FURTHER PHYLOGENETIC CONSIDERATIONS IN AMPHIBIANS
There is a problem in amphibian phylogeny. This problem is the relationship among the three orders of modern amphibians. There are two hypotheses about the origin of amphihians:
Some zoologists place anurans, urodeles . and caecilians into a single sub class Lissamphibia. It shows a common ancestry or modem amphibians. It suggests that they are more closely related to each other than to anv other group. Supporters or this classification give common characteristics between thesegroups. These characters are:
1. They have complex.stapes and operculum.
2. They use skin in gas exchange.
3. They have similar structures of the skull and teeth.
Other Zoologists think that modern amphibians were derived from two nonamniotic lineages. They noted difference in structure of vertebral column of these two lineages. They suggests that the three orders have separate origins. It shows that Amphibia is a paraphyletic group. It should be divided into multiple monophyletic taxa. This controversy is still going on.
Three sets of evolutionary changes occurred in amphibian lineages for movement onto land. Two of these changes occurred much earlier. Therefore, these changes are present in all amphibians.
1. One change was adaptation in the skeleton and muscles of these amphibians. These changes allowed greater mobility on land.
2. A second change occurred in jaw mechanism and moveable head. These changes helped the amphibians to uses insect efficiently as food resources on land. A jaw-muscle arrangement was derived from fishes.This method was adaptive by early tetrapods forfeedings on insects in terrestrial environments.
3. The third set of changes occurred in the amniote lineage. It is the development of on egg. This egg is resistant to drying. The aminiotic egg is not completely independent of water. But the extraembroynic membranes protect the embryo from desiccation. These membranes store wastes and promote gas exchange. This egg also has a leathery or calcified shell for protection. It is porous and allows gas exchange with the environment. These evolutionary changes gave rise to the remaining three vertebrategroup: reptiles, birds and mammals.