Origin of Amphibia (Tetrapoda)

Origin of Amphibia (Tetrapoda)

The primitive amphibians were the first vertebrates to initiate the conquest of land in the Devonian period. They arose from the Crossopterygian fishes. The most spectacular event in the phylogenetic history of vertebrates is the emergence of Amphibia from piscine ancestors. The event took place nearly 300 million years ago in the background of the dry and arid Devonian period. The close anatomical resemblances and palaeontological evidences provided by Romer, Watson, Stensio, Jarvik and others clearly establish that the amphibians arose from some fish-like ancestors. The structural and functional adaptations that were acquired by the first amphibians while invading land to face the problems of terrestrial life are as follows:

1. Head and lower jaw developed powerful musculature.

2. Pectoral girdle well developed with strong scapula and pelvic girdle tri-radiate with elaborate ilium.

3. Limbs well developed with powerful muscles to lift the body from the ground.

4. Vertebral column rigid and strong.

5. Well-developed lings for aerial respiration. Moist skin serves as an accessory respiratory organ.

6. Vascular system well developed incorporating a pulmonary circuit.

7. Middle Ear cavity with columella auris for transmission of sound waves.

8. Skin keratized to prevent desiccation on land.

The discoveries of the fossil footprints of Thinopus, an amphibian, and of skeletal remains of Elphistostege, an intermediate form between the crossopterygian osteolepid and the tetrapod, Ichthyostega, from the Devonian stratum strongly suggest that the tetrapods originated in the Devonian. There is no fossil record of Amphibia during the Silurian period and in the Carboniferous three major groups of Amphibia – the Labyrinthodontia, the Phyllospondyli and Lepospondyli were dominating the earth. So, the Devonian must be the period of amphibian emergence. When we discuss about the factors that become reason of origin then the climatic and ecological conditions of the Devonian period provide us with the impelling causes of the emergence of amphibians. The Devonian was a dry period when streams and ponds tended to dry up seasonally. Ancestors of amphibians (crossopterygians) with their lobed fins could move from drying pools where water was available. According to Romer, ‘land-limbs were developed to reach water and not to leave it’. Berrill (1955) believes that the enemies in water forced the crossopterygians to explore land. Other factors were the abundance of food on land, availability of atmospheric oxygen and especially, the recurrence of unfavourable environment. On coming to land the primitive amphibians acquired changes and became specialized and split up into three orders which took three different courses of evolution.

1. In Labyrinthodontia the skull was heavily armoured and the large teeth had enfolded enamel layer. While some were terrestrial, many of these remained fully aquatic. The evolution of Labyrinthodontia was essentially a process of reduction of ossification.

2. The Phyllospondyli including a group of small salamander like amphibian were contemporary with Embolomeri. It seems highly probable that salamanders (Urodela) and frogs (Anura) arose from them.

3. The Lepospondyli represent a residual group and the modern caecilians (Apoda) were derived from them.