Question

Consider the following:

           a. The first arthropods to appear in the fossil record are marine crustaceans and trilobites.

           b. terrestrial insects appeared relatively late in arthropod evolution.

c. although most crustaceans are aquatic, the crustacean groups called crabs and isopods each include both aquatic and terrestrial forms, and

           d. analyses of DNA sequence data suggest that crustaceans and insects are closely related.

  

Based on these observations, do you agree or disagree with the hypothesis that arthropods made the water-to-land transition more than once? Explain your reasoning.

Answer 1

Yes we can agree to the statement, because the earliest animals found in the fossil record were all aquatic organisms. Arthropods were the first animal to adapt to life on land. This required arthropods to evolve structures that would both support their bodies outside of buoyant aquatic environment and efficiently retain water on dry land. The hard water impermeable exoskeleton was an important adaptation that served both of these purposes. The evolution of the tracheal system in insects helped to obtain oxygen from air. Fossils of aquatic scorpions with gills appear in the Silurian and Devonian period and the earliest fossil of an air breathing Scorpion with book lungs dates from early carboniferous period.Evidences from modem phylogeny seems that land living clades evolved independently within the overall arthropod tree. On the basis of this the must have atleast 7 separated terrestrialisation events enacted by hexapods, myriapods, arachnids and already for groups of crustaceans. By the Silurian era myriapod and arachnid are unequivocally living on land and hexapods appear soon afterwards in the early Devonian period. Today's land living crustaceans do not appear to have been part of this radiation. The hexapods are numerically the most significant Terrestrial arthropods group today, the fossil record suggest that their transition on to land may begun slightly later. The non hexapod crustaceans had also attempted to invade the land more than one time. Precise dates for the origins of group like land crab and wood lice are unfortunately not well constrained., but we should reiterate that the is no evidence for these groups lived in land during palaeozoic.