods
Arguments on the precise relationship of the paired fins and tetrapod limbs
Modern lungfish & coelacanths are ancient lines or relics of fish which were common in the Devonian Their air/swim bladders homologous with lungs enabling their survival out of water
Lungs served as a bridge btn the 2 groups (fishes + amphibians)
Proposed the Dipnoi (Lungfish) as transitional group btn true fish and amphibians
Argued that early dipnoi were the primary form from which the amphibia originated.
Lungfish as Ancestral Tetrapods By the end of 19th C the Dipnoi resemblance to amphibians was disputed .It was argued to be a result of convergent evolution, and not an indication of true genealogical relationship.The Dipnoi could not be ancestral to amphibians Dipnoi had developed specialized characters such as crushing
plates of the jaw;These structure were unlikely possessed by amphibians
Dipnoans were a side branch that had independently acquired characters resembling those of amphibiansNot directly related to amphibians! The skull structure of dipnoi differed from that of amphibians.The phylogenetic tree shows dipnoi as descendants of the crossopterygians, branching off differently from the root taken by amphibians.The structure of the paired fins in dipnoi did not match the tetrapod limb, but rhipidistians offer a better model on which limbs derivation were based.

2nd theory: The crossopterygians as Ancestral to Tetrapods
Ancestors of the amphibians were the Rhipidistians - type of crossopterygian fish
These are lobe finned fish (quite closely related to the coelacanth line) which were common in the Mid-Devonian.Rhipidistians Lived In Freshwater Habitat These fishes possessed a swim bladder homologous with the early form of lungs.Although they also had gills, their lungs allowed them to absorb the oxygen necessary to live when they were on land.These fishes also developed moveable, paired fins to help them move on dry land.This offered a better starting point for the evolution of tetrapod limbs/legs The earliest known fossil amphibians are believed to be the ichthyostega
Possible explanation for evolution of tetrapod Limbs from the paired fins of sarcopterygian (lobe finned) fishes These fishes (notably Eusthenopteron) have bones in their paired fins that are very similar to the bones of tetrapod limbs.They have a single bone (similar to the humerus or femur) followed by paired bones (similar to the radius and ulna or fibula and tibia of
tetrapods)Eusthenopteron lacked digits - having fin rays instead> is a closely related fish that had 8 digits just like the earliest amphibians Ichthyostega was similar to Eusthenopteron

Rhipidistians and Amphibian evolution
The main reasons why this line of fish are considered the most probable ancestors of the amphibians The two groups share a number of important characters.the arrangement of skull bones is similar.the amphibian limb can be derived from the fin structure.the rhipidistians lived in shallow freshwater lakes and almost certainly had lungs and breathed air.both rhipidistians and early amphibians have a similar structure to their teeth - called a labyrinthodont pattern .It has a complex folding of the walls of the pulp cavity.rhipidistians were the only fish which had developed an opening to the nostrils
inside the mouth ;This is a feature found in all the land vertebrates.

Old amphibians - Ichthyostega
Had 4 short stubby legs with five toes and a tail fin. Had scales on their bellies and tails, strong backbone.Had streamlined bodies, long tails and fins for swimming but could hold themselves on land without difficulty.One of the striking features of the Icthyostegalia's were their teethTheir teeth showed deep in folding of the enamel which
is characteristic of the amphibian group known as Labyrinthodonts.

The crossopterygian fish Eusthenopteron is linked to the early  amphibian Icthyostega by a number of characteristics:
(1) same pattern of skull bones as Icthyostega
(2) internal nostrils  (found only in land animals and sarcopterygians - a taxonomic  group encompassing lungfish
and crossopterygians)
(3) teeth  like amphibians
(4) a two-part cranium (icthyostegids are  the only other vertebrates that have this characteristic)
(5) same vertebral structure.

Labyrinthodont or “stegocephalia
The study of early armored amphibians Labyrinthodont or “stegocephalia supports the theory of crossopterygian ancestry.The structure of the earliest amphibians could best be explained by supposing they had evolved from crossopterygians.Lungfish were specialized descendants of the earliest crossopterygians,Was a group from which the 1st amphibians also had evolved “not the parents but the sister group to the tetrapods”

Tetrapod Evolution Theories
1st theory suggests that the tetrapod development was in response to smaller bodies of water periodically drying.The fishes in those areas that had ability to move on land would crawl (walk) to other larger bodies of water.The 2nd theory proposed that the first land tetrapods arose from lunged, air gulping fishesTo avoid competition and predation in the aquatic habitats, they began to occupy the coastal regions.As their limbs developed, they strayed further on to land.
Land plants were flourishing and only a few arthropods occupied terrestrial habitats.
Therefore, food was plentiful, competitors were scarce and predators were non-existent.
This would have provided all types of selection pressure in favour of moving onto land.

Why Are Amphibian Populations Declining?
Many amphibians are suffering heavy population loss and development deformities
Factors contributing to amphibian decline;Habitat destruction/loss, alteration + fragmentation (impact 90% of threatened spp) Effects of Introduced (alien) species e.g. bullfrog;Over – exploitation (food + pet trade)Environmental pollution and Climate change Emerging infectious diseases e.g. Fungal attacks - Chytridiomycosis Batrachochytrium dendrobatidis

Amphibian conservation measures/strategies
Providing corridors to facilitate the migration of the populations of amphibians between each other.
Expanded understanding of the causes of declines and extinctions The need for further research into the causes of amphibian declines and extinctions Emerging amphibian diseases;Climate change
Environmental contamination ;Ongoing documentation of amphibian diversity, and how it is changing Exploration and biodiversity evaluation.Updating the Global Amphibian Assessment continuously .



References

Liem, K.F.; Walker, W.F. (2001). Functional anatomy of the vertebrates: an evolutionary perspective. Harcourt College Publishers

The Origin of the Larva and Metamorphosis in Amphibia". (1957)The American Naturalist. Essex Institute.