FURTHER PHYLOGENETIC CONSIDERATIONS IN THE ECHINODERMS
Most zoologists believe that echinoderms evolved from bilaterally symmetrical ancestors. Radial symmetry was evolved during the transition from active to more sedentary lifestyles. The oldest echinoderm fossils are about 600 million years old. It does not give direct evidence of this transition.
Relationship of ancient echinoderms and crinoids
Ancient fossils give some evidence about the origin of the water-vascular system and the calcareous endoskeleton. The crinoids are closely resembled the oldest fossils.
1. Crinoids use water vascular system for suspension feeding and filter feeding. They do not use water vascular system for locomotion. Thus filter feeding was the original function of the water vascular system.
2. The early echinoderms have a mouth-up position like crinoids. Therefore, they attached aborally. They used arms and tube feet to capture food and move it to the mouth
3. The calcium carbonate endoskeleton was evolved to support filtering arms. It also protects these sessile animals.
Evolution of modern echinoderms
Many modern echinoderms are motile. They have secondarily derived following characteristics:
1. Free -living lifestyle.
2. The mouth-down orientation. The mouth-down position has advantage for predatory and scavenging lifestyles.
3. Changes occured in the water-vascular system. There was evolution of ampullae, suction disks and feeding tentacles. These are adaptations for locomotion and feeding in more mobile lifestyle.
Some echinoderms like irregular echinoids and the holothuroids have bilateral symmetry. This bilateral symmetry is derived from pentaradial body. This observation also supports the idea that the free-living lifestyle is a secondary adaptation in echinoderms.
The evolutionary relationships among the echinoderms are not clear. Numerous fossils of echinoderms of the Cambrian period are present. But these fossils cannot develop definite evolutionary relationships among living and extinct echinoderms. Most taxonomists agree that the echinoids and holothuroids are closely related. But it is not clear whether the ophiuroids are more closely related to the echinoid/holothuroid lineage or they are related to asteroid lineage. The position of Concentricycloidea in echinoderm is also not clear.