The beginning of the Carboniferous period heralded the arrival of many new forms of life. Chief among these creatures were early reptiles, the decedents of amphibians. However, unlike their ancestors they evolved many new traits that allowed them to leave the water and become truly terrestrial. Although early reptiles could be distinguished from their amphibian ancestors by their size, their main distinguishing feature was their eggs. The embryos grew within a liquid like membrane separated into different areas, called an amnion, which was then surrounded by a tough outer shell. The shell, because of its microscopic pores, allowed air to enter and exit the eggs. The shell also kept moisture from draining out of the eggs which meant that reptiles could lay them on dry land, protecting them from waterborne predators. In addition to the shell, these amniotic eggs provided nourishment to the embryo through the yolk sac and an additional section for waste storage. Another useful feature of the egg was their larger size which allowed the embryo to develop fully within the egg and forgo the vulnerable larval stage. In addition to their eggs, reptiles also developed tough scales so as to fend off predators and to more importantly allow it to keep its body moisture inside so that it may venture away from water. This adaption allowed reptiles to explore more environments and thus find more opportunities for food, shelter, and reproduction.
References
Haines, T., & Chambers, P. (n.d.). The Complete Guide to Prehistoric Life. BBC
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[WGBH Deep Time Time Line]. (2001). Retrieved from http://www.pbs.org/wgbh/
evolution/change/deeptime/
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