The anatomical feature shared by all synapsids, and the one that gives them their name, is the 'lateral temporal fenestra' or 'synapsid arch'. This is a large hole or arch in the side of the skull, just behind the eye socket, which allowed for the development of larger, longer jaw muscles. The jaw could now be opened wider, and closed with far greater force, giving the synapsids an advantage over their ancestors in the capture and eating of prey. This arch has become more complex over time, now being formed from only one bone, the others having evolved into the bones of the mammalian inner ear.
Skull of an early synapsid, showing the position of the synapsid arch. Also notice the carnivorous teeth that are differently sized but not yet different in shape, but allowing for more complex feeding mechanisms to develop.
Adapted and reproduced with kind permission from Benton and Harper, 1997
The ancient synapsids were also amniotes, which meant that they no longer needed to return to the water to reproduce like their amphibian ancestors. Amniotes reproduce by means of a 'cleidoic' or closed egg, which is protected by a semi-permeable shell that keeps water in, but allows for exchange of gases such as oxygen and carbon dioxide to protect the developing embryo from the external environment, but also to keep it alive. There is no need to lay the eggs in water, and no larval stage, so the chances of young reaching maturity are increased. We know this because fossil eggs, although rare, are sometimes found in association with fossil bones, and in very rare cases special techniques can be used to see what is inside the egg, telling us about the reproductive cycle and the embryonic stages of the animals involved. Some descendants of the synapsids, the placental and marsupial mammals, have evolved to give birth to live young, an adaptation that further increases the juvenile's chances of survival. Learn more about mammals.
There have also been changes in the way in which the synapsids have moved over time. Early synapsids had a sprawling gait, not unlike that of modern crocodiles, where the limbs stick out to the side of the body. In this pattern of walking the body swings from side to side, resulting in a waddling motion. Later synapsids adopted a 'parasagittal' gait, like that of modern mammals, with all four feet positioned underneath the body. This gave later synapsids increased stability when standing as the knee joints were able to lock, the bones supporting the animal's weight. It decreased the 'waddle', making their locomotion more efficient, and allowing them to reach greater speeds, important both for avoidance of predators, and capture of prey.
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