Anamniotes

Transcription

Would you believe that walruses, rattlesnakes, and parakeets all once lived in the same house? Let's go back about 350 million years. Look around. Steamy swamps and rain forests of horsetails and ferns cover the region. Amphibians are the dominant land vertebrates. They range in size from newts to crocodiles. And all require water to do their egg laying. If they don't go to the water, their shell-less, jelly-like eggs will dry out. Because of this hazard, they spend most of their time living in or near fresh water. That is, until a breakthrough in evolution changes everything: the amniotic egg. The amniotic egg is shelled, waterproof, and can be laid on dry land. It is produced by the amniotes, a new group of animals named after their revolutionary egg. The first amniote is a tetrapod, a four-legged animal, resembling a small lizard. While some amphibians can walk around on land and bury their eggs in wet soil or highly humid areas, nothing before the amniotes has the ability to lay its eggs on completely dry land. Because of this evolved egg, the amniotes are the first animals with the ability to live a fully terrestrial life. But, despite their move inland, the amniotes have not abandoned their pond-dwelling upbringing. In fact, the amniotic egg brings the pond with them by enclosing the aquatic environment within its shell. This is achieved by four main upgrades that are unique to amniotic eggs. Let's take a closer look. The first development is the most obvious: the egg's protective shell. It's tough but flexible, and has a leathery surface, still seen in reptile eggs today. The shell protects the eggs from predators, bacteria, damage, and drying out. But, unlike the walls of a fish tank, the shell of the amniotic egg is porous, allowing oxygen to pass through so that the growing amniote inside doesn't suffocate. The next two developments are two separate membranes that work together like a pair of lungs. They bring oxygen into the embryo while removing carbon dioxide. The first is the chorion, which is the protective layer that oxygen passes through after entering the shell's tiny pores. You may recognize the chorion as the thin skin you peel away on a hard boiled egg. Think of this waterproof membrane as the in and out doors of the egg. It's the entrance for oxygen and exit for carbon dioxide. The membrane working with the chorion is the allantois. If the chorion is the doors, then the allantois is essentially the lobby of the building. It directs the oxygen and carbon dioxide while simultaneously storing unneeded waste from the embryo. The chorion and the allantois make sure the embryo has everything it needs and gets rid of anything it doesn't. The last and perhaps the most important development is the amnion, the membrane for which the egg is named. The amnion is also contained within the chroion and holds the fluid in which the embryo floats. Because it has left the watery world of the amphibians, the amnion is necessary for preventing the embryo from drying out. It is the transportable pond that allows the amniote to lay the egg on dry land. Its fluid also protects the embryo from any collisions or rough landings, like a shock absorber on your bike or car. Together, the shell and these four membranes create a safe, watery environment for the embryo to grow and develop. The new amniote offspring will continue the process of vertebrate evolution as it explores new land away from the water. They will spend the next million years splitting into two distinct groups: the synapsids and sauropsids. Synapsida is the group of animals that contain mammals, while sauropsida is the group that contains reptiles, birds, and dinosaurs. These two amniotic groups collectively contain the walruses, rattlesnakes, and parakeets we know today. Like a family reunion, with relatives of every shape and size, coming together from different corners of the Earth, these animals can all call one place home: the amniotic egg.