amniote

Examples of amniote in the following topics:

• Characteristics of Amniotes

  • The distinguishing characteristic of amniotes, a shelled egg with an amniotic membrane, allowed them to venture onto land.
  • The amniotes, reptiles, birds, and mammals, are distinguished from amphibians by their terrestrially-adapted egg, which is protected by amniotic membranes.
  • The evolution of amniotic membranes meant that the embryos of amniotes were now provided with their own aquatic environment, which led to less dependence on water for development, allowing the amniotes to branch out into drier environments.
  • The amniotic egg is the key characteristic of amniotes .
  • The key features of an amniotic egg are the chorion, amnion, and allantois.

• Evolution of Amniotes

  • Modern amniotes, which includes mammals, reptiles, and birds, evolved from an amphibian ancestor approximately 340 million years ago.
  • The first amniotes evolved from their amphibian ancestors approximately 340 million years ago during the Carboniferous period.
  • The early amniotes diverged into two main lines soon after the first amniotes arose.
  • In the past, the most common division of amniotes has been into the classes Mammalia, Reptilia, and Aves.
  • This chart shows the evolution of amniotes.

• Bilaminar Embryonic Disc Development

  • The floor of the amniotic cavity is formed by the embryonic disc.
  • The floor of the amniotic cavity is formed by the embryonic disc, which is composed of a layer of prismatic cells and the embryonic ectoderm.
  • As the embryo progresses in implantation, a small space appears in the embryoblast and forms the amniotic cavity.
  • The epiblast forms the floor of the amniotic cavity and is continuous with the amnion.

• Amnion Development

  • This cavity is roofed in by a single stratum of flattened, ectodermal cells called the amniotic ectoderm.
  • Outside the amniotic ectoderm is a thin layer of mesoderm (continuous with that of the somatopleure), which is connected by the body stalk with the mesodermal lining of the chorion.
  • When first formed, the amnion is in contact with the body of the embryo, but by about the fourth or fifth week, amniotic fluid (liquor amnii) begins to accumulate within it.
  • The amniotic fluid allows some free movement for the fetus during the later stages of pregnancy and also diminishes the risk of injury.

• Evolution of Reptiles

  • Dinosaurs and pterosaurs diverged from early amniotes and dominated the Mesozoic Era.
  • One of the oldest-known amniotes is Casineria, which had both amphibian and reptilian characteristics.
  • Soon after the first amniotes appeared, they diverged into three groups (synapsids, anapsids, and diapsids) during the Permian period.

• Development of the Urinary System

  • Urine excreted into the amniotic cavity by the fetus forms a major component of the amniotic fluid.

• Limitations of Phylogenetic Trees

  • For example, the phylogenetic tree shows that lizards and rabbits both have amniotic eggs, whereas frogs do not; yet lizards and frogs appear more similar than lizards and rabbits.
  • A tree may not indicate how much time passed between the evolution of amniotic eggs and hair.

• Onset of Labor

  • As labor progresses the amniotic sac can rupture and cause a slow or a fast gush of fluids.
  • Labor usually begins within a 24-hour period after the amniotic sac ruptures.

• Building Phylogenetic Trees

  • For example, all of the organisms in the orange region evolved from a single ancestor that had amniotic eggs .
  • Consequently, all of these organisms also have amniotic eggs and make a single clade, also called a monophyletic group.
  • Now, consider the amniotic egg characteristic in the same figure .
  • Lizards, rabbits, and humans all descend from a common ancestor that had an amniotic egg.

• Labor and Birth

  • At some point, the amniotic sac bursts and the amniotic fluid escapes.