From water to dry land

The sarcopterygians are unquestionably the nearest relatives of the tetrapods. In fact tetrapods are sarcopterygians in a strict phylogenetic sense. The rhipidistians were the first vertebrates to gradually step on shore and their descendants were the first tetrapods. But what drove them from the water to dry land?

The transition from living in water to living on land involves a variety of problems that need to be solved. Air is a much less dense medium than water and gives much less support. Some major structural modifications were needed for the first land vertebrates to be able to hold their body balanced and more power was needed to fight the stronger pull of gravity. Other modifications were also necessary involved with feeding, reproduction, sensing prey and predators and of course breathing mechanisms.

Limb modification: a) pectoral fin of the sarcopterygian Eusthenopteron b) equivalent forelimb of an early tetrapod, Eryops (Permission provided by Mike Benton)

The skeleton was modified to cope with the stronger forces of gravity and the vertebrae and surrounding musculature became more powerful. The lobed fin became a walking appendage with the addition of carpals, metacarpals and digits to the already present limb bones: humerus, radius and ulna in the pectoral fin and the femur, tibia and fibula in the pelvic fin. There were also modifications in the shoulder girdle, it became dislocated from the back of the skull and more strictly connected to the frontlimbs. The shoulder joint swung around so the now lengthened humerus pointed more sideways instead of backwards. New bones in the hand and wrist hinge allowed the hand to spread out and become a weight-supporting surface. Parallel modifications happened in the ankle joint, and the pelvic girdle became enlarged and firmly attached to the vertebrate column. There were also changes in the jaw structure, especially in the positioning of hinges at the back of the jaw. This might have been eased food-gathering or more likely buccal pumping or other forms of ventilation.

 

Structural modifications for terrestrialization from an osteolepiform (a and c) to a basal tetrapod (b and d): a) and b) seperation of shoulder girdle from skull and c) and d) enlargement of pelvic girdle (Permission provided by Mike Benton)

It is thought that the sarcopterygians were driven to shore because of a great supply of food in the form of vegetation and invertebrates that was mostly being unharvested. In other words, there were a lot of ecological niches that were empty and were ready to be filled. But the perfect modifications didn't just appear. They must have somehow evolved while the sarcopterygians were still aquatic animals. The most popular hypothesis is that the ancestors of the tetrapods lived in small freshwater and brackish pools and were forced to travel between pools in serious droughts. Therefore they needed to be able to travel over dry land, breath air and even feed and mate on the journey. Gradually they became less dependent on living in water although fresh water was still essential for reproduction, especially for the development and hatching of the young just like in modern amphibians.



Author: Snorri Sigurdsson (Email: ss4460@bristol.ac.uk)
Last updated: 14 November 2004
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