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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.
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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.
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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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