FURTHER PHYLOGENETIC CONSIDERATION
There is a fundamental question about the evolution of arthropod. The question is that whether or not the arthropod taxa represent different evolutionary lineages.
1. Polyphyletic origin
Many zoologists believe that the living arthropods should be divided into three separate phyla: C’helicerata, Crustacea and Uniramia. There are similar arthropods features in all three phyla. Therefore a polyphyletic origin of these groups shows convergent evolution. There can be dual origins of tracheae, mandibles and compound eyes. But man zoologists believe that convergence in all other arthropod traits is not possible.
2. Monophyletic origin
Meinhers of all four subphyla are present in the fossil record from the early Paleozoic era. There are currently no known fossils of arthropods from Precambrian times. Therefore, the fossil record does not help in discovering the evolutionary relationships among the arthropod subphyla. Thus zoologists rely on comparative anatomy, comparative embryology and molecular studies.
These are two important issues in the phylogeny of arthropods.
Issue 1: The question is whether or not the biramous limbs of crustaceans and trilobites are homologous structures. Their homology shows that trilobites are closely related to the crustaceans. Therefore, many zoologists take trilobites as an important ancestral group. Thus crustaceans and arachnids have arises form trilobites. But it is difficult to evolve Myriapoda and insects form trilobites.
Issue 2: The question is whether or not the mandible of crustaceans and trilobites are homologous structures. The mandibles in the groups of arthropods are structurally and functionally similar. But they have different muscle arrangements and articulation in mandibles. Therefore, some zoologist does not take mandible in these groups as homlogous structures. Therefore, the zoologists are working on the origin and homologies of compound eyes, tracheal systems and malpighian tubules.
Number of hypothesis has been given on the basis of these issues. These hypotheses show the relationship between the different sub-phyla of the arthropods.
1. First hypothesis shows that Uniramia and Crustacea have close relationship. These hypotheses develop homologies between these two groups. The homologous organs between these two groups are mandibles. compound eyes and some other structures.
2. Second hypothesis shows that the Uniramia diverge independently. It has not common ancestor with Chelicerata and Crustacea. Chelicerata and Crustacea are closely related to trilobites. Therefore, the mandible of the crustaceans and Uniramia are not homologous. On the other hand the biramous appendages of the crustaceans and trilobites are homologous.
This discussion shows that it is difficult to develop evolutionary relationship between the different groups of arthropods. More fossils and molecular evidences are required to develop relationship between them.