FURTHER PHYLOGENETIC CONSIDERATIONS
Relationship between Hemichordates and Chordates
It is difficult to establish evolutionary relationships between the hemichordates and chordates. Two common characteristics are present between these groups:
1. Dorsal tubular nerve cord.
2. Pharyngeal slits of hemichordates.
These two characteristics are the evidence of evolutionary ties between these phyla. But it is not clear whether these are homologous structures or not. Synapomorphies that differentiate chordates from hemichordates are: Tadpole larvae, Notochord, postanal tail and endostyle.
Evolution of chordates from sessile ancestor
The earliest echinoderms were sessile filter feeders. The lifestyle of adult Urochordates suggests a similar ancestry for chordates. The evolution of motile chordates from sessile ancestors took place by the development of a tadpole like larva. Increased larval mobility is an adaptation for sedentary species. It promotes dispersal of these species. The evolution of motile adult takes place by paedomorphosis. The development of sexual maturity in the larval body form is paedomorphosis. Paedomoorphosis takes place in animal kingdom especially among amphibians. Paedomorphosis produced a small sexually reproducing fish like chordate. This fish like chordate was the ancestor of higher chordates.
Evolution of vertebrates
The subphylum Vertebrata is the largest and most successful chordates. They developed following advanced characteristics:
1. Bony or cartilaginous vertebrae have completely or partially replace the notochord in vertebrates.
2. The anterior end of the nerve cord is changed into a brain. They have developed specialized sense organs on the head. These are evidence of a high degree of cephalization (concentration of all sense organs in the head).
3. The skeleton is modified anteriorly into a skull or cranium. That are eight classes of vertebrates. The vertebrates and chordates have bony enthskeletons. Thus the vertebrates have left an abundant fossil record. Ancient jawless fishes were common in the Ordovician period, approximately 500 million years ago. Over a period of 100 million years, fishes became the dominant vertebrates. Near the end of the Devonian period, 400 million years ago, terrestrial vertebrate appeared. Since then the vertebrates have spread on most of the earth’s habitats.